CHAPTER 5

Evaluating Airport Curbside Operations

This chapter presents measures of curbside roadway performance, definitions of curbside roadway sufficiency, and a hierarchy of analytical methods for estimating curbside roadway capacities and sufficiency. It also describes use of a macroscopic method, QATAR, for analysis of airport curbside roadways, and explains the use of this method. Appendix C documents the queuing theory and assumptions used in QATAR.

In evaluating airport curbside roadway operations, analyses of both the curbside lanes (where motorists stop to pick up or drop off passengers) and the adjacent through lanes are required. As described in Chapter 2, these analyses are necessary because double- or triple-parked vehicles impede or delay the flow of vehicles in the adjacent through lanes. As a result, the capacity of the through lanes decreases as demand for curbside space approaches or exceeds the capacity of a curbside roadway segment, causing double- or triple-parking.

Throughout this chapter, the term “parked vehicle” refers to a vehicle that has come to a complete stop and remains stopped to allow the loading or unloading of passengers and their baggage. Vehicles on curbside roadways are not “parked” in the same sense as vehicles in a parking lot or an on-street parking space because these parked vehicles may not be left unattended on airport curbsides. Within the airport industry, vehicles stopped or standing at curbsides are commonly referred to as parked vehicles.

As described in more detail later in this chapter, the capacity of curbside pickup and drop-off areas depends on the number of lanes airport management allows to be used for vehicles to stop, load, or unload. For example, at airports where double-parking is prohibited, curbside capacity equals the effective length of the lane next to the curb. At airports where double-parking is allowed, curbside capacity equals twice the effective length of this lane. Effective curb length differs from the actual curb length in that it only includes space where motorists can stop, load, or unload and excludes areas reserved for other uses (e.g., crosswalks, disabled motorists, or airport police vehicles) or where motorists prefer not to stop (e.g., adjacent to columns, along sharp curves, or uncovered areas distant from the nearest terminal doors).

In this chapter, methods of estimating the volumes, capacities, and sufficiency of the curbside lanes and the through lanes are presented separately. However, when estimating airport curbside roadway capacities and sufficiency, it is necessary to consider the operations of both the curbside lanes (the lanes predominately used for passenger loading or unloading) and the through lanes concurrently because the capacity and sufficiency of an airport curbside roadway system is determined by the component that has the lowest capacity or provides the poorest sufficiency.

The methods and data presented in this chapter represent the best available information concerning airport roadway operations and the consensus of the research team, the project panel, and prior research. It is suggested that additional research be conducted to refine the estimated airport curbside performance.

5.1 Performance Measures

The curbside utilization ratio is the recommended performance measure for airport curbside roadways. The curbside utilization ratio represents the ratio of the demand for curbside parking to its capacity and indicates the ability of a roadway to accommodate existing or projected requirements for vehicles loading or unloading at the curbside. It also indicates if spare capacity is available to serve additional demand and surges in demand.

As noted in prior chapters, roadway and curbside capacities are typically analyzed for the peak hour or design hour of a facility. For airport roadways, it is suggested that the design hour be a typical busy hour on the peak day of the week during the peak month. This suggestion is in contrast to planning for airfield and other airport facilities, which often considers the peak hour of an average day during the peak month. This approach recognizes there may be a limited number of times during the year when demand is higher, but constructing facilities to accommodate such infrequent occurrences may not be cost-effective. Other potential sources for a design hour could include the 30th busiest hour in the year (a peak period frequently selected in highway and transportation planning) and the 95th percentile hourly volume during the year, among others. Selection of a design hour may reflect a clock-hour or a busy 60-minute period (such as the busiest four consecutive 15-minute periods during the peak day of the peak month).

Typically, a curbside utilization ratio of 1.30 or less (65% of the combined capacity of the inner and second curbside lanes) is a desirable planning target for new curbside roadways. A curbside utilization ratio of 1.70 (85% of the combined capacity of the inner and second curbside lanes) is acceptable for existing facilities, recognizing that during peak hours and days of the year, demand will exceed capacity. However, individual airport operator policies regarding parking in multiple lanes may dictate different curbside utilization ratio planning targets.

The curbside utilization ratio is used as an indicator of curbside sufficiency, which provides an overall indication of the quality of the experiences of drivers and passengers using the curbside roadway. A curbside sufficiency of “under capacity” is a desirable planning target for a medium- or small-hub airport, both for the design of new curbside roadways and for analyzing an existing facility. This target should result in acceptable conditions during most of the year with a limited number of hours experiencing poorer conditions (such as around holidays). For a large-hub airport, a curbside sufficiency of “under capacity” is a desirable planning standard for the design of new curbside roadways. However, airport management could deem a curbside sufficiency of “near capacity” to be acceptable for design or existing operations depending on the desired customer experience, incremental cost to achieve an improved curbside utilization, the remaining useful life of the terminal and adjacent roadways, or number of hours per year during which “near capacity” conditions occur. A curbside sufficiency of “near capacity” may be considered acceptable for an existing curbside roadway at a large-hub airport, recognizing that on some peak days of the year, the level of service may decrease to “at capacity” or “over capacity.” Two separate sufficiency measures are used: roadway sufficiency for through traffic and curbside sufficiency for curbside loading/unloading traffic.

When additional performance measures, as described below, are required to supplement the curbside utilization ratio, the analysis is conducted using a microsimulation model. Such supplemental measures cannot be accurately determined without the use of a microsimulation model, either quantitatively or in the field (i.e., they are difficult to quantify using field surveys). For example, the use of a microsimulation model would help document the ability of an existing curbside roadway to accommodate future demand or quantify the benefits resulting from alternative curbside improvement options. A microsimulation model also captures the interaction effects between adjacent zones, such as queue spillback from one zone to the next or demand starvation downstream of a queuing bottleneck. These effects are not captured in simple macroscopic methods.

