Safety Performance Indicators for roads; Searching for a quantitative road safety assessment method. Safety Performance Indicators (SPIs) are factors that have a strong causal relationship with road safety. Typical of SPIs for roads is that different road features in varying combinations can influence the safety of a road. In a multi-year project SWOV performs research into data and existing initiatives in the field of SPIs. This is done with the purpose to develop a new method to quantitatively describe, evaluate and monitor the road safety of the road network and road design. In the second year of this research project the central question was: How can the road safety of existing and future roads be integrally measured and assessed ? For an answer to this question, the authors first searched for possible SPIs for the road safety of roads. They performed a zero measurement and a pilot monitor of the two most appropriate tools: the Sustainable Safety Indicator and VSGS (Safe Speeds, Credible Limits). The Sustainable Safety Indicator can be used to determine the extent to which the layout of a road conforms to the requirements of sustainable safety. VSGS is a tool to assess ‘safe speeds and credible speed limits’. In this framework, the following activities were carried out: 1. The Sustainable Safety Indicator and VSGS were used to assess a total of 154 roads in the Dutch towns Harderwijk en Leeuwarden; these roads were urban 50 km/h roads. 2. For each road the crash density was determined based on the registration of serious road injuries during the period 1998-2009. 3. The convergent validity of the instruments was determined. 4. The relationship between the scores of the instruments and crash density was determined (impression). This research failed to establish a relationship between Sustainable Safety Indicator scores, the VSGS scores and the crash rate. Because essential recent data on traffic volume and speeds driven was unavailable in this study, we have been unable to determine to what extent the Sustainable Safety Indicator or VSGS are valid instruments that can be used to classify composite features of road infrastructure as SPIs. However, a relationship was found between Sustainable Safety scores of roads, safe speeds and credible speed limits, which can be interpreted in terms of road safety. Recommendations of this study were: 1. Not all relations between the Sustainable Safety Indicator, VSGS and crash density that were found in this study are entirely explicable. Follow-up research, in which traffic volume and actual speeds driven are also included, is necessary in order to determine the extent to which these instruments can explain risk differences. 2. The ultimate goal is to determine SPI’s for urban infrastructure that can be used to quantitatively describe, assess and monitor the (road safety) quality of the road network and road design. This requires further study into among other things: a. The development of the traffic safety on different types of sustainably safe roads: is the road layout partly or fully in accordance with Sustainable Safety requirements ? Are these roads safer than roads without a sustainably safe layout? And: can these road safety relationships be determined with the Sustainable Safety Indicator and/or VSGS ? b. The relationship between the individual features that are included in the Sustainable Safety Indicator and VSGS on the one hand, and road safety on the other (which features are/are not decisive in the relationship with road safety ?) c. The question of whether the safety of a road is determined by the most extreme (high or low) value of a road feature, or by the calculated average for the entire road. In addition, additional research is required on the relationship between, the scores of the Sustainable Safety Indicator and VSGS in relation with speeds driven and traffic volume on the one hand, and the risk of a (serious) crash on the other hand. d. The suitability of the Sustainable Safety Indicator scores and VSGS-scores as an alternative for road crash data to assess the road safety of roads: are all features that are measured relevant as a measure of safety ? And: are the results of both instruments valid when it comes to a demonstrable relationship with road safety ? An important prerequisite is that the quality (registration rate, substantive registration of facts) of the crash registration increases.
Samenvatting