Speed is at the core of the problem of road safety, and speed management is a tool for improving road safety. Speed limits that are credible encourage drivers to comply with them, with consequent benefits for road safety. Credible speed limits have been found to be affected by the features surrounding the road by previous research in the Netherlands. However, not a great deal of empirical work has been done evaluating what a credible speed limit is for a given road layout and roadside environment based on motorists’ perceptions. This thesis builds a research model to link road environment, speed limit credibility, risk perception and speed limit compliance as a whole. The research presented here aims to verify the model. To do this, three separate experiments are used.
Experiment 1 investigates, using a questionnaire, whether the current national speed limit is credible for a variety of UK road environments, and what the difference is between the speed limit and the chosen, self-reported, driving speed. The survey results reveal that road layout and roadside environment affect the intrinsic perception of the appropriate speed and speed limit. Chosen speed limit and chosen speed are not identical in terms of compliance, but are correlated.
Experiment 2 provides various measurements of credible speed limits, and examines how to set more credible speed limits in order to improve driver compliance. A picture questionnaire, a driving simulator in automated conditions, and manual driving in a simulator are used for measurement. The experiment investigates how the layout of single carriageway roads and roadside environmental factors affect speed limit credibility, subjective risk perception and compliance with speed limits. Five indicators: 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, are used to evaluate a credible speed limit for a given road layout, which is used to define a credible speed limit. The method used for setting credible speed limits can be applied to other types of roads. The study develops the relationship between speed limit credibility, risk perception, and compliance with the speed limit.
Experiment 3 investigates how road warning signs affect perception (credibility, safety and necessity) and driving behaviour for a given road layout and roadside environment, using a questionnaire and driving simulator. The study finds that road warning signs affect driving speed, specifically by slowing down the driving speed and reducing the proportion of time spent driving in excess of the speed limit.
Combining the results of the three experiments, the research confirms that speed limit credibility is useful for speed management. The findings indicate that there exists a credible speed limit for each specific type of road that would lead to better speed management. As the credibility of the speed limit increases, drivers become more compliant with it. In terms of practical implications for road design, the research provides advice to local highway authorities on matching credible speed limits to rural single carriageway infrastructure in order to provide safe conditions for all road users. Speed limit compliance can be reinforced by using the most effective combination of warning and speed signs.
