Road safety by design; A decision support tool for identifying ex ante evaluation issues of road safety measures

Proefschrift Technische Universiteit Delft TUD, Delft


Jagtman, H.M.




During the last decade of the 20th century the road safety policy shifted from an approach in which the road users had to adjust to the traffic system towards the concept of an inherently safe road system. The Dutch sustainable safety concept, the Swedish vision zero concept and the British road safety program are examples of this concept. The basic idea of an inherently safe road concept is to prevent accidents from happening in the first place. For the limited number of accidents that remain, the severity level should be minimised. A traffic system, which is designed inherently safe, is further characterised by an integral system approach in which the infrastructure, the vehicles and the regulations are tuned to the users of the system. In the coming thirty years the inherently safe concept will be implemented in various European road systems. In the same period various intelligent transport systems (ITS) will be introduced in these systems. In order to design an inherently safe system with ITS, the possible effects of implementing these systems should be studied using a system approach, in which ITS has to be an element of the integrated traffic system. Effects on safety can result from the intended process (operation) of an instrument in road traffic or from a deviation from that process. Both type of effects need to be assessed before implementing an instrument in an inherently safe traffic system. Currently, ex-ante evaluations mainly aim at investigating the intended process. The development and introduction of intelligent transport systems (ITSs) and the necessity to integrate these ITSs as elements in the traffic systems for safety purposes inevitably increase the need for a revised and integrated safety approach towards evaluating the road elements, including ITSs, before using these. The project contributes to a process oriented approach for ex-ante evaluation of road safety instruments by identifying the safety issues that have to be evaluated. For broader safety science perspectives this project adopts a systems hazard identification method (HAZOP) to a more open process, namely road traffic. The aim of the integrated safety approach is to incorporate risk in the design and the development of the road system. The identified safety issues lead policy makers to the questions that need to be answered before implementing policy instruments into the inherently safe road system. For experts and researchers the issues resulting from applying the approach lead to scenarios that need to be tested during design of policy instruments (including intelligent transport systems). This project consists of three stages: development, application and validation of the integrated safety approach for identification of safety issues. In the first stage, development, the assessment framework is built. The effort is focussed on defining the intended traffic process and identifying deviations from this process. For identifying deviations the HAZOP (HAZard and OPerabity) method is adapted to the road traffic field. This so-called Traffic HAZOP uses a structured brainstorm process in order to identify and to qualitatively assess deviations in the traffic process. Before defining deviations, the traffic process in study has to be defined using a structured map, which contains relations between the main goals in transportation and linkages with possible road safety instruments, which is established through categorisation of these instruments into safety barriers. During application the framework is applied to currently used speed reducing measures and new measures using ITSs. The users for whom the framework should have added value in defining implementation issues of road safety policy tools will participate during this stage. The validation stage contains two steps: first, validation of the usability of the method for experts and policy makers and second, validation of the additional value of using the framework instead of the current ad hoc approach. The study resulted in a description of the integrated safety approach containing discussion of the structured map (for definition of the intended process) and the Traffic HAZOP (for identifying deviations). Furthermore, the approach was applied to two cases in which speed reducing instruments are studied. In one case study the use of speed humps to reduce speeds nearby a school was compared to the use of Intelligent Speed Adaptation, ISA. In this case study moreover three different groups were compared to analyse whether the composition of the team was of any relevance for the results. In the second case study a ISA pilot in the city of Lund was analysed in a retrospective manner. It was studied whether the method could identify any additional potential safety issues regarding the use of ISA in urban areas. This applications showed the applicability and added value of the method for policy making regarding traffic issues.

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