Hourly changes in accident risk for bus drivers

Traffic accident risk has in some studies been found to change with the time of day, after controlling for exposure, probably due to diurnal changes in the human body, which changes alertness. However, exposure data are not always of good quality, and culpability for accidents is not always taken into account. The change in culpable accident risk over the day for bus drivers was therefore investigated, with single accidents analysed separately, using induced exposure (non-culpable bus accidents) as well as general traffic density and number of buses on the road as controlling factors. It was found that the risk distribution was fairly similar to some previous results before controlling for exposure, but dissimilar to other, probably indicating that bus drivers have a somewhat different risk profile, but also that previous studies may not have controlled for exposure in a reliable way. When exposure was held constant, the risk distribution was different from all other studies. The three different exposure measures correlated strongly between themselves, and each would seem to be adequate for a basic control. However, although general traffic density was most strongly correlated with culpable bus accidents, the induced exposure parameter added some explained variance. Single accidents had a very different risk distribution as compared to other culpable accidents when exposure had been held constant. A number of unexpected effects were also noted, mainly that single accidents were associated most strongly with general traffic density.