CHAPTER 29
Glossary

Acceptable Gap Distance—The size of the gaps in major-road traffic typically accepted by drivers turning from a minor road to provide sufficient time for the minor-road vehicle to accelerate from a stop and complete a turn without unduly interfering with major-road traffic operations.

Accessible Pedestrian Signals (APS)—Equipment for use at signalized intersections that communicates pedestrian signal timing information in non-visual formats. Features include pushbutton locator tone, tactile arrow, pushbutton information message, automatic volume adjustment, alert tone, actuation indicator, tactile map, Braille and raised print information, extended button press, passive pedestrian detection, and clearance interval tones.

Advance Destination Guide Signs—Mixed text and arrow signs (including diagrammatic signs) that assist drivers in planning lane selections and exits, allowing them to focus more attention on the driving task and less attention on wayfinding.

Advance Stop/Yield Lines—Treatments that increase the distance ahead of a crosswalk at which drivers are required to stop or yield to allow pedestrians to cross.

AMBER Alert—An urgent broadcast regarding child abductions.

Annual Average Daily Traffic (AADT)—The total volume of road user traffic on a road for a year divided by 365 days.

Apparent Radius—The curve radius as seen from the driverʼs perspective, which, in some cases, can make the curve appear distorted, either flatter or sharper, depending on topography and other road elements.

Appropriate Message Length—Sign message lengths that drivers have time to read and comprehend as they pass the sign.

Arcminute—One-sixtieth (1/60) of one degree (1°).

Arrow Panel Visibility—A roadway sign condition dependent on a number of factors, including the capability of the lamps in the panel, the type of roadway, the physical location of the panel, and the panelʼs relation to horizontal and vertical curves, ambient light, and weather.

Arrow-per-lane (APL) Signs—Large or grouped signs providing every individual lane with its own arrow to improve driver navigation.

Behavioral Framework for Speeding—Conceptual overview of the key factors relevant to speed selection, as well as their relationship to potential speeding countermeasures.

Bicycle Lanes—Includes conventional and buffered bicycle lanes that use pavement markings to designate a portion of the roadway exclusively for bicyclistsʼ use.

Bilingual Information—Information that is presented in more than one language on changeable message signs (CMSs).

Blank-out/Blanking—The period of time, or scheduled phase, when sign readouts are not being used.

Bollard—A thick vertical post sometimes used to control pedestrian and vehicular traffic. Bollard-mounted lighting can be used to provide vertical illuminance on pedestrians at crosswalks for improved visibility.

Broad Spectrum—Light that contains a wide range of wavelengths (colors) across the visible spectrum. A broad-spectrum luminaire contains sufficient color content that humans can readily discriminate the colors of objects illuminated by it.

Buffered Bicycle Lanes—A treatment that pairs conventional bike lanes with a designated buffer space between the bike lane and the motor vehicle travel lane.

Bulbout—A curb extension going past the sidewalk or curb line into the street. Bulbouts reduce the street pavement width in order to improve pedestrian crossings by shortening crossing distances, reducing the time pedestrians are exposed to traffic, improving pedestrian and motorist visibility, and reducing traffic speeds.

Candela—The International System of Units (SI) base unit of luminous intensity.

Caution Mode Configuration—Arrow panel mode C, which provides flashing non-directional information to increase safety near highway work zones by providing early warning information to drivers indicating that caution is required while approaching and traveling through the work zone.

Changeable Message Sign (CMS)—CMSs are electronic, reconfigurable signs placed above or near the roadway and are used to inform motorists of specific conditions or situations. Also referred to as variable message signs (VMSs) or dynamic message signs (DMSs).

Clear Zone—The roadside border area that is available for drivers to safely stop or gain control of an errant vehicle. This area may include a shoulder, recoverable or non-recoverable slopes, and run-out areas that are smooth and clear of obstructions. See FHWA (2011).

Clearance Interval—The period of time necessary for safe transitions in right-of-way (ROW) assignment between crossing or conflicting flows of traffic, including pedestrian activity; a combination of the yellow clearance interval plus the red clearance interval or an all-red interval.

Clearing Distance—The distance a vehicle travels beginning at the time the signal changes to yellow and ending at the time the signal changes to red.

