Traffic calming, especially on major roads, is under increasing discussion in Germany. The discussion concerns environmental aspects and the increasing numbers of children fatalities. Previous research has focused on speed, accident rates etc in the area where traffic calming measures were implemented or on that area together with surrounding streets. The research presented in this paper tries to differentiate between the two types of areas. Three working hypotheses were drawn up to get a clear procedure and to deduct the necessary evaluation criteria. The hypotheses are: 1) a large proportion of non compliance with traffic regulations such as speed limits within the 30 kph-zones is related to a similar behaviour at the surrounding major roads; 2) the speed limit of 30 kph in the zones is supported usually by structural measures. This could cause drivers feel more aggressive on the 30 kph-zones roads as well as on the surrounding major roads. This in turn could lead to a more accidents and more severe accidents; and 3) the safety of cyclists and pedestrians decreases on surrounding major roads due to the changed traffic behaviour of drivers. The gain of traffic safety in the 30 kph-zones has to be compared with the loss of safety on the major roads. FolIowing before and after measurements in the town of Darmstad and analyses of 1160 accidents all of these hypotheses is falsified. Speed levels are the same. Frequency and severity of accidents has been reduced in the surrounding areas. However, negative effects within the traffic calming areas compensate this effect. It is concluded that structural changes in order to reduce speed level may give a false impression of increased safety. Junctions, especially at the exits of the 30 kph zones, must be designed carefully.