Strategic planning and design of rural roads can encourage drivers to choose speeds consistent with the posted limits, reducing the severity of speeding-related crashes.

More than 37,000 fatalities and 2 million injuries occur annually on highways and streets in the United States as a result of traffic crashes. The fatal crash rate in rural areas is 1.84 per 100 million vehicle-miles traveled, which is more than 2.5 times the urban fatal crash rate of 0.71. However, traffic safety improvement programs with strategies for rural areas may be especially successful in reducing crash frequency and severity.

Of particular interest among traffic-related fatalities in the United States are those attributed to speeding, which is approximately 27 percent of the total fatalities annually. Speeding-related crashes are those defined as driving too fast for conditions or exceeding the posted speed limit. Among rural traffic fatalities, approximately 28 percent are classified as speeding-related. In 2015, the percentage of speeding-related crashes that occurred on rural roadways with a posted speed limit of 55 miles per hour, mi/h, (88 kilometers per hour, km/h) or higher was 74 percent. Collectively, these data suggest value in effective speed-management programs to reduce speeding-related crashes on moderate- and high-speed, two-lane rural highways.

In January 2018, the Federal Highway Administration published a report, Self-Enforcing Roadways: A Guidance Report (FHWA-HRT-17-098), which provides details on how to produce “self-enforcing” roadways. A self-enforcing road, also called a self-explaining roadway, is planned and designed to encourage drivers to select operating speeds consistent with the posted speed limit. Engineers can apply the concepts to both planned and existing roadways. This article summarizes several of the concepts that transportation professionals may use to manage speeds on two-lane rural highways; however, many of the concepts may also be applicable to other roadway types.

Speed-Safety Relationships

With regard to crash severity, the operating speed of motor vehicles directly affects the crash outcome. Kinetic energy is directly proportional to the square of the operating speed; thus, higher operating speeds should result in more severe crash outcomes.

Traffic safety research reflects the physics involved. Research published in 2006 by the Transportation Research Board (TRB) determined that speed limit increases were associated with an increase in the probability of fatal injury crash outcomes, presumably because of increases in vehicle operating speeds. Likewise, a study published by TRB in 2008 found that increases in the posted speed limit were associated with an increased likelihood of a crash resulting in an injury or fatality on rural roads. In addition, a 2016 study by the Insurance Institute for Highway Safety concluded that fatality rates and risk increased as maximum speed limits increased in many regions of the United States.

The relationship between speed and crash frequency is less clear. Separate studies done in the 1960s concluded that as vehicle operating speeds deviated from the average speed of the traffic stream, crash involvement rates increased. Later efforts to replicate the findings were not consistent. In 2002, a study by the University of Minnesota concluded that studies with the familiar U-shaped speed deviation-crash involvement rate relationships were ecological fallacies because individual crash risk based on the speed dispersion among a group of vehicles in the traffic stream does not clearly distinguish between individual and group risk measures.

In summary, published research supports the notion that higher operating speeds are associated with more severe crash outcomes. Increasing posted speed limits has shown to increase operating speeds, leading to more severe crash outcomes. However, consistent findings related to the relationship between crash frequency and speed metrics have not been established.

Speed-Geometric Design Relationships

Geometric roadway design practices in the United States rely on design controls and criteria set forth in the American Association of State Highway and Transportation Officials’ (AASHTO) A Policy on Geometric Design of Highways and Streets, also known as the “Green Book.” The design speed (herein referred to as the designated design speed) is defined in the Green Book as “a selected speed used to determine the various geometric design features of the roadway.” The Green Book either explicitly or implicitly uses the design speed concept to establish horizontal alignment, vertical alignment, and cross section design elements. Examples include radius of curvature (R), stopping sight distance (SSD), braking distance (db), horizontal sight line offset (HSO), length of vertical curvature (L), maximum superelevation (emax), maximum side friction factor (fmax), and lane and shoulder widths.

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The inferred design speed, which FHWA’s Speed Concepts: Informational Guide (FHWA-SA-10-001) defines as “the maximum speed for which all critical design speed-related criteria are met at a particular location,” is equivalent to the designated design speed when either minimum or limiting values of design criteria are used. However, the Green Book recommends using design values that exceed minimum values, and in such cases, the inferred design speed will exceed the designated design speed.

