Setting Speed Limits for Safety
Engineering appropriate speed limits now is easier and more consistent, thanks to a powerful software tool called USLIMITS2.
Speeding--whether driving in excess of the posted speed limit or simply traveling too fast for road conditions--is a factor in an estimated one-third of all fatal crashes, according to the National Highway Traffic Safety Administration. Although agreement is almost universal on the relationship between speed and crash severity (the higher the crash speed, the more serious the resulting injuries and property damage), the relationship between speed and the probability of a crash is more complicated. Higher speeds require longer stopping distances and thus possibly result in more crashes. However, crash risk also involves a multitude of other factors, including roadway and environmental conditions as well as driver attitudes and behavioral traits that are independent of speed.
Unlike other driver behaviors that can have a negative impact on safety, such as distracted driving, speed is also associated with positive benefits, including reduced travel times, greater mobility, and increased economic productivity due to lower transport and inventory costs and larger market areas. Thus, speed management involves balancing safety and efficiency in travel.
Posting speed limits is the most widely used method for managing speed, yet they are only effective if set at the maximum reasonable and safe speed. The apparent subjectivity of the process of setting speed limits, however, can lead to inconsistencies in posted speeds within and between jurisdictions. This can also make it difficult to defend decisions on speed zoning when public pressure comes to bear.
In 2012, the Federal Highway Administration’s (FHWA) Office of Safety released an updated version of USLIMITS2, a Web-based software program designed to assist State and local agencies in setting appropriate--defined as safe, credible, consistent, and enforceable--speed limits. The following is a refresher on the science behind speed limits and a glimpse at how some States are using this software to improve their processes for setting speed limits.
Legal Framework For Speed Limits
As described in the Transportation Research Board’s (TRB) Special Report 254, Managing Speed: Review of Current Practice for Setting and Enforcing Speed Limits, published in 1998, motorists continually make decisions about appropriate driving speeds by factoring in the amount of risk they are willing to accept. Using the assumption that drivers are motivated by a strong incentive to complete their trips safely, a natural question is why not allow drivers to choose their own travel speeds? However, the report identifies three main reasons for officials to regulate drivers’ speeds:
- Inappropriate speed choices can impose risks and uncompensated costs on other drivers.
- Motorists often lack sufficient information (for example, capabilities of their vehicles, roadway geometry, or other conditions) to determine an appropriate speed.
- Motorists’ inability to judge the effect of speed on safety and their tendency to underestimate the effect of higher speeds on crash propensity impose unnecessary risks.
States establish their speed limits based on national standards outlined in FHWA’s Manual on Uniform Traffic Control Devices (MUTCD) and their own legal frameworks. Most States have statutes establishing default speed limits determined by road type and location. For example, State law might limit vehicle speeds to 30 miles per hour (mi/h) (48 kilometers per hour, km/h) in urban and residential areas, 55 mi/h (89 km/h) outside urban areas, and 70 mi/h (113 km/h) on freeways. However, States and most local governments also have the authority to change speed limits on the basis of an engineering study in sections of roads where the statutory limits do not fit specific conditions. The speeds posted in these speed zones designate the maximum reason-able and safe speed, based on favorable conditions.
Engineering Speed Limits
The MUTCD is the national standard for all traffic control devices on roads open to public travel. It requires that speed limits be posted in increments of 5 mi/h (8 km/h) and that speed limits in speed zones be based on an engineering study and analysis of free flow speeds (speeds that are unimpeded by other vehicles, stop signs, signals, or inclement weather).
The MUTCD recommends that agencies set speed limits within 5 mi/h (8 km/h) of the 85th percentile speed of free-flowing traffic. The 85th percentile speed is the speed that 85 percent of drivers travel at or below and is one of the best indicators of a reasonable and safe speed. According to FHWA’s report, Methods and Practices for Setting Speed Limits (FHWA-SA-12-004), motorists who drive faster contribute disproportionately to the risk of crashes. Setting a speed limit 5 mi/h (8 km/h) higher than the 85th percentile speed will make a few additional drivers legal, but setting it 5 mi/h (8 km/h) lower will make violators out of nearly half of all drivers.
