The ratio of crashes in darkness to those occurring in daylight has been used to assess the relative sensitivity of certain risk factors to ambient light level. When applied in a way that maintains control over exposure level, use of dark/light ratios can be helpful in identifying crash factors that are particularly sensitive to darkness. For example, during daylight saving time changeovers, dark/light ratios have been used to demonstrate the high vulnerability of pedestrians in darkness. In this report, we examine the application of the night/day ratio to evaluate changes in crash risk in darkness associated with vehicle characteristics. We find that correlations between driver age, vehicle cost, and patterns of driving suggest that links between vehicle equipment and crash risk in darkness cannot be asserted without also taking these factors into account. Younger drivers drive proportionally more miles at night, and show proportionally higher risk of fatal crash involvement at night than older drivers. Young drivers also drive proportionately less expensive vehicles than middle-aged and older drivers, and they drive an increasing proportion of originally expensive vehicles as the age of the vehicle increases. These changes in driver age demographics must be considered when evaluating nighttime crash countermeasures. (Author/publisher)
Samenvatting