A solution to increase compliance in urban areas might be to design roads as more "self-explaining". Roads are here defined to be self-explaining when they are in line with the expectations of the road user, electing safe behaviour (such as speed choice) simply by design. The aim of this study was to improve the understanding of the concept of self-explaining roads in urban areas by studying the relationship between speeds at a particular location, the speed limit and the road environment. In addition, the aim was to create a dialogue with municipalities regarding speed limits and actual speed levels in relation to the concept of self-explaining roads in urban areas. The relationship between actual speed levels, speed limit and traffic environment has been studied based on earlier speed measurements made by the NTF (National Society for Road Safety) in 23 different cities and a total of 69 measuring points. In addition, 113 traffic engineers studied photos from 20 different locations and made an assessment of the speed limit at the site, so-called blind estimation. This part was also performed by NTF. The results show that when the relationship between the speed levels and a number of site-specific background variables was studied, the actual speed levels were explained not only by road-specific variables such as speed limits, traffic flow and road type, but also variables that describe the surrounding environment such as type of area (inner city, residential areas, outside city areas) and presence of vulnerable road users. Blind-estimates of speed limits show that it is difficult to estimate a speed limit only by looking at a photo. Relying only chance, about 25 percent of the blind-estimates would have been correct, but now the hit-rate for the blind-estimates were on average 50 percent. Roads with speed limit 70 km/h were most easy to estimate (67% hit-rate) and roads with speed limit 60 km/h were most difficult (35% hit-rate). There was no difference in the hit-rate between women and men, nor in how many years they have worked in the area of traffic safety. However, there was a significant difference between those who have implemented new speed limits in a city, compared with those who had not. Traffic engineers who had implemented new speed limits had on average a higher hit-rate and made more correct estimates of the speed limits. A logistic regression showed that lane width was the most important factor when predicting the speed limit by only looking at a photo. The results showed that the probability of a correct estimate of the speed limit increases the narrower the road gets. In conclusion, the concept of self-explaining roads in urban areas is very difficult and with the existing speed limits it is not a reality today, from a national perspective. It is far from easy for the road users to assess the present speed limit by only looking at the road and traffic environment if they for some reason failed to notice the speed limit sign. (Author/publisher)
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