Supplemental performance measures include

• Number of vehicles parked in the second and third lanes. The number of through lanes blocked by parked or parking vehicles (and the proportion of the modeled hour during which this blockage occurs) is an indicator of the extent of roadway congestion. It is also an indirect indication of the ability of motorists to enter/exit and stop at their preferred curbside locations since it is difficult for motorists stopped in the curb lane to exit when triple-parking occurs without the intervention of traffic control officers.
• Queue length. Queue length is the number of vehicles waiting to enter the curbside roadway or a specific curbside parking area expressed in terms of the distance that the vehicle queue extends back from the curbside parking area or point of congestion. Queue lengths are estimated for different levels of probable occurrence. The mean queue length has a 50% probability of being exceeded sometime during the hour. The 95% queue length has a 5% probability of being exceeded. The 95% queue length is typically used for design purposes.
• Queuing duration. The queuing duration (in minutes) indicates how long the congestion will last, and is useful for comparing two potential design solutions, neither of which completely eliminates queuing. Ideally, the queuing duration is zero for a new curbside roadway and less than 1 hour for an existing curbside roadway.
• Average vehicle delay. Average vehicle delay consists of two components—through traffic delay and curbside loading/unloading delay.
• Through traffic delay. Through traffic delay is the extra amount of time, due to traffic congestion, required for a vehicle to traverse the entire curb length. To determine through traffic delay, the unimpeded travel time for through traffic on the curbside roadway is subtracted from the actual travel time to obtain the amount of through traffic delay per vehicle. In the design of a new curbside roadway, the delay to through traffic should ideally be near zero outside of crosswalks. For existing roadways, delays of up to 15 seconds per vehicle outside of crosswalks may be acceptable, recognizing that the delays could be significantly higher on peak days of the year. The acceptable amount of delay for through vehicles should be set by the airport operator based on the design of the landside circulation system and the number of other delays experienced by through vehicles on other portions of the roadway circulation system. For example, if through vehicles must pass several curbside loading/unloading areas, then delays at each curbside area are cumulative and may be less tolerable.
• Curbside loading/unloading delay. Curbside loading/unloading delay is the extra amount of time, due to traffic congestion, a vehicle requires to pull into a curbside stall, load or unload passengers, and exit. The minimum time necessary to drop off or pick up a passenger during uncongested periods (i.e., the average dwell time) should be subtracted from the total average observed time to obtain the amount of curbside loading/unloading delay. Curbside delays of up to 30 seconds are acceptable when designing a new roadway. Delays of up to 60 seconds per vehicle are acceptable for existing roadways.

As shown by the checkmarks in Table 5-1, use of these performance measures requires different analysis methods. When analyzing curbside roadways using microsimulation models, it is possible

Table 5-1. Recommended airport curbside performance measures.

to consider the number of vehicles parked in the second and third lanes, the length and duration of curbside queues, and average vehicle speeds (or delays). Without the aid of microsimulation models, it is difficult to accurately estimate vehicle parking patterns, travel times and delays, and queue lengths because of the relatively short distances on curbside roadways being analyzed and the difficulty estimating queue lengths through other means. Microsimulation also allows assessments of how traffic congestion on one curbside roadway link can impact upstream facilities or meter traffic bound for downstream facilities. When curbside roadways are being analyzed using the quick-estimation or macroscopic methods described in this chapter, the appropriate performance measures are curbside utilization and the corresponding levels of service. Use of the supplemental performance measures “through traffic delay” and “curbside loading/unloading delay” is not recommended due to the complexity of determining and confirming these delays.

5.2 Sufficiency Definitions for Airport Curbside Roadways

The primary element defining the sufficiency of an airport curbside roadway is the ability of motorists to enter and exit the curbside space of their choice (e.g., one near their airline door or another chosen destination). As roadway demand and congestion increase, motorists are required to stop in spaces farther away from their preferred destination. The motorist is required to either stop in a downstream or upstream curbside space, double-park, or, in an extreme case, circle past the curbside area multiple times while searching for an empty space.

The key performance measures defining the sufficiency of an airport curbside roadway are the

• Number of vehicles parked or stopped in the curbside lane, and the percent double- or triple-parked, or otherwise stopped, in a position that interferes with the flow of traffic in adjacent lanes. This number of parked vehicles is a function of curbside demand versus available capacity.
• Length and duration of queues at the entrance to the curbside area.
• Average delay encountered by private and commercial vehicles entering and exiting the curbside areas.
• Curbside utilization ratio, which is a comparison of the length of the vehicles stopped along the curbside and the effective length of the curbside (i.e., the total length less the space occupied by crosswalks or other areas in which vehicles, or certain classes of vehicles, cannot stop).

As stated, most of these measures are obtainable only through microsimulation modeling. Therefore, sufficiency definitions for airport curbside roadways shown in Figure 5-1 and presented in Table 5-2 are based on curbside utilization ratios. These definitions and ratios were validated using focus groups of airline passengers, airport landside managers, and commercial vehicle operators conducted as part of the research presented in ACRP Report 40: Airport Curbside and Terminal Area Roadway Operations.

ACRP Report 40 used level-of-service definitions on a scale of LOS A to F similar to those used in the HCM for a variety of facilities. To simplify the method and avoid confusion with the HCM measures based on quality of service, this report uses a sufficiency rating similar to that used in the HCM for planning analyses of signalized intersections.

5.3 Estimating Airport Curbside Roadway Traffic Volumes

Curbside roadway traffic volumes can be estimated using the same methods used to estimate airport terminal area roadway traffic (see Chapter 3): the traditional four-step travel forecasting method and the growth factor method. The key differences between estimating terminal area

Table 5-2. Sufficiency criteria for airport curbside roadways.

^a The ratio between the calculated curbside demand and the available effective curbside length.

roadway traffic and curbside roadway traffic include the need to prepare the following for curbside roadway traffic:

• Separate estimates of vehicles stopping in a curbside lane and through traffic vehicles. At small airports with a single terminal building and a short curbside area (e.g., less than 500 feet in length), the volume of through vehicles may equal the volume of vehicles stopping at the curbside. However, these volumes may differ at airports having (1) multiple terminal buildings or large concourses served by a common roadway, (2) a curbside area with inner and outer curbside roadways separated by a raised island with midpoint entrances and exits, or (3) curbside roadways that are used by non-curbside traffic (e.g., vehicles entering or exiting parking areas, rental car areas, or other facilities).
• Separate analyses of the departures curbside and arrivals curbside roadways. It is necessary to analyze these curbside areas separately because the departures and arrivals peak periods at an airport (and thus peak periods of curbside demand) occur during different hours of the day, and vehicle dwell times and space allocations (the proportion of curb length assigned to individual classes of vehicles) differ significantly at the departures and arrivals curbside areas, as described in subsequent sections of this chapter. At airports with dual-level curbside roadways, separate analyses of each level are required. At airports with a single-level curbside roadway, analyses of the peak periods for originating, terminating, and total passengers (originating plus terminating) are required.
• Separate analyses of terminal-adjacent versus island (or parallel) curbsides. The volume and mix of vehicles on center-island curbsides typically vary from those at the curbsides adjacent to the terminal building, depending on the assignment of vehicles or allocation of space to each curbside and curbside roadway. For example, certain classes of vehicles may be required to drop off and pick up passengers at a specific location, or through traffic may be directed to use certain lanes.

