Does daylight saving time increase traffic crashes? Seasonal confounding and a morning fatality effect.

Whether Daylight Saving Time (DST) transitions increase traffic crashes remains a contested policy question, with prior studies reporting 5%-7% increases in fatal crashes over the week following spring-forward but lacking a non-DST control group to isolate the clock-shift effect from seasonal confounding. We address this gap using a difference-in-differences (DiD) design with jurisdictions that do not observe DST as controls. An illustrative city-level analysis uses police-reported crash data from seven U.S. cities (2012-2025), including Tempe, Arizona - which does not observe DST - as a control; this analysis is reported in Appendix A. Our primary analysis uses the Fatality Analysis Reporting System (FARS), covering 375,658 fatal crashes across all 50 U.S. states and the District of Columbia from 2012 to 2022, with Arizona and Hawaii as non-DST controls, in a ±14-day window around each transition. In the illustrative city comparison, adding a non-DST city changes the positive uncontrolled estimate to a near-zero coefficient, although failed diagnostics preclude city-level causal inference. In the primary FARS DiD, for which pre-trend tests do not reject the null of no differential trends, the estimate for overall fatal-crash counts is also null (＋0.2%, p=0.954, ±14-day window). However, morning (6-10 AM) fatal crashes increase by ＋25.9% (p=0.007) in the 6:00-9:59 AM slot averaged over the ±14-day spring-forward window (post-transition days 0-14 vs. pre-transition days -14 to -1)-an effect that strengthens to ＋26.8% (p=0.002) once month-of-year fixed effects and day-of-week-by-DST interactions are added, remains similar when scaled by annual state-level vehicle-miles-traveled (＋22.8%, p=0.016), and is robust to bandwidth as narrow as ±3 days (＋45.8%, p=0.003) and to the exclusion of either non-DST control state. All FARS-based estimates pertain to fatal-crash counts, not crash risk per trip; annual VMT scaling cannot absorb short-run or intraday changes in traffic exposure. The post-transition pattern is not readily attributable to acute sleep deprivation alone and instead points to potentially interacting sleep, circadian, ambient-light, and traffic-exposure channels that this design cannot separately identify. The fall-back effect on total fatal-crash counts likewise vanishes with controls, a pattern consistent with seasonal confounding rather than a clock-shift effect. These findings suggest that prior uncontrolled comparisons may overstate DST's overall crash effect, while underscoring a targeted morning fatal-crash-count increase that merits policy attention.