EVALUATION OF LOWER SPEED LIMITS ON URBAN HIGHWAYS. FINAL REPORT

The objective of the research presented in this report was to evaluate the Nebraska Department of Roads (NDOR) method of establishing speed zones on state highways passing through urban areas and develop guidelines for establishing speed zones on these roadways. The evaluation was to determine the safety effects of speed zones established in accordance with the NDOR method relative to the safety effects of speed zones with lower speed limits. In addition, the NDOR procedure for conducting the test-car speed runs was evaluated relative to procedures used by other states. The accident experience in speed zones with reasonable speed limits was compared with the accident experience in speed zones with unreasonable speed limits. Reasonable speed limits were consistent with the speed limits determined by the NDOR method and unreasonable speed limits were lower than those determined by the NDOR method. Regression analysis was used to investigate the relationships between the accident experience and the traffic and roadway conditions in the zones. These relationships were used to assess the accident reduction potential of reasonable speed limits and to determine the speed limits that minimize road user costs. The results of the analysis of the accident experience in speed zones indicate that zones with posted speed limits equal to the reasonable speed limits proposed by the NDOR method of speed zoning are safer than zones with posted speed limits that are 5 and 10 mph below the reasonable speed limits. Speed zones with speed limits 5 mph below the reasonable speed limits were found to have 5% more accidents than zones with reasonable speed limits. Speed zones with posted speed limits 10 mph below the reasonable speed limits have 10% more accidents. The results of the cost-effectiveness analysis indicate that road user costs are minimized when the posted speed limits in speed zones are set equal to the reasonable speed limits proposed by the NDOR method. Speed zones with posted speed limits 5 and 10 mph lower than the reasonable speed limits have higher accident and travel time costs than zones with reasonable speed limits. A survey of traffic engineers knowledgeable about speed zoning procedures indicated that there is no generally accepted procedure for conducting test-car speed runs as part of speed zoning studies. The use of the test-car procedure to obtain input from local officials is unique to Nebraska. This use of the test-car speed runs is not recommended, because it is inherently biased toward the lower speed limits requested by local officials and serves no useful purpose in the determination of the most appropriate speed limit.