Closed-Loop Compensatory Component—Part of the steering control process in which drivers continually monitor and adjust for deviations in position on the road based on feedback from near-field visual cues.

Cognitive Preparation—The various active mental activities that can influence response times and decisions of drivers, including such things as driver expectancies, situational awareness, a general sense of caution, and where attention is being directed by the driver.

Color Spectrum—See Spectrum.

Complete Streets—A roadway design philosophy that aims to facilitate a coordinated multimodal transportation network using a framework of 10 elements.

Complexity—A function or level describing how much information is being provided and how difficult it is to process.

Complexity of Sign Information—The number of information units being presented as part of roadway sign messages.

Comprehension—The combination of completing a task at hand, e.g., reading a sign, plus the process of making the resultant decision, e.g., right or left turn in response to the signʼs information.

Cone—The portion of the roadway scene on the right-hand side of the roadway where a driver would typically look for road signs.

Conspicuity—The ease in seeing and locating a visual target, including signage, vehicles, bicycles, or pedestrians. In the context of road signs, it represents how easy it is to distinguish a sign from the surrounding visual environment.

Context Sensitive Solutions (CSS)—A design philosophy that aims to ensure that a project fits into its external context (i.e., environmental, social) and meets the needs of its users by consulting a broad set of stakeholders early in project development.

Continuation Distance—The distance that a vehicle travels prior to the descent of the entry gates at a railroad crossing.

Contraflow Bicycle Lanes—Refers to bicycle lanes installed on the left side of a one-way street to give bicyclists the option to ride opposite the flow of traffic in a designated bike lane.

Contrast Sensitivity—The ability to perceive subtle differences in shading, patterns, and colors between objects, and to recognize objects as separate from their background.

Conventional Bicycle Lanes—Bicycle lanes that are typically installed on the right side of the street between the adjacent travel lane, and the curb, road edge or parking lane; they are used to create separation between bicyclists and motor vehicles, visually remind motorists of bicyclistsʼ right to drive on the streets, encourage motor vehicles to stay in their lane when passing bicyclists, and increase bicyclist comfort and confidence on busier streets.

Counterbeam Lighting—A lighting technique whereby the light falls on objects from a direction opposite to the traffic. Counterbeam lighting is characterized by a luminous intensity distribution that is asymmetrical and has the maximum luminous intensity aimed against the direction of normal traffic flow.

Crash Likelihood—Factors that affect whether a crash occurs.

Crash Modification Factor (CMF)—A factor used to compute the expected number of crashes after implementing a countermeasure on a road or intersection. It is defined as the ratio of the expected crash frequency after a countermeasure has been implemented to the expected crash frequency without the countermeasure.

Crash Severity—Pre-, during-, and post-crash factors that affect post-crash survivability and injury severity.

Crest Horizontal Curve—A horizontal curve that also contains a vertical, concave down component of curvature.

Critical Gap—For design purposes, the critical gap represents the gap between successive oncoming vehicles that average drivers will accept 50% of the time (and reject 50% of the time).

Cross Section—The width of the lane.

Cross Slope—The transversal slope of the roadway (described as a percentage) with respect to the horizon.

Crossbuck—A railroad warning sign with two slats of wood or metal fastened together on a pole in a letter X formation, with the word “Railroad” on one slat and “Crossing” on the other, black letters on a white background. Crossbucks are sometimes supplemented by other warning devices such as flashing lights, a bell, a “Yield” sign, a “Stop” sign, and/or a descending gate to prevent traffic from crossing the tracks.

Curb—A raised or vertical roadway element used for drainage control, roadway edge delineation, right-of-way reduction, aesthetics, delineation of pedestrian walkways, and general assistance in orderly roadside development (AASHTO, 2018a).

Curb Extensions—A means of narrowing the road by extending the sidewalk or curb line into the street or parking lane, thus reducing pedestrian crossing time and exposure.

Curb Management—Includes employing best practices for curb space allocation and treatments, including considerations for providing safe access to a range of roadway facilities for all road users (ITE, 2018).

Decibel (dB) Level—A measurement that expresses the power or intensity magnitude of sound relative to a specified or implied reference level. A decibel is one-tenth of a bel, a seldom-used unit.