The MUTCD also lists other risk factors that may be considered, including road geometry, the pace speed (the 10 mi/h, or 16 km/h, speed range at which the most vehicles are driving), roadside development, parking practices, pedestrian activity, and crash experience. How-ever, it does not provide specific guidance on how to account for these variables.
In Methods and Practices for Setting Speed Limits, prepared in cooperation with the Institute of Transportation Engineers, FHWA reviewed national and international practices and then described the following methods for determining an appropriate speed limit for a given roadway section:
- Engineering approach. This approach typically involves a starting point (or base condition) from which engineers first identify a speed limit using the 85th percentile speed or road function, and then adjust it in accordance with other roadway characteristics, such as pedestrian or bicyclist activity, roadside hazards, and the presence or absence of medians.
- Expert system approach. Expert systems aim to emulate the human thought process in solving complex problems. In this approach--basically a computerized version of the engineering approach--a computer program uses decision rules developed by traffic engineers, enforcement personnel, and other practitioners to recommend a speed limit.
- Optimization approach. Here, the goal is to minimize the societal costs of transportation using models that estimate the effect of speed limits on travel time, vehicle operating costs, crashes, noise, pollution, and other factors. Although this method has some theoretical appeal, it is rarely used in practice because of its complexity.
- Injury minimization or safe system approach. In this case, the tolerance of the human body to injury during a crash is the primary factor in identifying appropriate speeds. This method has become popular in some parts of Europe, especially Sweden and the Netherlands, which have a legal framework that is different from that in the United States. (In the United States, the safe system approach cannot be the sole criterion for setting speed limits because State laws and the MUTCD require that the speed limit also be reasonable.)
Expert Systems for Setting Speed Limits
In 2003, FHWA released the first version of USLIMITS to assist practitioners in setting speed limits that are safe, credible, consistent, and enforceable. When used to determine appropriate speed limits, this Web-based expert approach provided a systematic, consistent method for examining and weighing factors in addition to vehicle operating speeds. ARRB Transport Research, headquartered in Melbourne, Australia, created the original version of USLIMITS by modifying a speed zone expert system developed for the Australian State road authorities. The researchers adapted the system logic developed by Australian experts to reflect practices in the United States, such as posting in 5 mi/h (8 km/h) increments.
Four years later, with funding from the National Cooperative Highway Research Program (NCHRP), a research team headed by the University of North Carolina Highway Safety Research Center produced an updated version with decision rules developed by U.S. experts. A project panel made up of three State department of transportation (DOT) members, an Institute of Transportation Engineers representative, and a county transportation representative oversaw development of USLIMITS2, which was released in 2007. In addition to the project panel, the research team convened a group of experts to identify key factors and decision rules through a series of panel meetings and case studies.
In 2012, the same team updated the user interface and moved the expert system to a server at FHWA. USLIMITS2 now can be accessed at http://safety.fhwa.dot.gov/uslimits.
The software employs a risk-based set of decision rules using roadway, traffic, and crash data input by the user to determine an appropriate speed limit for a specific segment of roadway. USLIMITS2 is applicable to all types of roads, from two-lane rural roads and residential streets to multilane urban freeways. Currently, however, USLIMITS2 is not designed to address speed limits in work zones, school zones, or other areas with variable speed limits.
USLIMITS2 provides recommended speed limits based on site characteristics, which include such variables as access density, pedestrian activity, roadside hazards, operating speed, and crash history. It also provides relevant warnings in a summary report that can suggest actions for addressing areas of concern.
As noted earlier, the 85th percentile speed, the speed that 85 percent of drivers travel at or below, is one of the best indicators of a reasonable and safe speed. The median or 50th percentile speed is used in recommending a speed limit when the crash risk on a particular road is significantly higher than the average for similar roads, or when other risk factors such as significant pedestrian activity are present. The researchers selected the median speed as the determining factor to better balance the speed driven by the majority of motorists with the needs of other road users or the local community.
The following case studies illustrate the application of USLIMITS2 and its data requirements using real-life examples.
