An evaluation of the implications of imposing speed limits on major roads

Abstract

The effectiveness of speed limits has been the subject of considerable debate over the years. In most cases in the past, speed limits have been changed because of a single factor (e. g. improving the safety of road traffic or saving energy). In this thesis an attempt has been made to evaluate the consequences of changing a speed limit using cost-benefit analysis which formed the principle objective of this study. The scope was confined to motorways and similar high-quality roads operating under free-flow traffic conditions where speed limits were believed to be most effective. To achieve the main goal, the effect of the speed limit on the mean speed of traffic was investigated which was the second objective of the study. The third objective was to find the effect of the speed of traffic, and especially the mean speed of traffic, on the frequency and severity of personal injury accidents. There was a need to investigate these two relationships as the literature was not consistent on these relationships. A hypothesis was proposed to achieve the second objective. This was tested by defining criteria that had to be met for each of the data collection sites and measuring the speed of vehicles. There were II sites in Tyne & Wear, England and 14 sites in the State of Bahrain. A statistical analysis was applied to the data collected. It was found, from both sets of data, that speed limits had a positive effect on the mean speed and the eighty-fifth percentile speed of traffic. Linear and non-linear (multiplicative) models were developed for each set of data. In addition to the speed limit, the trip length and the length of the section were shown to affect significantly the mean speed of traffic. The amount of change in the mean speed of traffic varied between the models tested but, generally, for every 4 to 5 km/h change in the speed limit the mean speed of traffic changed by, about, I km/h. In a similar way, a hypothesis was proposed to pursue the third objective. Criteria were established for the selection of suitable data collection sites and for the types of accidents. 9 sites were selected in Tyne & Wear and 10 sites in the State of Bahrain. Data was drawn from a5 year set of accident records in Tyne and Wear and a four year set in the State of Bahrain. A statistical analysis was applied to the data. The set of data from Tyne & Wear revealed no significant relationship between the mean speed of traffic and the frequency of accidents but the speed differentials affected the frequency of the personal injury accidents. The data from Bahrain showed that both the mean speed of traffic and the speed differentials of vehicles affected the frequency of the personal injury accidents. No significant relationships were found between the speed of vehicles and the severity of the personal injury accidents. The principle objective of the study was achieved by applying cost-benefit analysis to the consequences of changing the speed limit for a hypothetical typical section of road. The components of cost were the cost of travel-time, the vehicle operating cost, and the cost of accidents. No monetary values were assigned to the environmental effects so it was not possible to include them in the cost-benefit analysis but they were acknowledged. Any changes in air pollution and noise annoyance due to a change in the mean speed of traffic following a change in a speed limit were likely to be small and were not considered in the study. The significance of the uncertainty in the frequency and severity of personal injury accidents in relation to the mean speed of traffic was studied using 'break-even analysis'. Generally, it was believed that lowering the speed limit on motorways and similar high-quality roads would produce negative benefits, even if the frequency and severity of personal injury accidents decreasedw ithin expectedr anges. Increasing the speed limits would produce positive economic benefits but the conclusion was less firm than the previous case. Sensitivity analysis was applied to the variables used in the cost-benefit analysis. It was found that the net benefits were most sensitive to the estimation of the effect of the speed limits on the mean speed of traffic, the initial mean speed of traffic in the base year of the assessment, the travel-time cost, the changes in the frequency of the personal injury accidents, and changes in the number of fatal injury casualties per average personal injury accident as the speed limit varied (i. e. in descending order for most speed limits). The ranking of these variables differed as the speed limit was changed.