• Analysis of impacts of pedestrian crossings. At-grade pedestrian crossings may connect the terminal building with center-island curbside areas or adjacent parking facilities. At-grade crosswalks reduce both the effective curb length and the roadway capacity. The proportion of time that a crosswalk is occupied and blocked to vehicle traffic is a function of (1) the volume of pedestrians using a crosswalk, which varies according to its location and the facilities it connects; (2) the time passengers require to walk across the roadway (e.g., at a design walking speed of 3.5 feet per second per the FHWA Manual on Uniform Traffic Control Devices); and (3) whether vehicular and pedestrian traffic at the crosswalk is controlled by a signal, stop sign, traffic control officer, or is uncontrolled. While signalized pedestrian controls can be coordinated using a common background cycle to minimize stops, some signalized crosswalks, as well as crosswalks controlled by traffic control officers, or unsignalized crossings, are not coordinated and thus may create multiple stops for through vehicles.
• Separate analyses for each class of vehicle. Private vehicles, taxicabs, limousines, TNCs, door-to-door vans, courtesy vehicles, and charter buses/vans each have different dwell times, required vehicle stall lengths, and maneuvering capabilities. Furthermore, each service provided by these vehicles may have different operational methods and be governed by different airport regulations. For example, on an arrivals curbside, an airport operator may permit taxicabs, certain TNCs, or valet-parked vehicles to stand at the curbside for 30 minutes or more to ensure that waiting taxicabs, TNCs, and valet-parked vehicles are available for arriving customers. At some airports, customers using valet parking may drop off and pick up their car at the curbside, with vehicles temporarily stored at this location. Space for valet operations should be analyzed separately. Airport staff may allow charter buses to remain at the curbside for 10 to 15 minutes to ensure that all members of a large party have claimed their bags and boarded the vehicle, but may only allow hotel/motel courtesy vehicles to stop while actively boarding passengers. It is expected that many of these classes (e.g., taxicabs, TNCs, valet-parked vehicles, and courtesy vehicles) will eventually be operated using AVs, which could result in an additional class of service(s) with different operational methods that are governed by different airport regulations.
• Separate estimates of traffic volumes for each terminal building or concourse. The peak periods of activity for each airline serving an airport may occur during different hours of the day. At airports with multiple terminals or large concourse(s) dominated by a single airline, the largest traffic volumes (and curbside area requirements) may occur during a different hour (or different 15-minute period) at each terminal or near each concourse. The average vehicle dwell times at terminals primarily serving international passengers are longer than those at other terminals—about 30% longer at drop-off curbsides and 45% longer at pickup curbsides based upon data provided by San Francisco International Airport. In addition, motorists prefer to stop at the curbside area nearest the doors (or skycap podiums) serving their airline (or that of the passenger they are transporting). Thus, demands are not distributed uniformly along the length of a curbside—particularly at airports with multiple terminals or large concourses—but are concentrated at the curbside areas corresponding to the airlines serving the largest volume of passengers during the peak period.

  As a result, at airports with several terminals or multiple concourses, the traffic volumes and curbside area requirements that correspond to (or are generated by) each terminal or concourse should be estimated separately. These estimates can be prepared by allocating the total peak-hour traffic volumes to each curbside area according to the percentage of total demand served by each area during the peak hour. The percentage of total demand served by each area can be estimated by analyzing (in decreasing order of reliability) the proportion of (1) peak-period originating (or terminating) passengers served by each terminal building or concourse; (2) the number of scheduled aircraft seats served by a terminal or concourse during the peak period; (3) TSA screenings occurring, assuming that there are separate checkpoints for each terminal or concourse; or (4) the number of aircraft gates served by each concourse.

If the data are available from passenger surveys or other sources, it is preferable to prepare separate estimates of the traffic volumes occurring at each terminal curbside area by type of vehicle. This is because, as described in prior chapters, the travel mode choices and arrival/departure patterns of passengers on each airline may differ depending on the type of flight (e.g., domestic, international, or low-cost carrier), typical trip purpose (routes primarily serving business travelers versus those serving vacation or non-business travelers), a passenger’s place of residency (local resident versus non-resident), party size/presence of persons not accompanying the passenger aboard the aircraft, use and availability of parking, and other factors.

5.4 Estimating Airport Curbside Roadway Capacity and Sufficiency

Estimating airport curbside roadway capacities and sufficiency requires analyses of both the curbside lanes and the through lanes because the number of vehicles stopped in the curbside lanes affects the flow of vehicles in the through lanes; as curbside lanes approach or exceed capacity, the capacity of the adjacent through lanes is reduced.

5.4.1 Establishing Curbside Lane Capacity

Curbside lane capacity is typically estimated in terms of the area (and the number of lanes) that the stopped vehicles may occupy while loading or unloading. Since vehicles stop in a nose-to-tail manner at most airports, this area is described as the effective length of curb measured in linear feet. As described previously, effective length is defined as the total length of the lane minus (1) any space unavailable for public use because it is reserved for crosswalks, disabled motorists, or specific classes of vehicles (e.g., taxicabs or public buses) and (2) space located beyond the ends of the terminal building or adjacent to columns or other physical barriers that discourage its use by motorists because passengers cannot easily open their doors or easily enter/exit a vehicle.

The number of stopped vehicles that the curbside lane(s) can accommodate (i.e., the capacity of the curbside lanes) varies depending on

• The number of lanes in which airport operators allow vehicles to routinely stop to load or unload passengers and their baggage. Airport operators establish specific policies concerning double-parking that reflect the width of their curbside lanes, enforcement policies and capabilities, customer service, and use by private and/or commercial vehicles.
• The amount of curbside length occupied by a single stopped vehicle. To allow sufficient space to maneuver in and out of curbside loading or unloading spaces, drivers leave space between their vehicles and the vehicles immediately in front of and behind them. For example, if a single vehicle is 18 feet long, the total space required to accommodate it is about 25 feet. Thus, the curbside length occupied by a single vehicle is a function of the typical vehicle length, its maneuverability, and how close drivers are willing to park adjacent to vehicles.

In practice, the number of stopped vehicles is not distributed uniformly along the entire effective curbside length, as motorists prefer to stop at the first or initial doorway serving their airline, near skycap/baggage drop-off stands, or other preferred locations. Similarly, the number of stopped vehicles is not distributed uniformly throughout the peak hour, instead, traffic volumes typically fluctuate in 5- to 15-minute bursts, particularly at smaller airports, reflecting airline schedules.

