Towards A Checklist For Credible Speed Limits; Development Of An Assessment Method Based On Road And Road Environment Characteristics

One of the short-term possibilities of reducing the percentage of speeding offences is to improve the credibility of speed limits. A speed limit is more credible when the limit in force conforms more to what the road user intuitively considers to be safe for the relevant road section i.e. without knowing what the actual limit is. This is determined by a broad range of road and road environment characteristics. This report describes the first steps towards a checklist with which road authorities can quickly and simply see if the current speed limit of a particular road is credible. The checklist focuses on urban roads with a speed limit of 30, 50, or 70 km/h and rural non-motorways with a speed limit of 60, 80, or 100 km/h. Based on existing studies, ten road and road environment characteristics have been identified as influencing the credibility of speed limits. With regard to five of these characteristics, the road automatically meets the wishes for a credible speed limit when it has been laid out according to the general guidelines for road design of CROW National Information and Technology Platform for Infrastructure, Traffic, Transport, and Public Space. These five characteristics are: parking facilities, pedestrian facilities, cyclist facilities, road design including marking, and intersection type. The other five credibility characteristics are not, or only briefly, described in the CROW guidelines. These characteristics are as follows: road width, the straight sections of a road, physical speed reduction measures, openness of the road environment, and type of road surface. These are characteristics that, depending on which speed limit applies, entice higher or lower speeds; the authors call them 'accelerators' and 'decelerators'. The checklist focuses on this last mentioned group of road and road environment characteristics and, with them, requires a supplement to the guidelines. In the design of the checklist the authors have provisionally chosen a tree structure in which the five speed-ups/slow-downs are checked in succession. The tree structure allows the cohesion between the characteristics to remain visible, and shows better the holistic character of the credibility concept. The check-list has been tried out in practice by two road authorities. In general they were enthusiastic and judged the checklist as being useful and usable. They also identified a number of improvements that have since been carried out. In order to develop the checklist further it is useful to test it with a larger group of intended users. Moreover, it is important to realize that the checklist is not yet definite. The checklist presented is based on the limited knowledge available at this moment and must therefore be treated with the necessary caution. New knowledge about credibility can lead to adaptations and refinements.