Michigan: Racho Road
Complaints and enforcement challenges compelled authorities in the city of Taylor, MI, to select several corridors for reevaluation of existing speed limits. The traffic division of the police department evaluates requests for changes in speed limits in the city. The traffic division worked with a local engineering consultant to conduct an investigation. One of the corridors they selected was a 2-mile (3-kilometer) section of Racho Road that had an existing speed limit of 35 mi/h (56 km/h).
|
Factors Used in USLIMITS2 |
|
|
Road Type |
Site Characteristics |
|
Limited-access freeway |
Operating speed: 50th and 85th percentile speed |
|
Road section in undeveloped area |
Operating speed: 50th and 85th percentile speed |
|
Road section in developed area |
Operating speed: 50th and 85th percentile speed |
Racho Road is a four-lane divided roadway separated by a grassy median in the northern portion and a center two-way, left-turn lane in the southern segment. The roadway is an urban minor arterial serving low-density residential development and a major commercial shopping center. There are 25 access points and an average of one signal per mile (1.6 km) in the section under study. Although there is a 4-foot (1.2-meter) sidewalk in both directions that is offset 10–25 feet (3–7.6 meters) from the road edge, there is little pedestrian activity. The consultant and the traffic division used the USLIMITS2 tool to assist in the decisionmaking process.
Martin Parker, a senior engineer with Wade Trim, headquartered in Detroit, MI, and a national expert on speed zoning, conducted the study for the city of Taylor. His team used automated speed monitoring equipment and road tube sensors to collect speed data for a 24-hour period at three locations within the corridor. The median speed was 38 mi/h (61 km/h) and the 85th percentile speed was 44 mi/h (71 km/h). The 3-year crash rate for the study section was 340 per 100 million vehicle miles and the injury crash rate was 58 per 100 million vehicle miles.
According to USLIMITS2, the crash rate for the section was above the average for similar roads but below the critical rate of 364 per 100 million vehicle miles. The critical rate is the rate that is statistically higher than the average for similar roads with 95 percent confidence. The injury crash rate was below average.
USLIMITS2 recommended a speed limit of 40 mi/h (64 km/h). The Taylor city council approved the change in speed limit, and the city’s department of public works erected new signs.
Parker, who has conducted hundreds of these studies for Federal, State, and local agencies, observes, “Decisionmakers are more likely to accept the engineering study recommendations when they are supported by the expert-based USLIMITS2 system.”
Indiana: County Road 500/510 South
Changes in development along County Road (CR) 500/510 South in Tippecanoe County, IN, and complaints about the 55 mi/h (89 km/h) speed limit on a segment of the roadway prompted the county highway department and sheriff’s department to reevaluate the existing speed limit. CR 500/510 South is a two-lane roadway with 12-foot (3.7-meter) lanes and 6-foot (1.8-meter) grass shoulders. Roadside hazards such as telephone poles are located 15 feet (4.6 meters) or more from the edge of the road.
The study section is located south of the Lafayette city limits but is on the fringe of urban development. Over the years, the surrounding area gradually has transitioned from a rural undeveloped area to a mix of rural and residential development. The section of the road used in the study had fewer than 30 access points per mile (19 per kilometer), and carried an average of 4,200 vehicles per day. The median speed was 43 mi/h (69 km/h) and the 85th percentile speed was 49 mi/h (79 km/h). Police reported 29 crashes--six of which involved personal injury--over a 3-year period.
Because of CR 500/510 South’s varied characteristics, engineers used USLIMITS2 to evaluate the road under both undeveloped and developed scenarios. For the undeveloped scenario, the roadside hazard risk was rated a 3 based on guidance provided by USLIMITS2. The roadside hazard rating is a composite measure of roadside conditions, including shoulder width, side-slope, clear zone distance, and fixed object hazards. USLIMITS2 considers a rating of 3 or less to indicate good roadside conditions and low risk. For the developed scenario, both pedestrian activity and parking were negligible. According to USLIMITS2, the crash rate for the study section of 373 per 100 million vehicle miles was considerably above average for similar roads and above the critical rate. The injury crash rate of 77 per 100 million vehicle miles was near the average for similar roads in urban areas and above the average but below the critical rate for rural areas.
USLIMITS2 recommended a speed limit of 45 mi/h (72 km/h) for both scenarios. The program also recommended that the county undertake a comprehensive crash study to identify engineering or traffic control actions that could mitigate the crash problem. Based on the engineering investigation, the county engineer recommended--and the county council approved--reducing the speed limit to 45 mi/h (72 km/h).