Airports Where Double-Parking Is Prohibited

At airports where double-parking is prohibited, the number of vehicles that can be accommodated in the curbside lane is equal to the effective length of a single curbside lane divided by the

average length occupied by a single parked vehicle. Some airport operators restrict curbside parking or standing to a single lane for operational reasons (e.g., a narrow curbside roadway or curbsides used exclusively by commercial vehicles where double-parking is prohibited).

This description of the number of vehicles that can be accommodated in the curbside lane also applies to curbside roadways with a maximum of three lanes. This is because on a curbside roadway with three lanes only a single through lane would be available if double-parking were to occur, which would lead to frequent bottlenecks (e.g., when a double-parked vehicle or an open door of such a vehicle intrudes into the third lane). Thus, a single through/maneuvering lane for a significant portion of the curbside length is considered unacceptable, and double-parking is generally not tolerated on curbside roadways with a maximum of three lanes.

Airports Where Double-Parking Is Allowed

At airports where double-parking is allowed on the curbside roadways, the number of vehicles that can be accommodated at the curbside is equal to twice the effective curbside length divided by the average length occupied by a single parked vehicle. At airports where double-parking is regularly allowed, pavement markings typically have been installed designating the lane next to the sidewalk plus the adjacent lane for passenger drop-off or pickup, or where enforcement policies allowing double-parking have been established.

On roadways where double-parking is allowed, if the roadway were operating at full capacity, the stopped vehicles would likely not be evenly distributed along the length of the two curbside lanes; some motorists would choose to triple-park next to the most desirable doorways or other locations.

Additional Considerations

At airports with inner and outer curbside areas available for use by private vehicles, these areas have different effective capacities, even if they are the same length. Motorists prefer to stop at the most convenient space available (e.g., the inner curbside lane), even if they observe downstream congestion or delays on this roadway. Thus, if private vehicles are directed to both the inner and outer curbside areas it is necessary to consider the uneven distribution of demand between the two roadways, or alternatively, “discount” the capacity of the outer, less convenient curbside area. If one curbside is allocated to private vehicles and the second is allocated to commercial vehicles, such discounting is not required.

For example, motorists approaching the departures curbside at some airports can use either the curbside area adjacent to the terminal building or an alternative curbside area located within the adjacent parking garage. Passengers using the alternative curbside are provided with a grade-separated path to/from the terminal building and are offered skycap service on certain airlines. Notwithstanding the good access, good directional signage, and amenities available, motorists are reluctant to use the curbside area within the parking garage, even when the curbside area adjacent to the terminal is congested.

Consequently, it is suggested that, when calculating the capacity of a similar, remotely located curbside area, it is necessary to adjust (or discount) the actual length of curb space within a garage (or other supplemental location) to determine its effective capacity. This adjustment is necessary because, if both the primary and supplemental curbsides are allocated for private vehicle use, the supplemental curbside will provide less capacity (even though it may be the same length) than curb space adjacent to the terminal building because it attracts fewer passengers. This capacity adjustment factor is similar conceptually to the adjustments for lane utilization at various types of intersections as presented in the HCM. Again, if commercial vehicles are required to use a remotely located curbside (or passenger drop-off/pickup) area, it is not necessary to adjust its capacity; in this case, the use of this area is mandatory, not elective.

No published research provides guidelines on this adjustment factor, but the factor appears to vary according to the traffic queues caused by downstream congestion, local enforcement policies, availability of skycap service and dynamic signage, and the demographics of the passenger market (e.g., the proportion of frequent travelers or those traveling primarily with carry-on baggage). It is suggested that analyses be guided by field observations of existing conditions, which would reflect the unique characteristics of the airport and its passengers. If field data are unavailable, it is suggested that the capacity of the supplemental curb space located in a garage be reduced by 50% and that the capacity of an outer curbside be reduced by 20% to 30%.

Alternative Curbside Configurations

It is assumed in the previous discussions that vehicles stop in the curbside lane(s) in nose-to-tail configuration. However, at some airports, the curbside areas are configured with pull-through spaces or 45-degree stalls. (See Chapter 2 for illustrations of alternative curbside configurations.) In such configurations, the curbside capacity is the number of individual parking spaces as opposed to a curbside length. Such configurations also may result in dwell times and through-lane capacities that are different from those discussed in the following section.

5.4.2 Calculating Curbside Lane Requirements

Quick-Estimation Method

This method is appropriate for use during the early planning and design stages for a new curbside when little is known about the details of the curbside design or layout. This method is used to compute the curb length required to serve a given demand, but it does not provide specific results on performance, such as average delay or queuing probability.

A curbside lane can be considered as a series of stopping spaces, each capable of accommodating one vehicle. The average number of vehicles each space can serve during a given time period is inversely proportional to the average length of time (referred to as the vehicle dwell time) a vehicle occupies a space. For example, if the average vehicle dwell time is 3 minutes, then each curbside space can accommodate, on average, 20 vehicles per hour. If the peak-hour volume is 160 vehicles, then (with the assumed average dwell time of 3 minutes per vehicle), the required curbside length is equivalent to eight spaces or 200 linear feet (assuming an average space length of 25 feet for illustrative purposes). This can be represented mathematically as

R_a = V * D_i / 60 * L

where

This formula represents a condition where a single class of vehicles (vehicle type i) is using a curbside area (e.g., a curbside serving private vehicles exclusively), or where the requirements are developed assuming that all vehicles can be represented using average dwell times and a single stall length. More accurate estimates can be developed by considering, separately for each class of vehicle, the hourly volume, the distribution of dwell times (rather than average dwell time), and average vehicle length. Additional accuracy can result from consideration of the peak periods within the peak hour (e.g., analysis of the peak 15 or 20 minutes) and the non-uniform distribution of demand along the curbside lane caused by a concentration of traffic at specific

airline doors or other attraction points. The non-uniform arrival rate and distribution of vehicles can be reflected using statistical factors (e.g., a Poisson distribution).

Vehicle dwell time is one of the key factors determining demand for curbside facilities. Dwell times are affected by

• Enforcement policies of dwell time limits (i.e., strict enforcement leads to shorter dwell times).
• Local driver behavior (i.e., do drivers double-park in a way that allows other motorists to easily enter and exit the lane immediately adjacent to the terminal).
• Party sizes and amount of luggage (i.e., larger party sizes and passengers with more luggage take longer to load and unload).
• The time required for a customer of a vehicle-for-hire (e.g., taxicabs, limousines, and TNCs) to complete the payment transaction for the ride.

When conducting curbside operations or planning studies, an airport would ideally be able to conduct dwell time measurements to identify values specific to their passengers and policies. In the absence of such data, an analyst may wish to select a value based on those presented in Table 5-3, which summarizes dwell time measurements collected at selected airports.

