Speed and Crash Likelihood

There are three classes of scientifically robust evidence that collectively establish that higher travel speeds do result in a greater number of casualty crashes  

The case-control method. In this method scientists match their cases with controls and examine the differences. The University of Adelaide has conducted two comprehensive studies in South Australia, one in urban Adelaide and a later one on rural roads. Only the latter study is described in detail here, as the conclusions to be drawn from the results of both are the same. 

As part of its ongoing research into crash causes and consequences, the university maintains an in-depth crash investigation team that attends a crash scene as soon as possible after receiving notification of a serious casualty crash, typically from an ambulance call. The team collects a vast array of data and, amongst other things, uses computer modelling to estimate the precrash speed of the vehicle. 

For each case crash, the team then obtained 10 controls (830 in total). They went to each case crash location at the same time of day on the same day of week and under the same weather conditions as the case crash and measured the free speed of 10 vehicles travelling in the same direction as the crash vehicle. They stopped these drivers farther down the road and sought breath samples to eliminate any alcohol influence. 

The average (measured) free speeds of the controls were compared with the (computer-estimated) precrash speeds of the cases to compute 

---