This paper will affect the speed at which people elect to travel by vehicle and road design, by limiting speed and enforcing it, by traffic control, signs and markings etc. The speed at which people travel, in turn, affects road safety. In this context two questions arise: (a) How is the evolution of speed over time and space affected by what we do and (b) how does speed affect safety. The paper will review what is known, note the gaps inknowledge, and describe where opinions differ and why. Unfortunately, in spite of decades of speed measurement and monitoring, the evolution of speed over time is poorly documented and the understanding of what drives theevolution is largely missing. It is known that speeds evolve over time but do not know exactly why; it is known that there is some spillover of thechange from one road to another but cannot predict its size or extent. More is known in answer to question b. There can be no reasonable doubt thatif speed increases while other conditions (vehicles, roads, medical services) remain unchanged, what accidents occur will tend to be more severe. However, the prevalent and strongly held belief that the larger the speed the higher is the probability of accidents to occur is, at present, not well supported by research. Even so, given a change in mean speed the paper can predict the consequences in injuries and fatalities and it shows how todo it.
Abstract