Decision Sight Distance (DSD)—DSD represents a longer sight distance than is usually necessary and is used for situations in which (1) drivers must make complex or instantaneous decisions, (2) information is difficult to perceive, or (3) unexpected or unusual maneuvers are required.

Design Consistency—Conformance of a highwayʼs geometric and operational features with driver expectancy.

Design Speed—A selected speed used to determine the various geometric design features of the roadway.

Design Vehicle—A selection of vehicles expected to use a facility most frequently. The characteristics of these vehicles are used in the roadway design process.

Dilemma Zone—The portion of the roadway formed between (1) the clearing distance to the intersection (the distance the vehicle travels between the time the signal changes to yellow to the time the signal changes to red) and (2) the stopping distance (the distance traveled by the vehicle between the times the signal changes to yellow to the time when the vehicle actually stops) when the stopping distance is greater than the clearing distance. The size of the dilemma zone is relative to the situation; it is not a fixed area.

Driver Expectations—The driverʼs readiness to respond to situations, events, and information in predictable and successful ways.

Driver Fatigue—A general psycho-physiological state that diminishes an individualʼs ability to perform the driving task by reducing alertness and vigilance.

Drop-off—Deterioration of roadways caused when the edges of the pavement become destabilized and eroded, resulting in a difference in height between the pavement surface and the roadside surface.

Dynamic Characteristics—Message properties that specify character movement, such as the time to display each message phase, to display blanking between phases of a multiphase message, and to flash one or more lines of a message.

Dynamic Dilemma Zone—A road segment on approach to an intersection that varies in length based on fluctuations in vehicle speeds and number.

Dynamic Late Merge Systems—These systems, developed for use in work zone lane closure situations, utilize a series of changeable message signs and static work zone signs to provide merge information to the driver. The information is based upon the current traffic volume through the work zone and supports early merging when the traffic flow is light and late merging (closer to the gore point) when the traffic volume is heavier.

Dynamic Message Sign (DMS)—DMSs are electronic, reconfigurable signs placed above or near the roadway and are used to inform motorists of specific conditions or situations. Also referred to as changeable message signs (CMSs) or variable message signs (VMSs).

Effective Length of the Passing Lane—The physical length of the passing lane plus the distance downstream to the point where traffic conditions return to a level similar to that immediately upstream of the passing lane.

Effects of Roadway Factors on Speed—The impact of geometric, environmental, and traffic factors on driving speed under free-flow conditions in tangent roadway sections.

Empirical Bayes—A method in which empirical data are used to estimate conditional probability distributions.

Exit Gate Clearance Time—The amount of time provided to delay the descent of the exit gate arm(s) after the entrance gate arm(s) begin to descend at a railroad crossing.

Factors Affecting Acceptable Gap—These factors are the driver, environment, and other situational factors—such as traffic volume, wait times, familiarity with the roadway or oncoming vehicle size—that cause most drivers or specific groups of drivers (e.g., older drivers) to accept smaller or larger gaps than they would otherwise accept under normal conditions.

Fatal Accident Reporting System (FARS)—National Center for Statistics and Analysis (NCSA) data system.

Four-Quadrant Gate—A set of four descending gates to stop traffic at railroad crossings, which consists of one gate before and one gate after the railroad tracks for each of the two lanes of traffic.

Foveal Vision—Central vision of the eye. The fovea, located in the pit of the retina, is the source of the eyeʼs high visual acuity capability.

Free-Flow Speed—Free-flow speed is defined as conditions in which a driver has the ability to choose a speed of travel without undue influence from other traffic, conspicuous police presence, or environmental factors.

Functional Classification—The categorization of highways, roads, and streets by the character of service they provide. These categories are used in transportation planning.

Fundus—The interior surface of the eye, opposite the lens, which includes the retina, optic disc, macula and fovea, and posterior pole.

Gap—The time interval between two successive vehicles, measured from the rear of a lead vehicle to the front of the following vehicle, adapted from the Traffic Engineering Handbook (Pline, 1999).

Gate Delay—The length of time between the start of the flashing lights and the initiation of the descent of the entry gate arm at railroad crossings.

Gate Interval Time—The length of time between the initiation of the descent of the entry gate and the initiation of the descent of the exit gate at a crossing with a four-quadrant gate device.

