Talking on a Wireless Cellular Device While Driving: Improving the Validity of Crash Odds Ratio Estimates in the SHRP 2 Naturalistic Driving Study

Dingus and colleagues (Proc. Nat. Acad. Sci. U.S.A. 2016, 113, 2636–2641) reported a crash odds ratio (OR) estimate of 2.2 with a 95% confidence interval (CI) from 1.6 to 3.1 for hand-held cell phone conversation (hereafter, “Talk”) in the SHRP 2 naturalistic driving database. This estimate is substantially higher than the effect sizes near one in prior real-world and naturalistic driving studies of conversation on wireless cellular devices (whether hand-held, hands-free portable, or hands-free integrated). Two upward biases were discovered in the Dingus study. First, it selected many Talk-exposed drivers who simultaneously performed additional secondary tasks besides Talk but selected Talk-unexposed drivers with no secondary tasks. This “selection bias” was removed by: (1) filtering out records with additional tasks from the Talk-exposed group; or (2) adding records with other tasks to the Talk-unexposed group. Second, it included records with driver behavior errors, a confounding bias that was also removed by filtering out such records. After removing both biases, the Talk OR point estimates declined to below 1, now consistent with prior studies. Pooling the adjusted SHRP 2 Talk OR estimates with prior study effect size estimates to improve precision, the population effect size for wireless cellular conversation while driving is estimated as 0.72 (CI 0.60–0.88).