Transportation engineering researchers and practitioners often use operating speed models to assess geometric design consistency, most notably on two-lane rural highways. Many studies have estimated statistical models to predict vehicle operating speeds that may be used to evaluate highway geometric design consistency. In many of the models, transportation researchers and practitioners can input variables such as design features (for example, horizontal curve radius), posted speed limit, and annual average daily traffic into the models to determine the vehicle operating speed under free-flow conditions (for example, vehicle headways of 4 or more seconds). While the most common speed output from these models is the expected 85th-percentile operating speed, statistical models of mean speed and the standard deviation of speed exist. Applying operating speed models may confirm that designated design speeds, posted speed limits, and driver expectations will all be more consistent when the roadway geometry is designed to manage speeds.

Examples of speed-geometric design relationships are illustrated in Self-Enforcing Roadways: A Guidance Report. Among speed-based geometric features, the radius of horizontal curve and horizontal curve spacing have the greatest influence on driver speed choice. For more information, visit www.fhwa.dot.gov/publications/research/safety/17098/17098.pdf.

Self-Enforcing Road Concepts

According to National Cooperative Highway Research Program Report 504, “a design process is desired that can produce roadway designs that result in a more harmonious relationship between the desired operating speed, the actual operating speed, and the posted speed limit.” While transportation researchers suspect that achieving speed harmony in geometric design affects the safety of a road, the actual effects are unknown, as reported in “Geometric Design, Speed, and Safety,” published in Volume 2309 of the Transportation Research Record: Journal of the Transportation Research Board. However, when speed harmony exists, the road designs “look and feel” like the intended purpose and may be described as more self-explaining.

When the operating speeds of a roadway are inconsistent with the design speed, speed discord results. Speed discord is a roadway design that produces operating speeds that are higher than the posted speed limit. According to FHWA’s Speed Concepts guide, speed discord on two-lane rural highways often resulted from the use of above-minimum values of geometric design criteria. In such cases, the 85th-percentile operating speed along a roadway segment often exceeds the posted speed limit and the designated design speed.

“In the year 2017, a total of 1,137 lost their lives on Pennsylvania roadways. Out of the total number of fatalities, 463 were [the result of] speed-related crashes,” says Glenn Rowe, P.E., chief of the Pennsylvania Department of Transportation’s (PennDOT) Highway Safety and Traffic Operations Division.“When you consider over 40 percent of fatal crashes are relatedtospeed, it’s imperative that PennDOT implement cost-effective designs for self-enforcing or self-explaining roadways in order to reach our zero fatalities goal.”

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Self-Enforcing Roadways: A Guidance Report describes six concepts that may be used individually or in combination to design roadways that produce actual operating speeds consistent with the desired operating speeds. The concepts include: (1) applying the speed feedback loop process, (2)using the inferred design speed approach, (3)applying operating speed models to assess design consistency, (4)utilizing existing geometric design criteria, (5) using a combination of signs and pavement markings, and (6) setting rational speed limits.

What follows is a closer look at the inferred design speed approach, methods for design consistency, and setting rational speed limits.

Inferred Design Speed Approach

The inferred design speed approach applies only to design criteria that are based on the designated design speed. The designated design speed and inferred design speed will differ when using larger-than-minimum values (or lower-than-limiting values) of geometric design criteria. When applying this method, the inferred and designated design speeds are plotted on a two-dimensional graphic (speed versus roadway length) to evaluate design consistency. This graphic can provide information about setting an appropriate regulatory speed limit that is related to the anticipated operating speeds of a roadway. Large differentials between the inferred and designated design speeds will likely produce operating speeds that are higher than anticipated in the design process.

FHWA’s guidance report outlines a five-step process to develop an inferred design speed evaluation of an existing or planned roadway. The designated design speed is contained on a set of roadway construction plans, while the inferred design speed is computed using models (for example, stopping sight distance formula, point-mass model for horizontal curve design, and minimum length of crest vertical curve formula) contained in geometric design policy, such as the Green Book. Engineers need the geometric design features of the roadway to compute the inferred design speed, which is based on speed-based design elements (such as horizontal alignment, vertical alignment, and cross-section). The posted speed limit is based on an engineering field study or local statutes.