Gate-rushing—Gate-rushing is when drivers do not stop at railroad crossings and drive under the gate arms as they are descending or drive around gate arms that are already in the lowered position.

Glare—A visual phenomenon which occurs when the intensity of a light source within the visual field is substantially greater than the visual adaptation level, causing physical discomfort or pain (discomfort glare) and/or reduced visibility (disability glare).

Glare Screens—Visual barriers designed to shield drivers from glaring light from oncoming headlamps. Glare screens are often mounted on median barrier walls and come in a variety of forms, including vertical paddles, concrete barriers, wire or plastic mesh screens, etc. Shrubbery and other landscaping elements can also be used as glare screens.

Grade Separated Crossings—Treatments that completely separate pedestrians from vehicular traffic (may also be called overpasses, overcrossings, or bridges and underpasses, undercrossings, or tunnels).

Grade Severity Rating System—A simulation model that establishes a safe descent speed for the grade based upon a predetermined brake temperature limit.

Green Book—The incorporated reference establishing the criteria for acceptable standards, policies, and standard specifications, approved by the Secretary of Transportation in cooperation with the SDOTs, for construction and reconstruction projects on the NHS in the United States.

HAWK Signal—A new type of overhead beacon signal to assist pedestrians at unsignalized crosswalks on high-volume traffic streets.

High Pressure Sodium (HPS) Lamp—A type of lamp for street lighting that operates using an electric arc through sodium vapor under high pressure. HPS lamps glow with a characteristically yellow light.

Highway Safety Manual (HSM)—The recognized source of information and methods for quantitatively evaluating traffic safety performance on existing or proposed roadways in the United States. It is published by AASHTO.

Highway Systems—The combination of three major components—the road (local roads, collectors, arterials and freeways), traffic control, and users with or without a vehicle.

Horizontal Curves with Vertical Sag—A horizontal curve that also contains a vertical, concave up, component.

Human Factors—A scientific discipline that tries to enhance the relationship between devices and systems and the people who are meant to use them through the application of extensive, well-documented, and fully appropriate behavioral data that describe and analyze the capabilities and limitations of human beings.

Independent Alignments—Roadway design such that opposing lanes are developed independently of each other. The opposing alignments may or may not run parallel to each other. Also, they may be separated horizontally by geographic, landscaping, or other features. Similarly, they may be separated vertically on hillsides or other steep grades.

Induction Lamp—A type of lamp for street lighting that uses electromagnetic fields rather than electrodes to wirelessly transfer power to the interior of the lamp. Radio waves are typically

used to energize either a bulb filled with sulfur or metal halides or a tube based on conventional fluorescent lamp phosphors.

Information Units—A measure of the amount of information presented in terms of facts used to make a decision.

Intersection Sight Distance (ISD)—The stopping sight distance required at intersections. Actual ISDs will differ, depending on the type of intersection and maneuver involved.

Kinetic Energy Transfer—Kinetic energy is directly proportional to an objectʼs mass and velocity. The human body has limits for tolerating crash forces; when kinetic energy exceeds a certain threshold, a fatal or serious injury occurs.

Lag—The time interval from the point of the observer to the arrival of the front of the next approaching vehicle (Lerner et al., 1995, pp. 58–59).

Lane Drop Markings—Pavement markings that consist of short wide lines with short gaps used to delineate a lane that becomes a mandatory turn or exit lane.

Leading Bicycle Intervals—A treatment that can increase bicyclist conspicuity by placing them ahead of other vehicles and making them the only travelers in the intersection during the interval.

Legibility Distance—The minimum distance at which a sign must become legible to a typical driver. It is calculated as a function of the time it takes a driver to read the sign, interpret the sign, and execute maneuvers that comply with the signʼs message.

Legibility Index—The distance at which a given unit of letter height is readable.

Level of Service (LOS)—Quantitative performance measures that represent quality of service on a road based on speed (as a proxy for traffic flow and congestion) and number of vehicles on the roadway.

Long-Range Guidance—Driving preview time for drivers of at least 5 s.

Looming—One of several dynamic characteristics of message signs, this term refers to increasing the size of text or symbols over time in a message display.

