Simulation of rural travel times to quantify the impact of lower speed limits

Abstract

The number and severity of rural road crashes has been shown to decrease with reduced traveling speed. One method of reducing the traveling speed on rural roads is to reduce the speed limit of those roads. Despite the considerable road safety benefits resulting from reduced speed limits, public opposition to the change exists. One of the main concerns of the public is the increase in travel times associated with a reduction in speed limit. In South Australia the default rural speed is 100kph, however many undivided rural roads in the state are signposted at 110kph. This study quantifies this increase in travel time on an arbitrary rural road in South Australia if the signposted speed limit was replaced with the defult speed limit of 100kph. A Markov model of travel time on an undivided rural road is developed. The model includes factors such as vehicle speed distributions, passing lanes, traffic congestion, and speed advisory signs. Real data collected on South Australian rural roads that have had a similar reduction in speed limit are used to define the distribution of speeds of vehicles before and after the change in speed limit. The model shows that an increase in travel time is expected, however this increase is less than is first predicted by considering only the allowed speed limit.