How Many Road Categories Can We Distinguish

The importance of human factors in road design and traffic safety has been undoubtedly increasing recently. As part of the human-centered design, schemata as mental representations induce road user expectations as well as trigger behavioral patterns. In road design this concept is called “self-explaining roads, meaning that road users automatically drive according to an expected behavior and speed. This requires different categories of roads, each with homogenous characteristics and a remarkable difference between them. This paper investigates how many unique categories road users are able to distinguish. The paper first gives an overview of the design guidelines from a few countries highlighting the trends toward more self-explaining road networks. As a first step of the research a picture-sorting task was completed to find out how road users group 45 different road scenes, and how these groups correspond to the road categories according to current design practice. In addition, a questionnaire survey was used to determine how individual road scene images are grouped on the basis of speed choice, and how this grouping corresponds to road categories. In both cases, cluster analysis was used to form and analyze the categories. The main finding of both surveys is that road users have a limit in distinguishing road categories. This limit is not a strictly defined one, but it lies somewhere around 5 to 6. These results comply with the new guidelines in Germany and the Netherlands proving that the current initiatives to revise the Hungarian guidelines and the need for implementing the idea are justifiable.