SUMMARY

The concept of speed – namely, why and how do drivers choose their speeds, and what can be done to influence speeds – is a topic that currently receives a lot of attention as the profession seeks to design roadways to accommodate the mobility of both motorists and nonmotorized users alike. While speed is an operational metric, it is intertwined with the roadway design process. Design speed is related to particular geometric features of a roadway, though the speed at which drivers operate their vehicles on that roadway can differ from that design speed. In many cases, a segment is defined by its posted speed limit, because that is a tangible number that is usually readily identifiable. However, setting posted speed limit needs consideration of multiple factors, as described in NCHRP Project 17-76 (Fitzpatrick et al. 2021b, Fitzpatrick et al. 2021c). Cities and counties commonly have a de facto or prima facie speed limit that covers the entire jurisdiction except where signs are posted to display a different limit on a specific segment; as a result, streets with widely different characteristics could have the same posted speed limit, even if those characteristics prompt the driver to choose different operating speeds (whether higher or lower than the posted speed limit).

These variations in speed have led transportation practitioners to consider new ways to look at speed, and one that is increasingly common is the concept of target speed. While a single formal definition of target speed does not exist, common use implies that it represents the operating speed that the designer intends for drivers to use. The central issue to achieving target speeds involves the configuration and operation of roadways so that target speeds, compatible with context and all roadway users, are chosen by—and not forced upon—vehicle operators. However, much of the roadway context, especially the urban one, has already been established, so a large part of the effort of achieving target speeds involves retrofitting the existing environment. For practitioners to use this concept of target speed in redesigning existing roads, or even designing new roads, it is necessary to have a clear understanding of what combinations of roadway, roadside, and non-roadway factors influence operating speed, so that the desired target speed is achievable. That understanding can then lead to including consideration of those factors in the roadway design process, so that practitioners can regularly incorporate it into the planning, design, and operations of their roadways.

National Cooperative Highway Research Program (NCHRP) Project 15-76 was tasked with the following objectives: (a) to determine the effects of roadway, roadside, and non-roadway elements on operating speeds on roadways with a target speed between 30 and 40 mph and (b) to develop recommendations on how the findings can be incorporated into the roadway design process. The NCHRP 15-76 research team reviewed previous research results and current practices to get a sense of how target speed is defined and used in the profession and how the results from 15-76 could provide information and tools that practitioners need. They found that the use of the term “target speed” is increasing in the profession, but it is often either not defined, or it is described in the context of another speed concept. The research team developed a target speed definition based on the common uses of the term and its application within existing literature and policy, as well as the need to place the term in the context of the design process. Target speed can be used in the design process in conjunction with more established speed concepts such as design speed, operating speed, and posted speed limit.

The target speed definition developed in NCHRP 15-76 is as follows: target speed is a speed selected for a roadway during the design process that guides planning, design, and operational decisions and results in a roadway with a driving environment that encourages drivers to operate at or below that selected speed.

Speed is used in different ways within the broader project development process that includes planning and traffic engineering components as well as design. Similarly, multiple factors can influence a driver’s speed choice; however, the exact relationships may not be clear. Adding to the challenge of quantifying the relationship is the interaction between these factors and the overall visual scene for the driver. The contextual expectations of the road can communicate an appropriate speed to a driver, though the driver must be willing to accept that message in a contextual reaction.

In addition to research that describes findings related to operating speed and factors that potentially influence it, a number of jurisdictions include target speed and related concepts in their guidance. The research team identified several states (Florida, Massachusetts, Oregon, Washington, and Texas) among those whose departments of transportation (DOTs) had documented procedures or recommended practices for including target speed in the design process. Researchers also identified three cities (Austin, TX; Charlotte, NC; and Chicago, IL) with design guides or policies that included target speed. Each of these agencies that included target speed in their design processes also emphasized the use of a context classification to guide the selection of a target speed value. Each agency considered different factors in establishing their context classification criteria, but those factors typically included roadway cross-sectional elements (e.g., number and width of lanes, type and width of median, on-street parking), multimodal elements (e.g., bicycle lanes, bus lanes, sidewalk presence), and roadside or non-roadway elements (e.g., number of driveways, adjacent land use). The policies from state agencies also often included suggestions or recommendations for potential speed management treatments to consider for a given roadway to help achieve the selected target speed. Some policies also emphasized the importance of setting a target speed that is achievable, and they encouraged a review of treated streets to determine the effectiveness of implemented treatments and consider potential subsequent treatments.