In addition, because dwell times can reflect enforcement policies, which impact both dwell times and driver behavior (i.e., enforcement staff can intervene to allow vehicles to quickly enter or exit curbside spaces), an analyst may wish to use professional judgment and assume future dwell times that reflect different enforcement policies and thus, different dwell times. For example, an airport currently experiencing 3-minute average dwell times for passenger pickup may wish to assume a shorter dwell time (e.g., 90 seconds, or a value at the lower end of those observed at other airports) when planning a new terminal on the assumption that the airport would be able to achieve shorter (but still realistically achievable) dwell times through enforcement.

Table 5-4 presents data—gathered at the airports serving Memphis, Nashville, Portland, Raleigh-Durham, and San Diego—used to calculate curbside lane requirements by class of vehicle, the application of a Poisson distribution (or adjustment) factor, and the resulting curbside requirements. The table presents examples of typical curbside dwell times and vehicle stall lengths based on observations of curbside roadway operations at the airports, the estimated curbside requirements (i.e., design length) for five zones (two zones on the enplaning curbside and two zones plus a courtesy vehicle lane on the deplaning curbside). A comparison of the estimated requirements with the available curb length yields utilization factors for each of the five zones. As shown, two of the zones are substantially over capacity as evidenced by the utilization factors over 2.0.

Table 5-3. Average dwell times observed at selected airports.

Note: -- = Data not collected or vehicles are allowed to wait for customers at the curb (such as in a taxicab queue).

^a Range reflects differences between vans operated by off-airport parking companies (low end of range) versus those operated by hotels/motels (upper end of range).

The quick-estimation method involves the following steps:

• Step 1: Determine peak-hour traffic volume from field survey or estimates of future traffic.
• Step 2: Determine the vehicle mix. If vehicle mix is unknown, assume that private vehicles represent 55% to 80% of the total traffic volumes (with small- and medium-hub airports typically having a higher proportion of private vehicles), TNCs represent 15% to 20%, taxicabs and limousines represent up to 5%, courtesy vehicles represent 5% to 10%, and vans/buses/public transit represent 5% or less.
• Step 3: Determine the average vehicle stall length. Use the default values shown in Table 5-4 or the QATAR model (see Figure 5-2) or measure representative values, particularly for unusual vehicles (e.g., motorcycles, three-wheeled vehicles) or atypical parking configurations.

Table 5-4. Estimate of terminal building curbside requirements—sample calculation.

^a Represents calculated stall requirements adjusted to reflect random arrival of vehicles and non-uniform distribution of traffic volumes and demands using Poisson statistical probability factors.

^b Assumes that this mode makes a single stop at the curbside.

• Step 4: Determine vehicle dwell times using field measurements or the default dwell times shown in Table 5-4 or the QATAR model.
• Step 5: Calculate curbside stall requirements that are equal to the volume multiplied by vehicle dwell times divided by 60 minutes.
• Step 6: Determine curbside design stall requirements that are equal to the curbside stall requirements times a probabilistic factor applied to the total curbside stall requirements (if a mixed-use curbside such as a typical departures curbside) or to an individual class of vehicles (if curb space is allocated to this classification), ranging from 3.0 for curbside stall requirements of fewer than 5 curbside stalls to 1.2 for curbside stall requirements of 100 or more. (This assumes the peak requirements for all vehicle classes occur at the same time, which is reasonable if the curbside predominantly serves private vehicles, TNCs, and taxicabs. If a large volume of traffic is composed of buses, vans, or other vehicles or is composed of many individual curbside zones, it may be appropriate to analyze each class separately to determine the average number of stalls for each class, and then apply the probabilistic factor to the sum.)
• Step 7: Determine curbside design length that is equal to the number of design stalls times the average vehicle stall length.
• Step 8: Calculate the curbside utilization ratio that is equal to the curbside design length divided by the existing curb capacity (or effective length) considering whether double-parking is allowed by the airport operator. As defined previously in this chapter, a curbside utilization ratio equal to or less than 1.3 is considered acceptable for a new design, while a curbside utilization ratio equal to or less than 1.7 is considered acceptable for existing curbside roadways.

Macroscopic Method

Alternatively, the curbside lane can be considered a series of processing points (or servers) and traditional queuing analyses can be used to calculate the capacity of individual servers and the total capacity of the curbside lane. The macroscopic method (QATAR) described in the upcoming section, “Analytical Framework Hierarchy for Airport Curbside Roadways,” uses queuing analysis to estimate curbside capacity.

The following subsections describe the calculations of through-lane capacity and curbside capacity.

5.4.3 Calculating Through-Lane Requirements

The requirements for curbside roadway through lanes depend on the areas they serve. At airports with a single terminal building and a short curbside area, the volume of through vehicles may equal the volume of vehicles stopping at the curbside. As discussed in previous chapters, factors that may result in higher volumes of traffic in the through lanes include vehicles bypassing a curbside area (1) that does not serve their airline (e.g., a different terminal building or major concourse); (2) that is reserved for other classes of vehicles (e.g., authorized commercial vehicles); or (3) to enter or exit parking, rental car, or other land uses not related to curbside activities. As noted, bypass traffic proceeding to another terminal (as opposed to through traffic proceeding to a downstream portion of the curbside lane) may represent a significant portion of the total curbside roadway traffic volume. When these conditions occur, it is necessary to use the methods described in Chapter 4 to estimate the volume of traffic associated with the alternative land uses and/or to assign traffic volumes to each curbside roadway section (or airline) and class of vehicle.

The capacity of a curbside roadway through lane is measured using methods similar to those described in Chapter 4 for other airport terminal area roadways, adjusted to account for the presence of double- or triple-parked vehicles. As noted previously, double- and triple-parked vehicles block or delay the movement of vehicles in through lanes because through traffic must decelerate and maneuver around these stopped vehicles. As a result, through-lane capacity decreases when

curbside lane demand exceeds the available capacity of a specific curbside segment (as opposed to the entire curbside length), and vehicles are double- or triple-parked.

The reduction in through-lane capacity resulting from increased curbside lane demand can be estimated using commercially available microsimulation models capable of simulating airport curbside roadways or using QATAR (discussed later in this chapter). Alternatively, the approximations shown in Table 5-2 can be used to estimate curbside roadway lane capacities.

