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.
The relationship between actual speed levels, speed limit and traffic environment has been studied based on earlier speed measurements made by NTF (National Society for Road Safety) in 23 different cities and a total of 69 measuring points. In addition, traffic engineers from 73 different municipalities studied photos from 20 different locations and made an assessment of the speed limit at the site, so-called blind estimation.
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 on 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). 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 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.