Optimal social speed limits in a highway: suitability of the 120 km/h speed limit in Barcelona

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 correct 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. Through use of the standard engineering practice of assigning monetary values to loss of life, injuries and crashes, it is possible to insert traffic safety as an important factor, but it remains just one factor in the global decision. A specific traffic safety study has been performed for 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 gave 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). It is considered 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. For the covering abstract see ITRD E126595.