Setting Speed Limits on Rural Two-Lane Highways by Modeling the Relationship between Expert Judgment and Measurable Roadside Characteristics

Over recent years it has been assumed that posted speed limits should consider an integrated set of requirements such as the built-in road environment and the presence and characteristics of its users. In the present paper a multinomial logit (MNL) discrete choice model for selecting speed limits is presented as an exploratory method for relating measurable roadside characteristics and speed limits over the full length of rural two-lane highways. The model was developed using as a case study 34 km of rural roads in the region of Coimbra (Portugal). The choice of four traffic safety experts was recorded for each 200 m segment, in both directions, permitting the estimation of the MNL. Only straight and nearly straight roadway segments were considered, and speed limitations resulting from restrictive geometric properties of the segments were disregarded in this study. The explanatory variables were collected to describe the built-up characteristics of the different segments of the road and its surrounding environment. The model adjusted well to the data; and an external data set was shown to be consistent with the expert judgment. Variables that were added to translate lateral roadside constraints were those with a higher significance in explaining the choice of lower limits. Comparing the model with the actual speed limits posted in situ, it was possible to conclude that there is a clear mismatch between these limits and the surrounding environment with a significant tendency for lower posted speed limits compared with the limits at which experts believe that it would be safe to drive.