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This study recommends a method to establish maximum speed limits based on the 85th percentile of travel speeds. The conclusion is supported by an extensive literature search and analysis of traffic flow data collected by a unique Computer-Sensor System. Such data indicate that risk increases with deviation from mean speed. Such increase is minimal until approximately the 85th percentile, when the slope of the risk curve starts to rise sharply.and SUMMARY [my bolding added]:
This report documents work conducted by the Indiana Institute for Research in Public Safety under sponsorship of the National Highway Safety Bureau of the U.S. Department of Transportation to recommend a method for establishing maximum speed limits that could be widely implemented using existing technology and manpower resources.
The research plan includes an extensive review of existing research literature, an evaluation of identified methods, collection and analysis of data utilizing a Computer-Sensor System to validate existing methods, and the development of a programmed instruction text to implement the recommended method.
The literature review considered more than 300 documents relating to the history of speed limits; driver speed behavior and variables, other than speed limits which influence it; the relationship of speed, speed limits, and accidents; and methods for establishing speed limits. The review revealed three major approaches to establishing speed limits, one based on measure of prevailing speeds, another based on characteristics of speed distributions, and a third based on cost.
A national survey of practices used by states and cities to establish maximum speed limits was conducted to determine existing methods, technical resources, and the manpower involved in establishing speed limits. Questionnaires were sent to traffic engineers of all state highway departments, all cities over 100,000 in population (130), and 52 selected cities with populations under 100,000. The response (88%) indicated that the following items were most frequently considered in establishing speed limits:
The survey also showed the general availability of radar, vascar, ball-bank indicator, and vehicle counters to measure vehicle speed, traffic characteristics, and roadway features.
The various techniques for establishing speed limits identified as a result of the literature review and the survey of jurisdictions were subjected to a screening analysis to identify those methods worthy of further consideration for full-scale implementation. The analysis revealed these such techniques:
Data collected using the IRPS Computer Sensor System were analyzed to provide a further basis for selecting a recommended technique from the three identified by the screening analysis. The analysis clearly showed the strong relationship between deviation of the speed of the accident-involved vehicle from the mean speed of the traffic stream. The analysis also showed that the cumulative accident involvement rates were acceptably flat (i.e., independent of speed) until the speed deviation reached a point corresponding to the 85th percentile speed plus rounding and enforcement tolerances, after which it started to rise at a percipitous rate.
The final result of the study effort was to recommend that maximum speed limits be established on the basis of the 85th percentile of travel speeds. Such a limit is:
The recommendation of the 85th percentile is supported by a substantial portion of the technical literature as well as by the data and analyses of the present study.
The final project report is presented in four volumes. Volume I contains the technical portion of the report; Volume II presents an extensive review of the literature; Volume III provides an implementation program for the recommended method of establishing a speed limit; and, Volume IV contains a condensed explanation of the recommended method for the experienced traffic engineer.
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END OF SUMMARY