An empirical approach to determining speed limit credibility

There is now a body of literature on speed limit credibility, particularly in connection with speed management under the overall umbrella of 'Safe Systems'. However, there is rather little empirical work on the underlying factors that determine the credibility of speed limits and on how to enhance credibility for a given type of road. The study reported here aimed to investigate how factors such as road layout and the roadside environment together with drivers’ perception of risk affect speed limit credibility. It also aimed to provide a measurement of speed limit credibility and how to set more credible speed limits to improve drivers’ speed compliance. A picture questionnaire and a driving simulator in an automated condition in a simulator were adopted to obtain the measurements. The results suggest that certain road layout and environment features influence speed limit credibility. The research results show that five new indicators can be used as a reference for deciding on a credible speed limit in a given road environment: the most common choice of speed limit by drivers; the highest credible rating score value; indication of comfort with speed in automated driving; risk rating in the range from feeling safe to very safe; arousal indicated by skin conductance. Applying these indicators makes it possible to determine a limit that is credible for most motorists in a given road environment. Improving the credibility of a speed limit will lead to better speed management overall.