During the latest years VTI has carried out research within a project called "The impact of road surface on traffic" on commission by the Swedish National Road Administration (SNRA). The objective of the project is to improve and supplement the road user effect models in the SNRA pavement management system (PMS). Studies have been/are being carried out on the impact of road surface on vehicle speed, ride comfort, fuel consumption, noise properties and safety. The main objective with this study has been to provide new models describing the relation between traffic safety and the road surface condition. In the accident cost model used today the only road surface parameter included is rut depth and only ruts deeper than 10 mm are considered to have any influence on traffic safety. The relation is then linear and the accident cost increases with increasing rut depth. The investigation that has been carried out is based on data from the road surface measurements done by SNRA on the state roads during the period 1992 - 1998 and accidents reported by the police during the same period. Also data on precipitation has been used. Regression analyses have mainly been used. Dependent Y-variable has been accident ratio (accidents/ 100 million axles pair kilometres). Independent X-variables have been rut depth (mm) and/or unevenness given in the measure of unevenness called International Roughness Index (IRI in mm/m). The material has been divided into several different classes such as traffic flow and precipitation classes.