One of the main problems within pavement management and maintenance strategies is the relation between road surface condition and accidents, for example the relation between rut depth and accident risk. However, in a VTI study there were no results showing that deeper ruts tend to increase accident risk generally. Efficient and cost effective maintenance and rehabilitation of roads require access to objective and reliable analysis methods and tools. Pavement management systems including road user effect models for calculating the consequences and costs for road users of different maintenance strategies have been developed over the past years. There is, however, an identified need for improvement of existing road user effect models in many countries. The road administrations in Finland, Sweden, Norway and Estonia have brought up their concern of current models not functioning adequately. The main problems in these countries are the relation between road surface condition and accidents, the effect of the main condition parameters, i.e. rut depth, to road user cost, as well as the role of road user costs/effects for a road network that is in substantially good condition. These are problems that have to be solved in order to improve justification of road maintenance budget allocations. VTI was therefore commissioned to carry out a study to determine how rut depth affects the accident risk of road users. Separate analyses were done for data from Sweden, Finland and Norway, respectively. It was assumed that the accident risk also depends on other road condition variables, e.g. longitudinal unevenness, texture, cross fall, geographical position (country), vehicle flow, climate, weather conditions etc. All this data, in addition to rut depth, was delivered from each country providing a very good set of data for studying the effect of road condition on accident risk. As it was assumed that the relationship between accident risk and rut depth is not necessarily a linear function, rut depth was furthermore divided into a number of categories. It was also decided that separate model variables should be inferred for speed limit and AADT classes. Rut depth categories, and speed limit and AADT classes were chosen to match each country´s maintenance management strategies. The overall conclusion from the analysis is that the data does not support any general rules for a maintenance scheme. There are no results showing that deeper ruts tend to increase accident risk generally. Nor are there results that show that ruts have the same influence on the risk for different AADT classes at a given speed or vice versa. There appears to be an increased risk with ruts equal or more than about 15 mm in the highest speed class but the results differ between AADT classes and are not similar in a neighboring speed class making the results hard to understand and less usable for stating maintenance rules. For Norwegian data this trend cannot be seen for the highest speed class (equal or more than 90 km/h), but then this speed class is not comparable to the Swedish and Finnish highest speed classes (equal or more than 110 km/h roads and motorways, respectively). (Author/publisher) This report may be accessed by Internet users at http://www.vti.se/en/publications/road-user-effect-models--the-influenc…
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