Older drivers typically drive a shorter distance per trip and hence have lower accumulated driving distances per year. In general, drivers travelling more kilometres have reduced crash rates per kilometre compared to those driving fewer kilometres. Therefore, the low mileage of older drivers may exaggerate older driver risk per kilometre estimates. Several studies, using data from different countries, have tested this hypothesis. They all found that when driver groups were matched for yearly mileage, age-related increases in crash rates per km disappeared. That is, older drivers with an average or high annual mileage have crash rates that are comparable to those of younger adult drivers with the same annual mileage. Only drivers with a low annual mileage have more crashes per million driver kilometres, but this goes for younger drivers as well as for older drivers.
Crash rates can also be biased by the type of roads typically travelled by older drivers. Many avoid driving on motorways (with interchanges), the safest types of roads, and tend to drive on streets with intersections, which are, by their very nature, less safe and have more crashes. Hence, older drivers' risk estimates based on injuries or fatalities per mile driven will be overestimated when compared to those of younger drivers with higher yearly mileage on safer roads..