To fully investigate the relationships between speed and various roadway, roadside, and non-roadway factors, the research team identified potential study segments in eight states (California, Florida, Massachusetts, Oregon, Texas, Utah, Virginia, and Washington) to use for data collection and analysis. They also developed a list of 56 factors for analysis that included a variety of roadway factors from several sources. Examples include:

• Roadway characteristics data obtained from measurements made using aerial and street- level photographs.
• Roadside characteristics obtained from the United States Environmental Protection Agency (US EPA) Smart Location Database (SLD), which provided several variables that described the characteristics of the areas around the roadway segment (US EPA 2025a).
• Building height obtained from the United States Geological Survey (USGS) data catalog (Falcome 2016).

• Distance to buildings from the roadway center obtained from U.S. Building Footprint dataset (Microsoft 2019).
• School presence obtained from multiple sources.
• Traffic volume and posted speed limit data obtained through online databases from state and local agencies.

Researchers used the INRIX XD™ database to provide the speed data for the study segments, and they chose the free-flow speed measure in the INRIX XD™ database as the independent variable in the analyses. After multiple rounds of data collection and review, the research team developed a database of characteristics and speeds for 6,327 segments in the eight states for analysis.

After reviewing multiple combinations of factors for a variety of potential analyses, with agreement from the project panel to modify the subset of roadways considered in the project to include posted speed limits between 25 and 40 mph, inclusive, the research team decided to proceed with five evaluations that focused on particular variables as follows:

• Evaluation using the posted speed limit variable, to provide insight into the influence of the number on the speed limit sign.
• Evaluation using variables associated with transportation professionals, which included variables that could be under the influence of a transportation professional.
• Evaluation using demographics variables, which included any available variable that could help to identify conditions that are associated with different operating speeds.
• Evaluations focused on center line and edge line markings, which focused on the presence or absence of center line or edge line markings on the subset of the database containing two-lane roads with no median, an average lane width of no more than 13 ft, and a posted speed limit of 25 to 35 mph.
• Evaluation focused on average lane width, which explored the relationship between lane width and operating speed for segments with average lane width between 8 and 13 ft.

Key results from the analyses were as follows:

• In the posted speed limit evaluation, posted speed limit explained about 30 percent of the variability in operating speed for the 6,327 segments included in the database.
• In the transportation professionals evaluation:
  • Faster speeds were associated with wider bicycle lanes, increased building setback, presence of shoulder, increased shoulder width, higher volumes, and higher posted speed limit.
  • Slower speeds were associated with increased building height, presence of curb, higher access density (driveways and uncontrolled intersections), on-street parking, higher National Walkability Index score, higher roundabout density, higher traffic signal density, presence of speed humps.
  • Compared to segments with no bike lane, speeds were slightly lower when the bike lane was separated with a buffer and no vertical element, speeds were slightly higher when the bike lane was separated with pavement markings only, and speeds were higher when the bike lane was separated with a vertical element.

