Safety effects of route choice in a road network: Simulation of changing route choice

In the Netherlands, the concept 'Sustainably safe traffic' is the leading vision in road safety policy and research. The main goal of a sustainably safe road transport system is to reduce the annual number of road crash casualties to a fraction of the current levels. Important requirements following from this vision are that journeys should follow safe roads as much as possible, should be as short as possible, and the quickest and the safest route should coincide. This report focuses on the development of a method which enables the planner to find out the safety effects of existing route choices, and also of changes in route choice. Road safety can be described in various ways. It has previously been shown that micro-simulation models are a suitable aid for route choice studies. They make it possible to examine beforehand how the route choice will change as a result of new or adapted facilities alongside or on the roads, or in vehicles. Safety indicators are required when evaluating the safety effects of the route choice of (all) vehicles in a network, and when evaluating the effects of changes in these route choices. In this report these indicators are formulated and used in a test network in a micro-simulation model. We chose two types of road safety indicators: general and vehicle-dependant. The general indicators are independent of the traffic volume on a road network. They are derived from the route characteristics that are closely related to road safety, such as the route length or the number and types of transitions between different road types. These general safety criteria are rooted in the 'route diagram' which is a method of visualizing the Sustainable Safety character of a route. The optimal route diagram shows a journey that contains all road types in the correct sequence and in the correct proportions of length. The deviation from the optimal diagram determines how unsafe the presumed route is. Thus the route diagram expresses a qualitative safety that can be translated into quantitative criteria. The vehicle-dependant indicators allow for the real-time traffic situation on the network. They express the extent to which vehicles encounter other vehicles along a route and how these meetings end; they are 'conflict indicators'. The mass of the vehicles, their direction, speed, and lateral position largely determine the severity of conflicts. Here we are still speaking of calculated conflicts in a simulation model; in other words not of real conflicts, let alone near-misses. The results of the calculation methods used do not all give the same safety effects for a specific route choice. Further research is necessary to find the explanation for this and to determine the methods' utility. In principle, the route choice safety criteria are suitable for (computer) programs used in route planners. Applying the micro-simulation model to a test network is insufficient for deciding whether such models are a suitable road safety research instrument. For a well-founded decision, a micro-simulation must be tried on a real-life road network, and the registered safety, usually expressed in crashes, should be compared with the calculated safety. More research is needed to model serious conflicts between road users. It is especially important that the number and nature of calculated conflicts are similar to the real ones. That is why observations in real traffic are required.