Considering all the data together, except for an increase in the second hour, crash risk is statistically similar for the first 6 hours of driving and then increases non-linearly after the 6th hour. The 11th hour has a crash risk more than 3 times the first hour. Multi-day driving schedules are also associated with statistically significant crash risk increases of comparable magnitude to driving time. Non-sleeper operation crash risk is strongly associated with multi-day driving, somewhat more so than with driving time. Sleeper operation crash risk has strong association with driving time, with particularly increased risk in hours 8 through 11.

The safety implications of hours of service policies have long been an interest of safety researchers. There is a persistent literature which has sought to assess these safety implications by analysing crash data provided by carriers (e.g. Harris et. al. (1971); Jovanis and Chang, (1989); Kaneko and Jovanis, (1992); Lin, Jovanis, and Yang, (1993 and 1994). A major study of crash risk and driver performance was completed in the U.S. in the 1990’s by conducting a field experiment with instrumented vehicles and a set of drivers operating particular multi-day schedules (Wylie et. al. (1996)).

These are two examples of Cchanges in the U.S. hours of service policy in January 2004 argue for an assessment of the safety implications of the new policy. This paper presents an analysis of data collected from carriers during their operations in 2004. Each carrier was subject to the new hours of service policies implemented in January of that year. The analysis of the data sought to identify how specific hours of service policies were associated with crash risk. As such, particular attention is paid to driving time, as that measure was extended from 10 hours to a maximum of 11 hours in the new policy. Additionally an attempt is made to quantify the effects of multi-day driving, which includes an assessment of the regularity of the driving schedule (i.e. was the driving initiating driving at approximately the same time of day each day for several days) and time of day of driving if regular.f many U.S. studies that have sought this elusive relationship.