Wisconsin: Statewide Safety Study
In 2009, the Wisconsin Department of Transportation (WisDOT) released new Statewide Speed Management Guidelines, which provide step-by-step instructions for investigating and setting safe, enforceable speed limits. The guidelines recommend that all jurisdictions use USLIMITS2 as a starting point for determining appropriate speed limits.
Rebecca Szymkowski, State traffic engineer of operations at WisDOT, notes that both the department and local agencies are required by law to base all speed limit changes on a traffic engineering study. “USLIMITS2 is a tool that should be part of that study,” says Szymkowski. “It provides an objective method for considering the many factors involved and can help analysts in developing consistent and rational speed limit recommendations.”
WisDOT staff recently completed a statewide study of 34 noninterstate highway corridors identified by their regional offices as having suspected speed issues (for example, poor compliance or speeding-related safety problems). For each corridor, they questioned law enforcement personnel, collected speed data, reviewed crash data, and performed an analysis using USLIMITS2.
They also identified engineering countermeasures for reducing speed. The percentage of crashes that were related to speeding exceeded the statewide average in 13 of the 34 corridors in the study. The large majority of crashes occurred when the roads were wet or slippery because of snow or ice. Only about one-quarter of the study locations had appropriate speed limits.
John Campbell, senior traffic engineer with TADI of Cedarburg, WI, who conducted the study for WisDOT, concluded that “USLIMITS2 is a useful tool that is easy to use.”
In a related study, WisDOT compared five corridors across the State with irrational speed limits to similar corridors with rational speed limits. The researchers classified speed limits in each corridor as rational or irrational based on observed travel speeds and results from USLIMITS2. Posted speed limits that differed by more than 5 mi/h (8 km/h) from the limit recommended by USLIMITS2 were defined as irrational.
“Roadways with irrational speed limits tended to have larger differences in travel speeds and higher crash rates,” says Campbell. “These results are consistent with other studies that indicate that lower limits do not have much effect on the speed most drivers choose to travel and do not guarantee a safer road.”
Beyond the Limits
To minimize the number of speed limit changes, advisory speeds are used for spot locations, such as horizontal curves or hidden intersections, where reduced speeds are needed for safe operation. FHWA has developed a handbook of procedures designed to improve consistency in advisory speed signing and, it is hoped, in driver compliance with advisory speeds. The handbook, Procedures for Setting Advisory Speeds on Curves (FHWA-SA-11-22), provides guidelines for determining when an advisory speed is needed, engineering study methods for establishing the advisory speed, and strategies for selecting other curve-related traffic control devices.
Variable speed limits based on changing road, traffic, and weather conditions have the potential to improve safety and operations under adverse conditions when crash risks are higher. FHWA recently released Guidelines for the Use of Variable Speed Limit Systems in Wet Weather (FHWA-SA-12-022) for conditions such as snow, rain, or ice. The guidelines address the design, installation, operation, maintenance, and enforcement of variable speed limits in wet weather, especially at locations where the operating speed exceeds the design speed and the stopping distance exceeds the available sight distance.
Although speed limits are set to indicate the maximum speed based on favorable conditions, road design is based on a minimum speed that reflects comfort considerations and unfavorable conditions. In 2009, FHWA published an informational guide to help engineers, planners, and elected officials better understand design speed and its potential role in achieving desired operating speeds and setting rational speed limits.
These guidelines and reports are available on the FHWA Office of Safety’s Speed Management Safety Web site at http://safety.fhwa.dot.gov/speedmgt.
Last Word
The management of speed through appropriate speed limits is an essential element of highway safety. Speed limits are an act of the State traffic law system and need to be reasonable, safe, and enforceable.
According to Michael Griffith, director of FHWA’s Office of Safety Technologies, “The USLIMITS2 Web-based application provides State and local agencies with a tool for determining appropriate speed limits for specific roadway segments. It is easy to use and should be a part of any speed limit review. Engineers should collaborate with law enforcement and other stakeholders before a speed limit change is made. Enforcement is needed to ensure compliance with speed limits.”