Curbside roadway capacity must also be reduced when at-grade pedestrian crosswalks are present. As described above, the extent of the capacity reduction is a function of the volume of pedestrians crossing the roadway since the amount of time motorists must wait for pedestrians increases with pedestrian traffic. For example, if a crosswalk is controlled by a traffic signal, and if the signal allocates 25% of the green time during each hour to pedestrians, then the capacity of the curbside roadway would be 25% less than if there were no crosswalk. If, instead of a signal, crosswalk operations are controlled by a traffic control officer, then a similar approximation can be made by observing curbside roadway operations. If the crosswalk is uncontrolled, then the behavior of motorists (do they stop when a pedestrian enters a crosswalk?) and the volume of pedestrians need to be considered. For curbsides with multiple crosswalks, the combined impact of all crosswalks should be considered.

5.4.4 Additional Considerations in Estimating Commercial Ground Transportation Vehicle Curbside Requirements

The analytical methods used to estimate curbside traffic volumes presented in Chapter 4 are applicable to private vehicles and commercial ground transportation vehicles, the volumes of which can be directly correlated to airline passenger demand (e.g., limousines, taxicabs, and door-to-door vans dropping off passengers). However, these analytical methods are not applicable to vehicles that are allowed to remain at the curbside for extended periods (e.g., taxicabs, door-to-door vans, and TNCs operating in a demand-responsive mode standing in queues waiting to pick up passengers) or that operate on a scheduled or de facto scheduled basis (e.g., courtesy vehicles that generally operate on fixed headways regardless of the number of passengers transported).

Allocation of Curb Space

Generally, airport operators do not reserve space for commercial ground transportation vehicles dropping off airline passengers, with the exception of vehicles, such as public buses, that drop off and pick up passengers at the same curbside space. The amount of space allocated to commercial ground transportation vehicles picking up passengers is generally determined by airport management considering such factors as

• Customer expectations. Deplaning airline passengers generally expect on-demand taxicabs as well as TNCs to be available immediately adjacent to the baggage claim area or visible from the exit doors. Passengers who have reserved luxury limousines or use curbside valet parking expect a higher level of service than those choosing public transportation (e.g., baggage assistance, shorter walking times, minimal wait time).
• Operational needs. To minimize the wait times of deplaning passengers, taxicabs are generally allowed to wait at the deplaning curbside area in queues of 3 to 10 vehicles. The number of taxicabs in the queue is a function of airport policy, the proximity of a taxicab hold area (where additional taxicabs may wait until dispatched to the curb), and the availability of curb space. At airports where TNCs are permitted to operate using a PIN code issued to customers or a similar system, the amount of space allocated to TNCs is a function of the volume of customers, the number of authorized TNCs, and airport policies.
• Space requirements. In analyzing the amount of space to be allocated to each class of commercial vehicle operator (e.g., hotel/motel courtesy vehicles), the number of vehicles that will use

• the space concurrently (which is based on the number of operators and the frequency with which they serve the airport) and the permitted vehicle dwell times and vehicle sizes must be considered.
• Vehicle maneuverability. In determining the amount of curb space to be allocated to each class of commercial vehicle operator, consideration should be given to the maneuverability requirement of the vehicles used (e.g., vans, minibuses, or full-size buses) and, if appropriate, requirements of access to baggage compartments or baggage trucks. For example, a 45-foot-long full-size bus requires about 60 feet to stop parallel to a curb space, while a 60-foot articulated bus requires 90 feet or more. If a bus has an under-the-floor baggage storage compartment, curb spaces should be configured so that columns, sign poles, or other obstacles do not interfere with the opening of the baggage compartment.
• Vertical clearances. The ability of a full-size bus or other large vehicle to use a curbside area may be limited by the vertical clearance available (including low-hanging signs or drainage structures). For example, the minimum vertical clearances required are 13 feet for a full-size bus, 11.5 feet for the shuttle buses used by rental car companies, and 9 feet to 10 feet for courtesy vans serving hotels/motels. These dimensions can vary for those vehicles using compressed natural gas, having rooftop air conditioners, or having rooftop antennae. Some multilevel roadways cannot accommodate full-size buses or over-the-road coaches used by charter bus operators.
• Competition. Commercial vehicle operators compete with private vehicles, other operators providing the same service, and operators providing services that are perceived as being similar (e.g., taxicab and TNC operators). Each commercial vehicle operator generally wishes to be assigned space nearest the busiest terminal exit doors or space that is equivalent to or near the space provided to their competitors to maintain a “level playing field.”
• Airport management policy. Some airport operators have policies that encourage the use of public transportation and, thus, assign public transit vehicles the most convenient or most visible curb space.
• Revenues generated by commercial vehicle operations. Airport operators receive significant revenues from public parking and rental car concessions. As such, the courtesy vehicles serving on-airport parking lots and rental car facilities may be assigned higher priorities than other courtesy vehicles, including those serving privately operated parking or rental car facilities located off-airport.

Number of Curbside Stops Made by Commercial Vehicles

An additional factor to be considered when estimating the curbside roadway lane requirements of commercial vehicles is the number of stops each vehicle makes. For example, a single courtesy vehicle or public bus may stop two or more times along a terminal curbside, depending on the length of the curb and airport policies. The calculation of curbside lane requirements for each courtesy vehicle, for example, must be adjusted to account for the number of stops.

5.5 Analytical Framework Hierarchy for Airport Curbside Roadways

Airport curbside roadway operations—particularly the reduction in through-lane capacity that results from increased curbside lane demand—can be analyzed using the quick-estimation method described below, the macroscopic method (QATAR) described in subsequent sections, or commercially available microsimulation methods used to simulate airport curbside roadways.

5.5.1 Quick-Estimation Method

The quick-estimation method is used to measure both the curbside utilization ratio (i.e., the ratio between curbside demand and curbside capacity) and the maximum through capacity for five-, four-, and three-lane curbside roadways. The sufficiency for a curbside roadway system is defined as the worst result of the curbside lane sufficiency and through-lane sufficiency.

Estimates of the maximum flow rates (i.e., service flow rates) on curbside roadways at each sufficiency level can be determined using the data provided in Table 5-2. These data were established from observations of curbside traffic flows conducted as part of prior research and analyses of curbside roadway traffic flows conducted using microsimulation of airport roadway traffic. Figure 5-3 depicts the relationship between curbside roadway traffic flow rates and utilization factors for five-, four-, and three-lane curbside roadways.

Since as used in Table 5-2, “capacity” varies depending on whether an airport operator allows vehicles to double-park, the policy of the airport being analyzed should be reviewed.