Low Pressure Sodium (LPS) Lamp—A type of lamp for street lighting that operates using an electric arc through sodium vapor. LPS lamps operate at lower pressure than HPS lamps, and they are nearly monochromatic yellow in color.

Luminaire—A lighting fixture that consists of one or more electric lamps, lamp housings, reflectors, masts, wiring, and other necessary parts.

Luminaire Cutoff Pattern—The distribution of intensities from a luminaire around the point at which the luminaire emits no light. A luminaire with full cutoff projects less than 10% of rated lumens beyond 80 degrees from nadir and no light at or above 90 degrees from nadir. The nadir is defined as the angle that points directly downward (0 degrees) from the luminaire.

Luminance—Luminance is the luminous intensity per unit area of light measured as candelas per square meter (cd/m²).

Luminous Intensity—A measure of the perceived power emitted by a light source in a particular direction per unit solid angle (e.g., cd/°).

Lux—The International System of Units (SI) unit of illuminance and luminous emittance.

Maneuver Time (MT)—The amount of time required to safely complete a maneuver. MT is primarily affected by the physics of the situation, including vehicle performance capabilities, tire-pavement friction, road-surface conditions (e.g., ice), and downgrades, and to a lesser extent by driver-related factors (e.g., deceleration profile), although these factors are highly situation-specific because the maneuvers encompass a broad range of actions (e.g., emergency stop, passing, left turn through traffic).

Manual on Uniform Traffic Control Devices (MUTCD)—An FHWA publication that defines the standards for installing and maintaining traffic control devices on all streets, highways, pedestrian and bicycle facilities, and site roadways open to public travel in the United States.

Mental Models—The system userʼs internal understanding and representation of an external reality.

Mesopic Lighting—Light conditions under which visual sensitivity is shifted toward the blue/green portion of the visible spectrum compared to vision under photopic (daytime) lighting, thus making objects and clothing in this part of the spectrum more visible under these conditions. See photopic lighting.

Metal Halide Lamp—A type of lamp for street lighting that operates using an electric arc through mercury vapor under high pressure. Metal halide salts added to the mercury glow at different wavelengths, yielding a relatively white light.

Most Meaningful Information (MMI)—Information sought by drivers for a particular road location and point in time through scanning the road environment in front of, behind, and to the sides of the vehicle they are driving.

Multimodal Network—A road network that supports different modes of travel (e.g., bicycles, wheelchairs, transit, walking, driving, freight).

Nighttime Driving—The situation in which motoristsʼ visibility while driving in darkness on rural roads is limited; roadway features, objects in the roadway, or pedestrians ahead are less visible depending upon headlamp intensity, ambient lighting, and the presence or absence of oncoming headlamp glare.

Open-Loop Anticipatory Control Process—Part of the steering control process in which drivers predict road curvature and required steering angle based on far-field visual cues.

Optic Flow—The visual pattern caused by moving forward, in which points close to the point of expansion move outward more slowly than points more peripheral to it. This information is directly used by the driverʼs visual system to perceive motion.

Passing Lane—A lane added in one or both directions of travel on a two-lane, two-way highway to improve passing opportunities.

Passing Sight Distance (PSD)—The amount of distance ahead a driver must be able to see in order to complete a passing maneuver without cutting off the passed vehicle before meeting an opposing vehicle that appears during the maneuver.

Pavement Classifications—The type and texture of paving material affect how light reflects from the road surface. The International Commission on Illumination (Commission Internationale de lʼEclairage or CIE) has developed four road surface classifications. Class R1 surfaces have primarily diffuse reflectances, Class R2 surfaces have mixed diffuse and specular reflectances, Class R3 surfaces have slightly specular reflectances, and Class R4 surfaces have mostly specular reflectances (see Illuminating Engineering Society of North America, 2000).

Pavement Drop-off—Drop-offs are caused when the edges of pavement are destabilized and eroded, resulting in a difference in height between the pavement surface and the roadside surface.

Pedestrian Hybrid Beacons (PHB)—Lighting treatments to reduce pedestrian crash potential at crosswalks by alerting drivers to the presence of pedestrians in crosswalks.

Perception-Identification-Emotion-Volition (PIEV) Time—The total time from perception to completing a reaction.

