Optimal Social Speed Limits in a Highway: Suitability of the 120 Km/h Speed Limit in Barcelona

Optimal Social Speed Limits in a Highway: Suitability of the 120 Km/h Speed Limit in Barcelona


F Robusté, M Velez, A López-Pita, Universitat Politecnica de Catalunya, ES



This article is intended as a small contribution in the study of the thorny problem of road safety, scientifically questioning the existing maximum speed limit on highways where the road layout, vehicle power, physical environment and driver ability permit high driving speeds.

Speed limits in Spain are the most commonly ignored driving rules. When a rule is not respected by almost anyone, its enforcement is difficult since it would mean giving tickets to everyone. In a mature society, that fact points out that perhaps the people do not perceive the rule as ?valid?.

The question which arises is: if 120 km/h is not perceived by people as the ?right? speed limit, what should it be? This paper presents a methodology used to determine the ?optimal? speed in a highway that has a physical layout to permit high driving speeds. ?Optimal? speed is defined as the speed presenting the least social costs.

To determine the optimal speed, it has first been necessary to perform a socio-economic analysis of factors which vary according to speed. The factors studied are: time savings, comfort ? driver preference, accident rate and environmental impact. For each of the factors, ad hoc models are calibrated.

Of course, the main deterrent to higher speeds (besides the highway physical layout) is the social cost (and alarm) of accidents: high speeds increase the variability range of car speeds, hence the standard deviation of the speed and thus they increase the probability of accidents, growing both in number and fatal outcome. Through use of the standard engineering practice of assigning monetary values to loss of life, injuries and crashes, we are able to insert traffic safety as an important factor, but it remains just one more factor of the global decision. A specific traffic safety study has been performed for the selected stretch of the highway.

The numerical application has been performed on the A-7 highway in Barcelona, Spain. A segment of this highway one hour?s drive north of Barcelona was chosen for its straight layout and little agitation traffic. The model yields a result of 124 km/h as the optimal social speed (currently, the average speed in that section of the highway is 134 km/h). The conclusion is that in the analyzed stretch of the A-7 highway, current speed limits could be increased to 140 km/h if these new speed limits were really enforced.

The paper also comments on the generalized speed limit violation in Spain.


Association for European Transport