Designing for Target Speed, Volume 2: Implementation Guide (2026)
Chapter: 1 Introduction
CHAPTER 1. INTRODUCTION
Overview
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 non-motorized 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. 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. Target speed is defined as 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.
The objective of this Guide is to give practitioners and designers a process and tools to achieve target speeds through the geometric design process. To achieve target speeds, the design process places greater emphasis on the combined effect that roadside and roadway features have on the speed that drivers choose. Designing roadways for a target speed involves the simultaneous implementation of a combination of roadway and roadside design elements, which this research has summarized in Chapter 3. This Guide addresses the target speed design considerations of roadway corridors with existing or proposed speed limits between 25 and 40 mph.
Traditional approaches to road design do not account for the effect of various geometric and non-geometric elements that are not identified in the AASHTO’s A Policy on Geometric Design of Highways and Streets, commonly known as the Green Book. (AASHTO 2018) Apart from the fact that many roadway corridors have land uses and associated features that are not within the practitioner’s control, the design engineer can adjust certain roadway design features with a consciousness of their effect on target speed.
The target speed is a speed selected in the design process for a roadway 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.
The profession is in the process of adding roadway context classification considerations to the design process, as a complement to roadway functional classification but not replacing it completely. (Stamatiadis et al. 2022) The relationships of roadway hierarchy, function, and context reside in the principles of target speed.
The process of selecting a target speed for design is necessary at the start of project planning and scoping. This should include a speed study that is the first step in identifying what types of design elements are most appropriate for the selected or anticipated road environment and reducing the inclusion of elements that are inconsistent with that environment. Such inconsistencies lead to mismatched expectations for road users.
Scope of the Guide
This Guide was developed to provide guidance for practitioners on incorporating target speed into the design process for roadways with existing or proposed speed limits between 25 and 40 mph. New tools are provided for traffic planners and road practitioners to make informed deliberate choices of road elements to achieve target speeds that match the roadway function and road environment.
Emerging practice as prescribed herein aligns the functional classification structure with the road environment context to reflect the facility’s purpose while accounting for the built environment that frames a given corridor segment.
The Guide serves the development of speed-based design criteria for new roads and roads being widened, reconstructed, repurposed, or modified, to more closely align with their functional and road environment context classification. It provides tools to cover the range of target speeds from 25 mph to 40 mph for suburban and urban contexts.
Target Audience
This Guide provides information on the process, data needs, and analysis methods for practitioners that participate in the project development process, specifically designers, planners, and traffic engineers. Practitioners should be aware that Federal, State, and local laws and policies may supplement or supersede this Guide’s guidance and should be familiar with them while applying the content of this Guide.
Organization of Guide
This Guide is organized into four chapters:
- Chapter 1. Introduction: Overview of speed terminology and the importance of target speed; importance of including target speed within the design process for typical roadway project development.
- Chapter 2. Target Speed in the Design Process: Presentation of road classification by context and type; introduction of the 10-step process for applying target speed to the roadway design process; documentation of transportation expectations as the basis for selecting a target speed.
- Chapter 3. Achieving Target Speed: Presentation of speed-influencing treatments and road elements for achieving target speed.
- Chapter 4. Case Studies: Examples of target speed procedure applications in three case studies.
Speed Terms
The term “speed”, as used in the context of this Guide, refers to the velocity of motorized vehicles on roadways. Speed is measured and governed in several ways, so it is important to have clarity in definitions of speed terms to be able to effectively communicate with both practitioners and non-technical audiences. Table 1 summarizes commonly used speed terms and their definitions considered in this document.