Perception-Reaction Time (PRT)—The time a driver takes to process information, typically defined as the period from the time the object or condition requiring a response becomes visible in the driverʼs field of view to the moment of initiation of the vehicle maneuver. Per AASHTO (2004), bits of information on a scale from 0 to 6 bits is processed by the average driver at about 1 and 1.5 bits of information per second for unexpected and expected situations, respectively.

Perceptual Requirements—The visual information about the roadway and surrounding environment that drivers need to judge road curvature, determine lane position and heading, etc.

Phase (for message signs)—The text that is displayed at a single point in time on a message sign.

Photopic Lighting—Light conditions under which visual appearance is stronger at the yellow portion of the visible spectrum compared to vision under mesopic (nighttime) lighting when visual sensitivity is shifted toward the blue/green part of the spectrum. See mesopic lighting.

Pilot Program—A funded program intended to promote, demonstrate, and document the application of a new transportation design or initiative.

Point of Expansion—During forward motion, the point in the forward field that appears stationary relative to the observer (the observersʼ actual destination), and from which all other points are seen as moving away.

Post-Mounted Delineators (PMDs)—A type of marking device used to guide traffic; a series of retroreflective devices mounted above the roadway surface and along the side of the roadway to indicate the alignment of the roadway.

Preview Sight Distance (PVSD)—PVSD is a measure of driver sight distance based on the assumption that “the driver views or previews the roadway surface and other cues that lie ahead to obtain the information needed for vehicular control and guidance” (Gattis and Duncan, 1995).

Proven Countermeasures—FHWAʼs Proven Safety Countermeasures initiative is a collection of 28 countermeasures and strategies effective in reducing roadway fatalities and serious injuries on highways in the United States. The collection can be accessed here: https://highways.dot.gov/safety/proven-safety-countermeasures/.

Psychomotor Requirements—The control actions (e.g., steering-wheel movements; foot movements to press brake, etc.) that drivers must make to maintain vehicle control or to facilitate other information acquisition activities.

Raised Crosswalk—Treatments that elevate the crosswalk to the level of the sidewalk and are used to reduce vehicle speeds as they approach the crosswalks (also referred to as speed tables or raised intersections).

Raised Medians—Treatments that provide pedestrians with a protective environment in the middle of an intersection or mid-block crosswalk, reducing pedestrian exposure to traffic and allowing them to focus on crossing one direction of travel at a time (also referred to as pedestrian refuge islands, safety islands, crossing islands, or center islands).

Raised Pavement Markers (RPM)—A variety of three-dimensional devices used in conjunction with pavement markings to mark lane boundaries. They often have a reflective surface to increase visibility and produce a noticeable vibration or physical sensation when in contact with vehicle tires.

Rectangular Rapid-flashing Beacons—Lighting treatments to reduce pedestrian crash potential at crosswalks by alerting drivers to the presence of pedestrians in crosswalks.

Red Light Running—Situations when drivers enter a signalized intersection when a red light is being presented.

Retroreflective Raised Pavement Markers (RRPM)—Raised pavement markers affixed to the road surface that are designed to reflect light directly back to the light source.

Retroreflectivity—The property allowing a surface to reflect a large portion of its light directly back to or near its source.

Road Diet—Redesigning a roadway to reduce the number of vehicle lanes in a street.

Road User Exposure—The amount of time when an existing or intended type of road user can be exposed to a potential crash. This value is based on user modes, characteristics, numbers, and how long individuals spend in the roadway environment of interest.

Roadway Shoulder—See Shoulder.

Roundabout Intersection—As defined by the MUTCD, roundabouts are circular intersections with yield control at entry, permitting a vehicle on the circulatory roadway to proceed, and deflecting the approaching vehicle counter-clockwise around a central island (FHWA, 2009).

Roving Eye Treatments—Pedestrian or driver signals which include a pair of animated eyes as part of the lighted display, intended as a reminder to watch for vehicle movement (for pedestrians) or to watch for pedestrian movements (for drivers).

Rural Areas—AASHTO (2018a) defines rural areas as those places not included in urban areas.

Safety System Approach—A holistic and comprehensive approach that serves as a guiding framework to make places safer for people, which focuses on both human errors and human vulnerabilities.

