Risk-Exposure Density and Mileage Bias in Crash Risk for Older Drivers

Driver crash rates per mile indicate high crash risk in older age. A reliance on mileage alone may underestimate risk exposure of older drivers as they tend to avoid highways and travel more on non-freeways (e.g., urban roads) that present greater hazards. We introduce risk exposure density as an index of exposure that incorporates mileage, frequency of travel, and travel duration. Population-wide driver fatalities in the U.S. during 2002?2012 were assessed per driver age (16?20, 21?29, 30?39, 40?49, 50?59, 60?69, >70 years) and gender. Mileage, frequency, and duration of travel per person were used to assess risk exposure. Mileage-based fatal crash risk increased greatly among male (RR=1.73; 95% CI:1.62,1.83) and female (RR=2.08; 95% CI:1.97,2.19) drivers from age 60?69 years to age >70 years. Adjusting for their density of risk exposure, fatal crash risk increased only slightly from age 60?69 years to age >70 years among male (RR=1.09; 95% CI:1.03,1.15) and female (RR=1.22; 95% CI:1.16,1.29) drivers. While ubiquitous in epidemiology research, mileage-based assessments can produce misleading accounts of driver risk. Risk exposure density incorporates multiple components of travel and reduces bias caused by any single indicator of risk exposure.