A comparison of accidents during daylight and darkness was made for both rural and urban roads. Accident rates on all types of rural roads were higher during darkness than during daylight. Critical accident rates during darkness for various types of roads were calculated. In urban areas, a larger percentage of accidents occurred on wet pavements during darkness than during daylight, but there were no significant differences for rural roads. No significant difference was found between average speeds during conditions of daylight and darkness. On rural roads, imposition of the 24.6-m/s (55-mph) speed limit resulted in a reduction in accident rates for both daylight and darkness. However, there were changes in the percentages of wet-pavement accidents on rural roads. For the entire rural system, there was a decrease in the percentages of wet-pavement accidents during daylight and an increase during darkness.

Report Date


Report Number

No. 545

Digital Object Identifier



Offered for publication by the Transportation Research Board