To establish the sufficiency for a given curbside demand and traffic volume, the data in Table 5-2 should be used as follows:

1. Calculate the curbside utilization ratio.
2. Select the corresponding curbside utilization ratio for the curbside lane as shown in Table 5-2, rounding up to the next nearest value, and note the corresponding sufficiency level. For example, for a four-lane curbside roadway with a calculated ratio of 0.6, the sufficiency level is “under capacity.”
3. Determine the through-lane sufficiency for the through lanes by (1) selecting the maximum service flow rate row in the table corresponding to the appropriate number of lanes on the entire curbside roadway (include all curbside lanes and through lanes), and (2) comparing this rate to the volume of traffic on the curbside to calculate the volume/capacity ratio. For example, the roadway capacity is 2,680 vehicles per hour for a four-lane roadway with curbside lanes operating at a sufficiency of “under capacity.” If this roadway were serving 2,500 vehicles per hour, it would have a v/c ratio of 2,500/2,680 or 0.93.

4. Use Table 5-2 to determine the through-lane sufficiency that corresponds to the calculated v/c ratio for the through lanes. A v/c ratio of 0.93 corresponds to a sufficiency of “at capacity.”
5. The sufficiency for the entire curbside roadway system is determined by the component—either the curbside lane or the through lanes—with the poorest sufficiency. Considering the previously mentioned example, if the curbside utilization factor corresponds to “under capacity” and the peak-hour traffic volume, when compared to the through-lane maximum service flow rate, corresponds to “at capacity,” the sufficiency for the curbside roadway system is “at capacity.”

The maximum service flow rates shown in Table 5-2 apply to all vehicles on the curbside roadway, including those stopped in the curbside lane. These flow rates need not be adjusted for heavy vehicles or driver familiarity because they were developed from observations of traffic operations on airport curbside roadways.

5.5.2 Macroscopic Model—Quick Analysis Tool for Airport Roadways

Developed through prior research and updated as part of this research project, QATAR allows airport planners and operators to determine the ability of a curbside roadway to accommodate changes in traffic volumes, airline passenger activity, vehicle mix, curbside allocation plans, and curbside enforcement levels. QATAR also allows the user to observe how airport curbside roadway levels of service are expected to vary as these input factors change. Appendix C presents additional information on the methodology and mathematics used in QATAR.

In the analysis procedure used in QATAR, it is assumed that (1) vehicles begin to double-park and potentially triple-park, if allowed, as the number of vehicles stopping in a zone approaches the zone’s capacity (or length), and (2) the capacity of the adjacent maneuver and travel lanes decreases as the number of double- and triple-parked vehicles increases. The propensity of arriving vehicles to double-park (reflecting the percentage of occupied curbside spaces) can be modified by the QATAR user to reflect local conditions and policies.

Using a multiserver (or multi-channel) queuing model, QATAR calculates

• The number of vehicles stopping in each curbside zone to drop off or pick up passengers. The number of spaces occupied simultaneously (assuming a 95% probability), when compared to the number of available spaces, defines the level of service for the curbside lane.
• The number of through vehicles proceeding to/from adjacent zones. The capacity of the through lanes is determined by the total number of travel lanes and the curbside utilization ratio. The number of through vehicles plus the number of parking vehicles, when compared to the capacity of the available through and maneuvering lanes, defines the sufficiency of the bypass lanes. As described previously, an increase in the curbside utilization ratio (i.e., an increase in the amount of double- and triple-parking) causes a reduction in the capacity of the through lanes.
• The peaking characteristics of the roadways, assuming that the volumes will not be exceeded 95% of the time during the analysis period. Traffic volumes on curbside roadways are not uniform throughout an hour-long period, or other analysis period, and peak periods of activity or microbursts of traffic occur frequently.

Inputs

Figure 5-4 and Figure 5-5 present examples of QATAR input sheets (including the suggested default values for dwell times and vehicle stall lengths). As shown, the following information is required to use QATAR:

• Curbside geometry—The physical characteristics of the curbside, including length, number of lanes, driver-side parking versus passenger-side parking, and number of roadway lanes approaching the curbside area. The curbside can be divided into a series of zones, each with its own characteristics.

• Hourly traffic volumes—The hourly volume of vehicles entering the curbside.
• Through vs. curbside traffic volumes—The proportion of vehicles using the roadway that stop at the curbside. If the user has divided the curbside into zones, the proportion (or volume) of vehicles stopping in each zone is required.
• Vehicle mix—The mix (i.e., classification) of vehicles in the traffic stream entering the curbside (either the actual volume or the percent of vehicles by vehicle classification). If the user has divided the curbside into zones, the proportion (or volume) of vehicles by classification stopping in each zone is required, or the user can determine that the proportion is constant in each zone.
• Dwell times—The user can accept the default values in QATAR or enter vehicle dwell times by vehicle classification.
• Vehicle stall length—The user can accept the default values in QATAR or enter vehicle stall lengths by vehicle classification.
• Adjustment factors—The user can enter adjustment factors in QATAR to reflect the effect of pedestrian crosswalks, regional conditions/driver behavior, and a weighting/calibration factor.

Outputs

Figure 5-6 presents an example of a QATAR output sheet.

As shown, QATAR yields the following outputs:

• Sufficiency—A graphic depicting the sufficiency for the curbside areas and roadway through lanes in each zone.
• Volume/capacity ratio—A tabular presentation of the volume/capacity ratio for the through lanes in each zone.
• Curbside utilization ratio—A tabular presentation of the curbside utilization ratio for the curbside area in each zone.

In some cases, the capacity of the roadway approaching the curbside may dictate the capacity of the curbside roadway segment. For example, the capacity of a five-lane curbside section with a two-lane approach roadway may, during periods of low curbside demand, be governed by the ability of the approach roadway to deliver vehicles to the curbside.

Limitations of the Analysis Tool

QATAR is used to analyze the macroscopic flow of vehicles but not the operation of individual vehicles (as would a roadway traffic microsimulation model). As such, QATAR does not

• Replicate or analyze operations, such as individual vehicles maneuvering into or out of curbside spaces, improperly parked vehicles, vehicle acceleration/deceleration characteristics, or how these characteristics vary by vehicle size or type.
• Analyze how roadway congestion or queues affect traffic operations in the zones located upstream of those being analyzed or meter (i.e., restrict) the flow of vehicles into downstream zones. The QATAR model looks at each zone independently and does not model interactions between zones.
• Analyze the performance zones that operate in a linear queue fed by dispatch from an offsite hold lot, such as a taxi queue or TNC PIN-system queue.
• Represent pedestrians crossing a curbside roadway (properly or improperly) or vehicle delays caused by pedestrian activity other than to allow the user to estimate the approximate decrease in roadway capacity.
• Evaluate the potential capacity decreases of specific curbside geometries. Rather, a single, continuous, linear curbside roadway is assumed in the model. If the curbside roadway consists of one or more parallel curbside roadways, QATAR should be used to analyze each parallel curbside roadway separately.

