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NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 1 SUMMARY Several factors are considered within engineering studies when determining the posted speed limit for a speed zone. National Cooperative Highway Research Program (NCHRP) Project 17-76 investigated the factors that influence operating speed as well as safety and used that knowledge to develop the Speed Limit Setting Procedure (SLS-Procedure) so engineers can make informed decisions about the setting of speed limits. The SLS-Procedure was automated with the Speed Limit Setting Tool (SLS-Tool). The SLS-Tool is spreadsheet based and is included with the NCHRP Research Report 966: Posted Speed Limit Setting Procedure and Tool: User Guide (1). Currently, the predominant method for setting speed limits uses the 85th percentile speed. It is viewed as being a fair way to set speed limits based on the driving behavior of most drivers (85 percent), who represent reasonable and prudent drivers since the fastest 15 percent of drivers are excluded. The 85th percentile speed is also believed to represent a safe speed that would minimize crashes. Criticisms of the 85th percentile speed method include a concern that drivers may not see or be aware of all the conditions present within the corridor, and such an approach may not adequately consider vulnerable roadway users such as pedestrians and bicyclists. Other concerns are that drivers are not always reasonable and prudent, or they only consider what is reasonable and prudent for themselves and not for all users of the system; and the use of measured operating speeds to set speed limits could cause increased speed over time (i.e., speed creep). Drivers frequently select speeds a certain increment above the posted speed limit, anticipating that they will not receive a ticket if they are not above that assumed enforcement speed tolerance. Also, most of the early research justifying the use of the 85th percentile speed was conducted on rural roads; therefore, the 85th percentile speed may not be appropriate for urban roads. The research team considered the breadth of approaches available for the setting of speed limits and the need to develop a methodology that could be used for any roadway type. The research team selected a decision-rule-based procedure for the SLS-Procedure. Given the increased emphasis on designing for the context of the roadway, the research team decided that the SLS-Procedure should be sensitive to context and use the expanded functional classification scheme available in NCHRP Research Report 841: Development of Crash Modification Factors for Uncontrolled Pedestrian Crossing Treatments. The roadway types and roadway contexts available within the expended functional classification scheme were collapsed into four Speed Limit Setting Groups (SLSGs): Limited Access, Undeveloped, Developed, and Full Access. Unique decision rules were developed for each SLSG. For the SLS-Procedure, the research team recommended consideration of the measured operating speed as the starting point for selecting a posted speed limit but that the measured operating speed be adjusted based on roadway conditions and consideration of the crash experience on the segment. The guiding principles developed by the research team for the SLS-Procedure included that the procedure: ï· Use a data-driven approach with research-based decision rules. ï· Produce consistent results for a given set of conditions. ï· Incorporate contemporary policies, guidelines, and practices. ï· Consider driversâ speed choice and roadway safety. ï· Provide transparency in the decision process.
NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 2 ï· Consider all roadway types and roadway contexts. ï· Vary the decision rules to account for the diverse characteristics of each speed limit setting group. ï· Consider agency data and human resource constraints. ï· Include inputs and outputs on the same screen to demonstrate the relationship between each roadway characteristic and selection of the suggested speed limit. ï· Allow for future modifications to accommodate new knowledge. ï· Create efficiencies in the decision process, where possible. The SLS-Procedure starts with identifying the roadway segment context and type, which determine the appropriate SLSG. For that SLSG, the roadway characteristics and crash potential for the segment are used to identify the speed distribution that should be considered and whether the closest 5-mph increment value or a rounded-down 5-mph increment value should be used. Based on the previous steps, a potential suggested speed limit is identified. Figure 1 illustrates the procedure. Figure 1. Overview of SLS-Procedure to calculate the suggested speed limit. In NCHRP Project 17-76, the research team focused a portion of the Phase II efforts on collecting data for suburban and urban roads to fill the known research gap for city streets. The developed databases for Austin, Texas, and Washtenaw, Michigan, were used to investigate the relationships among crashes, roadway characteristics, and posted speed limits. The team found that crashes on city streets were lowest when the operating speed was within 5 mph of the average operating speed. Therefore, the research team recommended that the 50th percentile speed be a consideration within the SLS-Procedure, especially for the SLSGs of Developed and Full Access. The evaluation of the Austin, Texas, and Washtenaw, Michigan, data supported including the following variables within the decision rules: signal density, access density, and undivided median on four-lane (or more) streets. Findings from the literature were also used to develop the decision rules. CalculatedÂ valueÂ basedÂ onÂ considerationÂ ofÂ roadwayÂ contextÂ &Â type,Â speedÂ distribution,Â andÂ safety ConsiderationÂ ofÂ drivers'Â speedÂ selectionÂ onÂ theÂ segment;Â ConsiderationÂ ofÂ crashÂ riskÂ basedÂ onÂ roadwayÂ characteristics ContextÂ =Â rural,Â ruralÂ town,Â suburban,Â urban,Â orÂ urbanÂ core;Â TypeÂ =Â freeway,Â majorÂ arterial,Â minorÂ arterial,Â collector,Â orÂ local RoadwayÂ ContextÂ andÂ TypeÂ SpeedÂ Distribution Safety SuggestedÂ SpeedÂ Limit
NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 3 Presenting a workshop was a requirement of the research. Members of the research team conducted several workshops and presentations during the development of the SLS-Procedure, and these presentations provided opportunities to obtain feedback on the potential format of the procedure. The presentations with the panel were especially influential in setting the direction for the SLS-Procedure and SLS-Tool. NCHRP Project 17-76 concluded with the development of two publications: ï· NCHRP Research Report 966: Posted Speed Limit Setting Procedure and Tool: User Guide. ï· NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool (this document).