This paper was presented at the "Speed limits, safety and special conditions" session. These proceedings are available on CD-ROM. The influence of vehicle travel speed on traffic safety has attracted a lot of interest especially since the latest speed limit increases in many jurisdictions of the United States. There are different opinions regarding the effect of travel speed on traffic safety. Some previous studies found that average travel speed related to traffic accidents while others indicated that speed differentials mainly affected traffic safety. This paper examines travel speed and accident data on Saskatchewan highways. It is found that both average travel speeds and speed differentials are closely correlated with casualty accident rate. It was also found that speed differentials seem to increase as the average travel speeds increase at road sections with lower speed limits and decrease as the average travel speeds increase at road sections with the highest speed limit. This result is contrary to the findings of some previous studies. The reasons for such relationship between travel speeds and speed differentials are explored. The paper also reviews international experience on speed limit changes. It suggests that two ypes of speed limit changes should be differentiated. Speed limit changes for large systems and speed limit changes of individual road sections may have different impacts on traffic safety. Generally, speed limit increase (decrease) for a large system tends to lead to an increase (decrease) in travel speed and casualty accidents, while speed limit changes of individual road sections may have varying impacts on traffic safety depending on specific road situations. The safety impacts of speed limit changes for individual road sections therefore should not be generalized. (A)
Abstract