As such, QATAR produces an approximation of airport curbside roadway operations intended for planning purposes. If more detailed analyses are desired, including the modeling of interaction effects between zones, the user is encouraged to use a microsimulation model capable of simulating airport curbside traffic operations.

Interpreting the Results

Although it provides an approximation of airport curbside roadway operations, QATAR allows a user to identify which aspect of a curbside roadway (insufficient parking capacity or insufficient roadway capacity) is likely the cause of poor conditions. With that, QATAR can then be used to quickly test numerous alternative curb allocations, dwell time assumptions, and other factors to determine their potential benefits.

Certain vehicles (e.g., courtesy vehicles or door-to-door vans) may make multiple stops along the terminal curbside area, especially at large airports. Vehicles making multiple stops can be represented properly (using Option C—one of the available input sheet options in QATAR) because the total volumes of vehicles stopping in each zone need not equate to the total curbside roadway traffic. However, with Option C, QATAR requires percentages of vehicles to sum to 100% and vehicles making multiple stops may not be accurately represented, particularly if they account for a significant percentage of the total vehicles entering the roadway.

5.5.3 Use of Microsimulation Models

Guidelines

Chapter 4 provides guidelines on the use of microsimulation models for analysis of airport roadways.

It is suggested that the capability of a software package be confirmed prior to considering its use in analyzing airport curbside roadway operations, as some packages do not accurately simulate the parking or maneuvering movements that occur on an airport curbside roadway.

The following guidelines are provided for calibrating a microsimulation model for airport curbside roadways:

• If double- or triple-parking is allowed, verify that the model correctly predicts the average number of double- and triple-parkers during the peak hour (compare one-hour model simulation to one-hour field counts).
• If queuing occurs on the existing curbside roadway, count the throughput in the through lanes and the number of vehicles processed per hour in the curbside lanes under such congested conditions.
• Validate through-lane flow rates. Enter demands into the simulation model and verify that the maximum through-lane flow rate for the peak hour predicted by the model matches the field counts. Adjust mean headways in the model until the model through-lane volumes match the field counts. A difference of 5% to 10% between model through-lane volumes and field counts is acceptable.
• Validate curbside processing capacity. Enter demands in the simulation model and compare the curbside processing rate to field counts. Adjust average dwell times in the model until the processing rate over the peak hour matches the field counts.

These guidelines are in addition to guidance published elsewhere (see FHWA guide on microsimulation model validation).

Curbside Performance Measures for Analyses Performed Using Microsimulation

The performance measures presented in Table 5-1 are intended to help select the appropriate curbside analysis method. When curbside roadways are analyzed using microsimulation methods, the performance measures presented in Table 5-2 can be used to compare curbside roadway alternatives in the context of level of service.

The measures listed in Table 5-1 do not directly correspond to quantitative values equaling a specific level of service. For example, duration of queuing is a potentially useful measure in the context of comparing alternatives (e.g., if one curbside roadway alternative would result in 2 hours of queuing, while another would result in 1 hour of queuing), but the magnitude of the queuing itself could be relatively minor, so reporting a result of “under capacity” for one alternative and a result of “near capacity” for the other could be misleading. Similarly, the queue length measure can provide an easy way to compare alternatives, but a relatively long queue could be a better condition than a relatively short queue if the rate at which vehicles are served at the curbside is relatively high for the alternative with the longer queue.

Together, length of vehicle queues and average speed—two measures that are typically microsimulation software outputs—can provide a time in queue measure that can be used to compare and evaluate analyses of curbside roadway prepared using microsimulation models. Because of the wide range of motorist expectations regarding traffic conditions when they arrive at an airport curbside, a range of thresholds for time in queue between acceptable and unacceptable operations were identified, with unacceptable operations corresponding to the threshold

of capacity (volume-to-capacity ratio equal to 1). For the lowest of these thresholds, the time in queue was identified as 50 seconds. This time (50 seconds) is consistent with the delay threshold for unsignalized intersections operating at capacity and considered to be a reasonable lower threshold. For context, consider a small-hub airport, such as Billings Logan International Airport. Most of the time, there is no queue leading to this airport’s curbside, even during peak periods in bad weather. If a queue did develop such that motorists would have to be in the queue for 60 seconds, it would seem unacceptable in that context.

For the upper bound of acceptable/unacceptable thresholds, a comment expressed in at least one focus group conducted as part of prior research—moving is acceptable, not moving is not acceptable—was used. From a motorist’s perspective, it would seem as if a queue were not moving if a person could walk faster than the vehicles were moving. Using an arbitrary queue length of 1 mile and brisk walking speeds of 3 to 4 mph, the time spent in such a queue would be between 15 and 20 minutes. The 20-minute time in queue appears to be a reasonable upper bound for a threshold between acceptable and unacceptable (anecdotal experience suggests that queues of this length likely occur at large airports somewhat regularly). This time in queue is not intended to represent the longest queue time during the busiest days of a year, when delays may be even greater. Also, higher values of time in queue could be used by airport operators who observe higher thresholds at their locations.

Service thresholds corresponding to conditions that are “well under capacity” (the equivalent of the HCM’s LOS A) have also been defined. It is suggested that time in queue should not be zero, but should seem to a motorist as if it were nearly zero. A simple way to identify this lowest value would be to take 10% of the “at capacity” value. For the “at capacity” threshold of 60 seconds, a time in queue of 6 seconds or less would correspond to “well under capacity”—from a practical perspective, that would essentially mean no queue or perhaps one vehicle waiting, which is consistent with the original basis for this threshold. With an “at capacity” threshold set at 20 minutes, the “well under capacity” time in queue would, therefore, be 120 seconds. Although 120 seconds in a queue seems high compared to, for example, a signalized intersection delay, for a motorist approaching a curbside anticipating a wait of up to 20 minutes, a 2-minute wait would seem remarkably short.

Once the upper and lower level-of-service bounds are identified, the values for the other sufficiency levels can be calculated using a straight-line projection between the two points, with the “under capacity” level representing the midpoint between “well under capacity” and “at capacity.” The results of these estimates, assumptions, and calculations are presented in Table 5-5. The information can also be presented in graph form, as shown in Figure 5-7. As noted, the values of the time in queue can easily be extrapolated upward from the 20-minute level to any value.

Table 5-5. Time spent in queue for various sufficiency levels.

Note: Input data are to be taken from microsimulation modeling output.

^a Analyst must first select a value for the maximum acceptable time spent in queue for the subject airport. Then, using queue length and average speed outputs from the microsimulation model, the level of service can be identified.