Design Consistency Methods

Design consistency methods also may be used to assess the relationship between posted speed limits, designated design speeds, and operating speeds. Operating speed prediction models are needed to apply the methods. A synthesis of these models is available in TRB’s 2011 Transportation Research e-Circular “Modeling Operating Speed” at http://onlinepubs.trb.org/onlinepubs/circulars/ec151.pdf. Design consistency methods can be evaluated manually using a series of equations, or using an automated method, such as the Design Consistency Module of the FHWA Interactive Highway Safety Design Model (IHSDM).

The design consistency method requires detailed geometric design data. These data often are not available in early project planning efforts. However, as the project development progresses, engineers can integrate computer-aided files (or geometric design data) into IHSDM. The Design Consistency Module estimates 85th-percentile operating speeds, including acceleration and deceleration rates approaching and departing curves, and compares operating speeds on successive design elements. It also compares the designated design speed to the predicted 85th-percentile operating speed. Speed differences of less than 6 mi/h (9.6 km/h) are considered good, while speed differences greater than 6 mi/h (9.6 km/hr) are considered fair (if less than or equal to 12mi/h, 19.3 km/h) and poor (if greater than 12 mi/h, 19.3 km/h).

Self-Enforcing Roadways: A Guidance Report outlines a three-step process to apply the design consistency approach. The steps are:

1. Acquire roadway geometricdesign criteria (for example, radius of curve, tangent lengths, vertical grade, lane and shoulderwidths, and roadside hazard rating).
    
2. Determine the appropriate operating speed model to apply based on the existing or planned geometry.
    
3. Set the posted speed at a level consistent with the expected operating speed.

Setting Rational Speed Limits

Another approach to developing self-enforcing or self-explaining roadways is to set speed limits that are reasonable, rational, and consistent with the features of the roadway. FHWA has a Web-based tool, USLIMITS2, which provides guidance regarding appropriate posted speed limits for all road types. USLIMITS2 determines rational speed limits through expert knowledge of speed limits and a series of decision rules and procedures applied to a particular scenario.

The three-step process in Self-Enforcing Roadways: A Guidance Report outlines how to apply USLIMITS2. Inputs to the program include operating speed information, traffic volume, terrain, access density, roadside characteristics, the statutory speed limit, presence of onstreet parking, pedestrian and bicycle activity, crash history data, and geometric design information.

Safer Speeds, Safer Roadways

Speeding is cited as a contributory factor in nearly 27 percent of all fatal crashes reported in the United States, and a significant number of these incidents occur on rural roadways with posted speed limits that exceed 40 mi/h (64.3 km/h). As a result, managing speeds on two-lane rural highways is likely to be an effective safety management strategy. Self-enforcing or self-explaining roadways present one possible approach to manage speeds by encouraging drivers to choose a speed that is compliant with the regulatory speed limit.

“Properly designed self-enforcing roadways can be effective in producing speed compliance and less severe crash outcomes,” says Monique R. Evans, P.E., director of Eastern Federal Lands Highway Division at FHWA. “And FHWA’s Self-Enforcing Roadways: A Guidance Report provides guidance to transportation agencies on how to design self-enforcing roadways.”

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Eric Donnell, Ph.D., P.E., is a professor of civil engineering and director of the Thomas D. Larson Pennsylvania Transportation Institute at Pennsylvania State University. His research and teaching interests include geometric design of highways and streets, speed management, and traffic safety.

Kristin Kersavage is a Ph.D. candidate in civil engineering at Pennsylvania State University. She received her bachelor of science degree from George Washington University. She has worked on transportation engineering projects for both the public and private sectors.

Abdul Zineddin, Ph.D., is a transportation specialist with FHWA’s Office of Safety Research and Development at the Turner-Fairbank Highway Research Center. He oversees the speed management research program. Zineddin holds bachelor of science, master of engineering, and doctorate degrees in civil engineering from Pennsylvania State University.

For more information, see www.fhwa.dot.gov/publications/research/safety/17098/17098.pdf or contact Abdul Zineddin at 202–493–3288 or abdul.zineddin@dot.gov.