PhD ThesisThe 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.University of Bahrai