For situations where the community desires, or a safe system approach suggests, a speed limit lower than that recommended by USLIMITS2, the solution is not to simply lower the speed limit. Studies show that setting speed limits lower than the prevailing speed without changing the road environment will have little effect on speed and might increase crash risk. Instead, agencies should consider alteration of the road layout and other engineering countermeasures to reduce speeds to a level that would support the lower limit. Engineering countermeasures for reducing speeds and improving road safety will be explored further in a future issue of Public Roads.
References
American Association of State and Highway Transportation Officials. Highway Safety Manual, Appendix 3E—Speed and Safety. Washington, DC, 2010.
Campbell, J., Safety Impacts of Rational vs. Irrational Speed Limits. Wisconsin DOT, Madison, WI, June 2011.
Federal Highway Administration. Effects of Raising and Lowering Speed Limits on Selected Roadway Sections (FHWA-RD-97-002). Washington, DC, 1997.
https://www.fhwa.dot.gov/publications/research/safety/humanfac/rd97002.cfm
Federal Highway Administration. Guidelines for the Use of Variable Speed Limit Systems in Wet Weather (FHWA-SA-12-022). Washington, DC, 2012.
http://safety.fhwa.dot.gov/speedmgt/ref_mats/fhwasa12022/
Federal Highway Administration. Manual on Uniform Traffic Control Devices (MUTCD), 2009 edition. Washington, DC, 2009. http://mutcd.fhwa.dot.gov/kno_2009r1r2.htm
Federal Highway Administration. Methods and Practices for Setting Speed Limits (FHWA-SA-12-004). Washington, DC, 2012. http://safety.fhwa.dot.gov/speedmgt/ref_mats/fhwasa12004/
Federal Highway Administration. Procedures for Setting Advisory Speeds on Curves (FHWA-SA-11-22). Washington, DC, 2011.
http://safety.fhwa.dot.gov/speedmgt/ref_mats/fhwasa1122/
Forbes, G.J., Gardner, T., McGee, H., and Srinivasan, R. Methods and Practices for Setting Speed Limits: An Informational Report (FHWA-SA-12-004). Washington, DC, April 2012.
http://safety.fhwa.dot.gov/speedmgt/ref_mats/fhwasa12004/
Milstead, R., Qin, X., Katz, B., Bonneson, J., Pratt, M., Miles, J. and Carlson, P. Procedures for Setting Advisory Speeds on Curves (FHWA-SA-11-22). Washington, DC, June 2011.
http://safety.fhwa.dot.gov/speedmgt/ref_mats/fhwasa1122/
Organization for Economic Co-Operation and Development. Speed Management. Paris, 2006.
http://www.internationaltransportforum.org/Pub/pdf/06Speed.pdf
Stuster, J., Coffman, Z., Warren, D. Synthesis of Safety Research Related to Speed and Speed Management (FHWA-RD-98-154). Federal Highway Administration, Washington, DC, July 1998.
https://www.fhwa.dot.gov/publications/research/safety/98154/
Transportation Research Board. TRB Special Report 254: Managing Speed—Review of Current Practice for Setting and Enforcing Speed Limits. Washington, D.C., 1998.
http://onlinepubs.trb.org/onlinepubs/sr/sr254.pdf
Wisconsin Department of Transportation. Wisconsin Statewide Speed Management Guidelines. Madison, WI, June 2009.
http://town-dayton.com/wp-content/uploads/2013/01/speed-guide.pdf
Davey Warren is director of research at Brudis & Associates, Inc. He has more than 37 years of experience in traffic and safety research with a significant component focused on speed management. He received a B.S. in civil engineering from North Carolina State University and an M.S. in transportation from the University of Maryland.
Guan Xu is manager of the Speed Management program in FHWA’s Office of Safety and a member of the USDOT Speed Management Team. She leads FHWA’s efforts to implement infrastructure actions in the USDOT Speed Management Strategic Initiative. She has more than 25 years of experience in traffic operations and safety. She holds an M.S. in civil engineering from the University of Cincinnati.
Raghavan Srinivasan is a senior transportation research engineer at the University of North Carolina Highway Safety Research Center. He has more than 15 years of research experience in human factors and traffic safety. He has a Ph.D. in civil engineering (transportation) from the University of California, Davis.
For more information, visit http://safety.fhwa.dot.gov/speedmgt or contact Guan Xu at guan.xu@dot.gov.