• • Speeds were found to be slower on 2-lane roads as compared to 4-lane roads by 3.4 mph.
• In the demographics and transportation professionals evaluations:
  • Faster speeds were associated with more occupied housing units and a larger percent of two-plus-car households.
  • Slower speeds were associated with increased building height and higher National Walkability Index score.
• In the evaluation focusing on average lane width, subsets of the main database were created to better focus on segments with between 8 and 13 ft lane widths. The average lane width variable was found to be not statistically significant segments for segments with 2 lanes per direction. The additional travel surface space for motorists provides room for drivers to move away from other vehicles, which could encourage higher speeds. Average lane width was found to be significant for two-lane undivided roads (roads with 1 lane per direction). The speed difference between an 8-ft and 13-ft lane on two-lane undivided streets without bike or parking lanes is about 1.3 mph. These road segments could have curbs or shoulders and with posted speed limits of 25 to 35 mph.
• In the evaluation focusing on center line and edge line markings, for two-lane roads with posted speed limit between 25 and 35 mph, the absence of center line markings was associated with slower operating speeds, as was the absence of edge line markings.

Overall, the analyses in this study found these variables to have the greatest influence on operating speed on roadway segments with a 25 to 40 mph posted speed limit:

• Signal density. This variable ranged between 0 and 22 signals per mile with an associated difference of 15.1 mph between when 0 signals per mile was present compared to when 22 equivalent signals per mile was present.
• National Walkability Index. For this database, the range of values was 2.83 to 20.0. The segments with a value of 20 would have operating speeds that are about 8.2 mph lower as compared to those segments with a National Walkability Index value of 2.83. The influence of intersection density, proximity to transit stop, and diversity of land use are considered within the National Walkability Index.
• Access (driveways and uncontrolled intersections) density. More access points were associated with slower speeds, as anticipated. For the range of access points present in this database (0 to 116 driveways or uncontrolled intersections per mile), the speed difference was 4.1 mph slower for the segments with 116 access points per mile than for the segments with no access points.
• Building height. The segments with the tallest buildings were estimated to have speeds that were 5.2 mph slower.
• Building setback. The segments with buildings within a few feet had operating speeds that were estimated to be 2.4 mph slower.
• Presence of speed humps. Speeds were slower when a speed hump is present on the segment by about 3.6 mph. This database included 88 segments with speed humps that were on 25-mph (40 segments), 30-mph (34 segments), and 35-mph (14 segments) roads. None of the 40-mph segments had a speed hump.

• Shoulder width. Speeds were higher with increasing shoulder width. For the range of shoulder width values present in this database (0 to 18 ft), the speed change was 5.6 mph between the two extents of shoulder width.
• AADT. For the range of AADT values present in this database (350 to 59000 veh/day), speed is 3.2 mph faster on the roads with the largest AADT. The finding that speeds are higher on roads with larger AADT is counterintuitive; therefore, these results may suggest that AADT is a surrogate for other factors, rather than an indication that higher traffic volumes produce higher speeds.
• Center line markings and edge line markings. When considering the operating speed on two-lane undivided road segments with a 25 to 35 mph posted speed limit and average lane widths between 8 and 13 ft, the analysis found that the presence of edge line and center line markings was associated with higher operating speeds on the order of 2.1 mph for edge line and 5.2 mph for center line markings.

Variables that may have potential to be influential, but were not statistically significant or the influence was of a smaller difference in operating speed than anticipated for this database, included the following:

• Roundabout density.
• Posted speed limit.
• Bicycle lane width.
• Number of schools within 0.5 mile.

In addition to the research analysis and findings, the research team developed a guide for practitioners to use to incorporate target speed into the design process. This guide provides an avenue for implementation of the research, and it is intended for use as a resource to either supplement an agency’s existing policies and guidelines on the design process or to serve as an example that an agency could use to develop a new or revised policy that includes target speed in their design process. The guide emphasizes the need to select a target speed value early in the design process and to select a value that is attainable. The process in the guide also includes a step for reviewing the performance of the subject roadway to evaluate the effectiveness of the design and consider whether additional treatments might be useful.

NCHRP Project 15-76 concluded with the development of two publications:

• NCHRP Web-Only Document 460: Designing for Target Speed, Volume 1: Operating Speed and Road Elements (this document).
• NCHRP Web-Only Document 460: Designing for Target Speed, Volume 2: Implementation Guide (Brewer et al. 2026).