Safe System Approach (SSA)—A roadway design philosophy that aims to maximize safety performance by reducing kinetic energy transfer and anticipating and accommodating human limitations and errors. This philosophy uses a framework based on six principles and five elements.

Safe System Hierarchy—An assessment tool based on the hierarchy of controls for workplace safety used to prioritize countermeasure selection using four tiers: remove severe conflicts, reduce vehicle speeds, manage conflicts in time, and increase attentiveness and awareness.

Safe System Matrix—A quantitative and qualitative tool from the Safe System Assessment Framework. It is used to assess the extent to which a project aligns with Safe System principles.

Safety Edge—A wedge-shaped asphalt material placed between the roadway and the shoulder, which can be used as a drop-off countermeasure.

Self-enforcing Roads—These roads use a target design speed to select a roadwayʼs geometric properties and support road usersʼ selection of this speed with traffic signs, pavement markings, and other roadside elements.

Separated Bicycle Lanes—One or two-way exclusive bikeways parallel to the roadway yet physically separated from moving traffic.

Serial Processing—A chain of events in which one step does not begin until the previous step is complete, which is used to model some driver behavior.

Shared-Use Lanes—Roadways or lanes used concurrently by vehicles, bicyclists, or pedestrians in either rural or urban areas.

Sharrows—Shared-lane markings.

Short-Range Guidance—Preview time for drivers of up to 3 s.

Shoulder or Roadway Shoulder—A portion of the roadway contiguous with the traveled way for accommodation of stopped vehicles, for emergency use, and for lateral support of the sub-base, base, and surface courses. It may also be used by non-motorized traffic.

Shoulder Drop-off—A difference in height between the pavement surface and the roadside surface caused when the edges of pavement become destabilized and eroded.

Shoulder Rumble Strips (SRSs)—A raised or grooved pattern on the shoulder of a travel lane to provide a tactile or audio alert to the driver.

Sight Distance (SD)—The distance that a vehicle travels before completing a maneuver in response to some roadway element, hazard, or condition that necessitates a change of speed and/or path. SD is based on (1) a perception-reaction time (PRT) required to initiate a maneuver (pre-maneuver phase) and (2) the time required to safely complete a maneuver (MT).

Sight Distance at Left-Skewed Intersections—The available sight distance to the driverʼs right side for a vehicle crossing a major road from a left-skewed minor road (where the acute angle is to the right of the vehicle).

Sight Distance at Right-Skewed Intersections—The available sight distance to the driverʼs left side for a vehicle crossing a major road from a right-skewed minor road (where the acute angle is to the left of the vehicle).

Sign Comprehension—The road userʼs ability to interpret the meaning of a sign. The ability to comprehend and use signs is associated with three stages: legibility, recognition, and interpretation. Sign comprehension can also consist of the sign reading task plus the process of making the resultant decision, e.g., right or left turn in response to the signʼs information.

Sign Design—Design parameters of signs that impact the legibility of text placed on the sign, including retroreflectivity, legend color, font size, and font style.

Sign Legend—The text and/or symbols composing the message of a sign.

Sign Legibility—Specific design characteristics of signs that contribute to the driversʼ ability to perceive and understand the signʼs message.

Sign Legibility Index—An index created by the USSC to calculate sign letter height. To determine letter height, divide the viewer reaction distance by the appropriate legibility index value (which varies depending on illumination, font style and case, as well as font color contrast to background).

Sign Recognition—The ability of the driver to readily distinguish the sign, especially in the context of other signs and stimuli.

Small Target Visibility Method—One method used to calculate or measure road lighting levels based on the visibility of a small target. Visibility is calculated using target size and reflectivity, road reflectivity, veiling luminance, and other factors.

Spectral Power Distribution (SPD)—The distribution of the power of each wavelength in the visual spectrum produced by a light source. The spectral power distribution of a luminaire affects the perceived color of objects illuminated by it and may affect the ability to detect, identify, or discriminate objects under mesopic lighting conditions.

Spectrum—The full range of wavelengths (colors) of light produced by a light source. For example, a low-pressure sodium luminaire contains nearly monochromatic yellow light, while an LED light source contains a variety of wavelengths with an abundance of blue relative to the other wavelengths in the spectrum.

Speed Management—A comprehensive strategy that addresses all factors that influence speeding behavior, including speed limits, roadway designs, traffic control, enforcement, and public awareness.

