An Unbiased Estimate of the Relative Crash Risk of Cell Phone Conversation while Driving an Automobile 2014-01-0446

A key aim of research into cell phone tasks is to obtain an unbiased estimate of their relative risk (RR) for crashes. This paper re-examines five RR estimates of cell phone conversation in automobiles. The Toronto and Australian studies estimated an RR near 4, but used subjective estimates of driving and crash times. The OnStar, 100-Car, and a recent naturalistic study used objective measures of driving and crash times and estimated an RR near 1, not 4 - a major discrepancy. Analysis of data from GPS trip studies shows that people were in the car only 20% of the time on any given prior day at the same clock time they were in the car on a later day. Hence, the Toronto estimate of driving time during control windows must be reduced from 10 to 2 min. Given a cell phone call rate about 7 times higher when in-car than out-of-car, and correcting for misclassification of some post-crash calls as pre-crash, the final required downward adjustment of the Toronto and Australian RR estimates is about 7 times. The Toronto adjusted RR is 0.61 and the Australian adjusted RR is 0.64, which now agree with the OnStar RR estimate of 0.62. All five adjusted RR estimates for cellular conversation while driving in automobiles are near 1, with a pooled RR of 0.61 (95% confidence interval 0.51 to 0.74). Talking on a cell phone while driving an automobile does not increase crash risk relative to not talking.