A method to assess road safety of planned infrastructure

This case study of Maastricht was part of Workpackage 9 ‘Town studies' of the research project Developing Urban Management And Safety (DUMAS), commissioned by the European Commission. One of the main objectives of DUMAS was to develop an assessment framework to encourage the use of objective measures in urban safety management initiatives. The objective of the 'Maastricht Town Study' was to develop a generally applicable methodology for assessing the influence on traffic safety of local initiatives, taking the 'Maastricht case' as a typical case. The town of Maastricht is situated in the South of the Netherlands, and it was chosen for this study because it has some serious problems, concerning a motorway running through the city. This motorway not only has a through-function, but is also used as distributor road. Besides the mobility and accessibility problem, the situation also involves a major environmental problem of air pollution and noise nuisance. To solve these problems, three alternative plans have been prepared. These three infrastructure plans have been analysed as to their traffic safety consequences by estimating the number of injury accidents in the year 2010. The three variants were named after the most important intervention: the ‘Traverse variant', the ‘Northern Bridge variant', and the ‘Eastern Diversion variant' (by-pass). In the analysis, not only the effects of the measures concerning infrastructures were taken into account, but also the predicted accident reduction, due to the introduction of the ‘sustainable safety' principles, was included. At this moment, the introduction of ‘sustainable safety' plays an important role in the Netherlands to improve road safety and is therefore one of the main institutional issues. A sustainably safe road traffic system is one in which the road infrastructure has been adapted to the limitations of human capacity, through proper road design. The key to arriving at a sustainably safe road system lies in the systematic and consistent application of three safety principles: -functional use of the road network by preventing unintended use of roads; -homogeneous use by preventing large differences in vehicle speed, mass and direction; -predictable use, thus preventing uncertainties among road users, by enhancing the predictability of the road's course and the behaviour of other road users. The three safety principles require the specification of the intended function of each road and street. Roads are built with as major function the so-called traffic function, enabling people and goods to travel. Three options are distinguished: -the flow function, enabling high speeds of long-distance traffic and, frequently, high volumes; -the distributor function, serving districts and regions containing scattered destinations; -the access function, enabling direct access to properties alongside a road or street. Based on the distinguished road functions, five road categories have been defined: through-road, distributor road, urban and rural, and access road, urban and rural. To assess the road safety consequences of the three plans concerning infrastructure in Maastricht, a quantitative method has been introduced. Themethod consist of three steps: STEP 1:Determination of national safety indicators per road category; STEP 2:Calculation of local safety indicators and comparison with national indicators; STEP 3:Estimating the number of accidents for different scenarios in the prognosis year with a new set of sustainable safety indicators per road category. The basic formula used in these three steps is: Nacc = indic * v_k in which: Nacc = number of injury accidents [] indic = safety indicator [/106 km] per road category v_k = vehicle kilometres [106 km] on this road category In the first step, ‘indic' is calculated for each of the five defined road categories, based on a representative sample of the national road network in a reference year. In the second step, the first step is repeated for the studied road network in the reference year. Then the national safety indicators are compared with the local safety indicators, resulting in a correction factor for each national safety indicator. In the third step, local safety indicators are estimated for the prognosis year by applying the correction factors on a set of estimated national sustainable safety indicators in the reference year. Subsequently, the number of injury accidents is estimated with the corrected safety indicators for all considered variants in the prognosis year. In order to apply the described method, one needs to gather accident data, road category data, and traffic volume data. Basically, all information needed was available for the Maastricht case. However, to perform the required calculations, it is necessary that all data is linked to one digital map only. Unfortunately this was not the case. Each data type, categorisation data, traffic volume data, and accident data was linked to its own map. The development of a common map and the data manipulation is described extensively. It was found impossible to perform the second step of the method in the Maastricht case. As a consequence, the number of injury accidents in the prognosis year was estimated by using uncorrected national safety indicators. In the Maastricht case, the estimated numbers of injury accidents were not significantly different for the three infrastructure variants studied. However, in the case of the Eastern Diversion variant, the number of kilometres driven over the total network is smaller than for the other two variants. This may have environmental advantages. The quantitative results of the method presented is very useful for policy makers to identify traffic safety consequences of local infrastructure plans. The method is particularly suited for comparing the traffic safety effects of a number of alternatives to solve one particular traffic problem. The setup of the method is relatively simple and probably applicable in other countries as well. Clearly, national safety indicators should be available, to be able to apply the described method.