Speed Perception—A driverʼs judgment of how fast he or she is traveling.

Stopping Distance—The distance traveled by a vehicle beginning from the time a traffic signal changes to yellow and ending at the time when the vehicle actually stops.

Stopping Sight Distance (SSD)—The distance from a stopping requirement (such as a hazard) that is required for a vehicle traveling at or near design speed to be able to stop before reaching that stopping requirement. SSD depends on (1) how long it takes for a driver to perceive and respond to the stopping requirement (PRT) and (2) how aggressively the driver decelerates (MT). This distance can be calculated as the sum of driver perception-reaction time + vehicle deceleration, under a range of visibility/traction conditions.

Suburb—Roads and streets that are typically within the outlying portions of urban areas with low to medium development density, mixed land uses, multi-family residential structures, and non-residential development (e.g., town centers and big box stores; AASHTO, 2018a).

Task Analysis—Identification of basic activities performed by drivers as they navigate different driving scenarios by successively decomposing driving segments into tasks and subtasks/information processing elements.

Title II of the Americans with Disabilities Act (ADA) of 1990—Title II of the ADA prohibits discrimination on the basis of disability by public entities and is implemented in 28 CFR Part 35. 28 CFR 35.151 New construction and alterations is a section of ADA regulations that is particularly relevant for highway engineers, and can be found at https://www.ecfr.gov/current/title-28/chapter-I/part-35/subpart-D/section-35.151/.

Traffic Calming—The physical design and other measures put in place on existing roads to reduce vehicle speeds and improve safety for pedestrians and cyclists, such as speed humps, speed tables, raised intersections, horizontal shifts, and roadway narrowing.

Traffic Engineering—The definition from ITEʼs Traffic Engineering Handbook is “that branch of engineering which applies technology, science, and human factors to the planning, design, operations and management of roads, streets, bikeways, highways, their networks, terminals, and abutting lands” (Pline, 1999).

Train Delay—The amount of time between the gate descent and the train arrival at railroad crossings.

Truck Escape Ramp—A facility designed and constructed to provide a location for out-of-control trucks to decelerate to a stop, which is also available for use by other vehicles.

Turnout—A widened, unobstructed shoulder area or lane that provides opportunities for slow-moving vehicles to pull out of the through lane and passing opportunities to following vehicles.

Two-Quadrant Gate—A set of two descending gates to stop traffic at railroad crossings, which consists of one gate before the railroad tracks for each of the two lanes of traffic.

Urban Areas—AASHTO (2018) defines urban areas as those places within boundaries having a population of 5,000 or more.

Veiling Luminance—Uniform luminance that washes over the retina, causing a reduction of contrast. Veiling luminance is caused when the eye is exposed to a light source that is substantially more intense than the adaptation level.

Viewer Reaction Distance—The distance a viewer will cover at a given rate of speed and reaction time, which can be calculated by speed of travel (ft/s) times perception-reaction time (s).

Visual Acuity—A measure of the ability of the eye to distinguish shapes and the details of objects at a given distance.

Visual Conspicuity—Characteristics of a sign that enable a driver to differentiate the sign from its surrounding environment.

Variable Message Sign (VMS)—VMSs are electronic, reconfigurable signs placed above or near the roadway and used to inform motorists of specific conditions or situations. Also referred to as changeable message signs (CMSs) or dynamic message signs (DMSs).

Warning Time—At railroad crossings, the time between the initiation of the flashing light traffic control devices and the arrival of the train.

Warrant Criteria—Criteria used to determine whether street lighting is warranted at an intersection or other location. Various jurisdictions use nighttime/daytime crash ratios, average daily traffic, and other criteria to determine whether street lighting is likely to provide sufficient safety improvements to justify the expense of installing and operating lighting at the location.

Wayside Horn—An audible warning horn mounted at rail-highway crossings to alert drivers when a train is approaching.

Work Zone Speed Limits—Reduced speed limits that are used in work zones to maintain safe traffic flow.

Yellow Timing Interval—Duration of the yellow signal indication (also referred to as the “yellow change interval” or “yellow clearance interval”).

Zip Merging—Vehicles merging one by one in an alternating pattern.