Table 1. Definitions of selected speed terms.
| Term | Definition | Source |
|---|---|---|
| 50th percentile (median) | The speed at or below which 50 percent of the total observed values fall in a sample of measured spot speeds. | NCHRP Report 966 (Fitzpatrick et al. 2021a) |
| 85th percentile | The speed at or below which 85 percent of the total observed values fall in a sample of measured spot speeds. | NCHRP Report 966 (Fitzpatrick et al. 2021a) |
| Contextual expectations of a roadway | Includes what the driver expects along with what is expected of the driver. It may be commonly or colloquially referred to as the “look and feel” of a roadway. It extends beyond the roadway’s functional purpose of carrying traffic within the vehicle travel lanes, encompassing the overall visual and sensory experience presented to drivers, and communicating an appropriate speed to a driver. | NCHRP 15-76 Definition (Brewer et al. 2026) |
| Contextual reaction | How the driver actually responds to the roadway, particularly the choice of speed, given the contextual expectations. | NCHRP 15-76 Definition (Brewer et al. 2026) |
| Design speed | A selected speed used to determine the various geometric design features of the roadway. | AASHTO Green Book (AASHTO 2018) |
| Free-flow speed | The average speed of vehicles on a given segment, measured under low-volume conditions, when drivers are free to drive at their desired speed and are not constrained by the presence of other vehicles or downstream traffic control devices (i.e., traffic signals, roundabouts, or STOP signs). | NCHRP Report 966 (Fitzpatrick et al. 2021a) |
| Operating speed | The operating speed of a road is the speed at which motor vehicles generally operate on that road. In a general sense, the term operating speed refers to the speed at which drivers are observed operating their vehicles. The 85th percentile of a sample of observed free-flow speeds has been typically used as a descriptive statistic for establishing the operating speed associated with a particular road segment; however, other percentiles have also been used. | NCHRP Report 966 (Fitzpatrick et al. 2021a) |
| Posted speed | Numeric speed limit value displayed on regulatory speed limit signs. | NCHRP Report 966 (Fitzpatrick et al. 2021a) |
| Speed | Rate of movement of a vehicle. | NCHRP Report 966 (Fitzpatrick et al. 2021a) |
| Spot speed | Instantaneous measure of speed at a specific location on a roadway. | NCHRP Report 966 (Fitzpatrick et al. 2021a) |
| Statutory speed limit | Statutory speed limits are established by State legislatures and are enforceable by law. Such limits typically vary by highway type (e.g., interstate) or by location (e.g., urban district). | NCHRP Report 966 (Fitzpatrick et al. 2021a) |
| Term | Definition | Source |
|---|---|---|
| Target speed | A speed selected in the design process for a roadway 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. | NCHRP 15-76 Definition (Brewer et al. 2026) |
| Time-mean speed | Arithmetic mean or average of several spot speed measurements (or the average of speeds of vehicles passing a given point along a roadway over a certain time period). | NCHRP Report 966 (Fitzpatrick et al. 2021a) |
Examples of Target Speed
Recently, several state and local agencies have developed target speed policies and practices, which can emphasize different aspects of the design process and the intended outcomes of including target speed. Example statements of the target speed definition are noted below.
- Nevada: Target speed is the highest speed at which vehicles should operate on a roadway in a specific context. It should be set so the kinetic energy in a crash is such that fatal and serious injury crashes do not result when crashes occur and must be consistent with the level of multimodal activity generated by adjacent land uses, to provide both mobility for motor vehicles and a supportive environment for pedestrians, bicyclists, and public transit users. Ideally, the target speed, speed limit, and design speed should all be the same. (Nevada DOT 2022)
- Massachusetts: A target speed is the highest operating speed at which drivers should ideally operate on a roadway given a specific context. Over time, roadway features or uses can change, which may mean that practitioners need to revisit appropriate target speeds, rather than just defaulting to existing speed limits. In practice, these kinds of roadway evolutions can mean that while a driver may be adhering to the speed limit, they are still traveling at a speed too high for the road environment. (Massachusetts DOT 2023)
Objectives and Guiding Principles – Target Speed
Target speed is a proactively determined value and is not obtained solely through observing current or historical operating speeds. The target speed should match the form and function of the roadway so that speed controls and speed management measures are appropriate for the road type and can be readily understood by road users. (Main Roads Western Australia, 2022) The road environment should clearly match and reinforce the selected target speed.
