To the Editor:
We read with interest the article by Young,1 in which he surmises that the imbalance he found in his study between case and control windows of driving exposure might suggest a positive bias in a case–crossover study, examining the association between cell phone use and crash risk conducted by us previously.2 However, his assumption that we examined phone conversation only is not supported by our article. The phone companies provided us with the precise time when a call was placed (dialing task) or received (answering task), the length of time on the phone (conversing task), and the time at which the phone call was completed (hanging up task). We also had data on phone messaging, both sent and received (tasks of writing or accessing messages). Unfortunately, due in part to uncertainty in determining time of collision precisely, we could not tease out what aspect of phone use might lead to crashes. In that regard, naturalistic designs are a superior methodology. Young cites several papers featuring this design that found no increased risk of crash due to conversation per se.3–5 However, these same studies did find an increased risk of crash or near-crash due to cell phone tasks, such as texting, dialing, and reaching. In this respect, the findings are consistent with ours, and the odds ratios obtained by these authors were within or above our 95% confidence interval of 2.2–7.7.
The maximum case and control period we considered was 10 minutes. For those driving <10 minutes at the time of crash (63% of case–cross-over participants), only the actual minutes driven before the crash were taken into account, both for the case and control windows. Our crashes were predominantly weekday crashes during peak hours, when driving habits are more likely to be routine. Cases confirmed they were driving during the relevant control periods at the time of hospital interview, thus reducing recall bias. Taking these factors into account, the likelihood of misclassifying time driven during the control period is low. This is in contrast to Young's study, which examined a 100-day, 24-hour pattern of driving. It would have been interesting for Young to assess driver recall in his sample to test concordance of self-report with the naturalistic data over a short time frame.
Suzanne Patricia McEvoy
Mark Robert Stevenson
The George Institute for International Health
University of Sydney
Sydney, New South Wales, Australia
1. Young RA. Cell phone use and crash risk: evidence for positive bias. Epidemiology. 2012;23:116–118.
2. McEvoy SP, Stevenson MR, McCartt AT, et al.. Role of mobile phones in motor vehicle crashes resulting in hospital attendance: a case-crossover study. BMJ. 2005;331:428–430.
3. Klauer SG, Dingus TA, Neale VL, Sudweeks JD, Ramsey DJ. The impact of driver inattention on near-crash/crash risk: an analysis using the 100-car naturalistic driving study data. Report No. DOT HS 810 594. Washington, DC: National Highway Traffic Safety Administration; 2006.
4. Olson RL, Hanowski RJ, Hickman JS, Bocanegra J. Driver distraction in commercial vehicle operations. Report No. FMCSA-RRR-09-042. Washington, DC: US Department of Transportation; 2009.
5. Hickman J, Hanowski RJ, Bocanegra J. Distraction in commercial trucks and buses: assessing prevalence and risk in conjunction with crashes and near-crashes. Report No. FMCSA-RRR-10-049. Washington, DC: US Department of Transportation, Federal Motor Vehicle Carrier Safety Administration; 2010.