A Case Control Study of Speed and Crash Risk, Technical Report 3: Speed as a Risk Factor in Run-off Road Crashes

In the U.S.A., the imposition and subsequent repeal of the 55 mph speed limit has led to an increasingly energetic debate concerning the relationship between speed and the risk of being in a (fatal) crash. In addition, research done in the 1960s and 1970s suggested that crash risk is a U-shaped function of speed, with risk increasing as one travels both faster and slower than what is average on a road. Debate continues as to the causes of this relationship, and there is reason to suspect that it may be an artifact of measurement error and/or mixing of different crash types. This report first describes two case-control analyses of run-off road crashes, one using data collected in Adelaide, Australia and the other using data from Minnesota. In both analyses the speeds of the case vehicles were estimated using accident reconstruction techniques while the speeds of the controls were measured for vehicles traveling the crash site under similar conditions. Bayesian relative risk regression was used to relate speed to crash risk, and uncertainty in the case speeds was accounted for by treating these as additional unknowns with informative priors. Neither data set supported the existence of a U-shaped relationship, although crash risk clearly tended to increase as speed increased. The resulting logit model was then used to estimate the probability that a given speed could be considered a casual factor for each of the 10 Minnesota crashes.