Importance of Target Speed
Studies that have examined the relationship between uniformity of speed suggest that speed dispersion about the mean speed does indeed play a role in crash probability on urban streets, particularly on urban arterials, but that many other factors are also likely to affect crash probability. (Transportation Research Board 1998) A recent NCHRP study that used crash data for urban arterials confirmed the relationship between speed variability and crash occurrence (Park et al. 2020, Fitzpatrick et al. 2021a). The authors observed that when the range of operating speeds is great within a segment, more crashes occur. Intuition is that larger spread (or
variability) in operational speed is indicative of reduced smoothness in operations, with higher potential for speed differentials. That condition, in turn, may result in an increased risk of crash occurrence. They noted that another possible explanation is that the associations found could indicate that sites with more speed variability tend to be those with mixed visual cues or prone to ambiguous contextual situations (e.g., wide streets in a residential setting) that may result in different drivers choosing different speeds, and perhaps by doing so a larger proportion of the driving population could be more likely to exceed roadway conditions and thus increase their risk of crashing. (Park et al. 2020)
In the past, practitioners sought to promote good traffic operational and safety performance for a roadway facility primarily by meeting dimensional design criteria. Traditional applications took the approach that, if the geometric design of a project met or exceeded specific dimensional design criteria that are derived from the design speed of a roadway, it would likely perform well. In some cases, this may have led to overdesign, resulting in projects that were more costly than they needed to be or were inappropriate for the road environment, factors that until recently were not explicitly accounted for. (AASHTO 2018) Practitioners are encouraged to review the guidance in the current edition of the AASHTO Green Book, which discourages above-minimum design criteria for low-speed facilities.
Establishing appropriate operating speed on a roadway segment or corridor depends largely on the context class of road involved and its design features, traffic density, geographic location, and land use, and to a lesser extent on documented roadway functional class or even the type of vehicles using the road. Hence, a desirable approach is to design or redesign roads so that their context, function, and design characteristics are more apparent to drivers and more closely aligned with desired motorist driving speeds – the target speed. As a long-term approach, agencies are more commonly defining roads by the roadway context to encourage homogeneous corridors that result in less variation in speed and observed speeds aligned with target speeds. (Stamatiadis et al. 2022)
Road Elements Affecting Target Speed
NCHRP Project 15-76, Designing for Target Speed (Brewer et al. 2026), investigated and documented relationships between road elements and operating speed. In additional to gaining insights into relationships from previous studies, NCHRP Project 15-76 conducted an analysis of speeds and segment characteristics of 6,327 roadway corridors with speed limits between 25 and 40 mph. Results of this research should not be used for speed prediction; rather, the results provide an appreciation of which road elements may be influencing the speed that drivers are selecting on a two or four-lane arterial roadway segment.
Typically, a project that focuses on target speed is for an existing roadway where much of the roadway context has already been established, so a large part of the effort of achieving target speeds involves making changes to the existing environment. The results from NCHRP Project 15-76 also reflect existing roadways, so the elements found to affect speed have been used on active roadways. Chapter 3 provides a detailed presentation of elements found to be associated with operating speed, which could then be considered during the choice of target speed.
How to Use this Guide
Correlations exist between a roadway context and its intended function. This Guide provides practitioners with methods of classifying roadways and selecting design elements that more closely align with drivers’ expectations. By using the context and functional classification of the road as combined influences on road designs, the road authorities can reconcile the often-mismatched expectations of users that contribute to an undesirable variation in operating speeds. Closing the gap between driver expectations and current operations is one benefit that the procedures of this Guide provide.
In principle, the procedure described in this Guide systematizes the incorporation of target speed in the design of roadways in several logical steps:
- Documenting the existing road environment, roadway functional/context class and operating conditions and the associated roadway attributes.
- Accounting for the roadway features and context classification that influence operating speed then choosing a target speed.
- Adjusting, to the extent feasible, the road elements and attributes that contribute to achieving a target speed, recognizing the collective and cumulative effect of those attributes on operating speed.
- Setting the target speed as a design speed then applying design criteria values appropriate for that speed.
- Evaluating the built condition to validate treatments and measures applied to achieve target speeds.
Details of this procedure are described in Chapter 2.
