Rainfall as a Shared Environmental and Occupational Road Hazard: Wet-Weather Crash Risk for Commercial and General Traffic in Harris County, Texas, 2006–2021

Wet-weather driving is a shared environmental and occupational hazard; transportation workers’ occupational fatality rate is nearly four times the all-worker average. We quantified wet-weather crash frequency in Harris County, Texas, over 2006–2021 using a stratified matched-pair design with hourly precipitation and Texas Crash Records Information System data to compute a relative risk factor (RRF), a comparative index of crash frequency relative to matched dry hours, not an exposure-normalized rate. RRF exceeded 1.0 in every year, peaking in 2011 (1.44) and 2020 (1.38); 2017, the wettest year of the study period (2024 mm), registered 1.13. RRF was 1.26 below 6 mm/h and 1.34–1.40 above. Highest estimates: males aged 18–29 (1.32), 05:00–06:00 (1.62), January (1.35), tollways (1.61). Truck-involved crashes rose less (1.20) than non-truck crashes (1.28; ratio 0.94, 95% CI 0.90–0.97). RRF fell with injury severity (1.31 no injury; fatal 0.89, 95% CI 0.74–1.07), which is not evidence that rain reduces severity. RRF size depends on the estimator (overall 1.28; 1.56 as the ratio of totals). Intervals assume independent pairs; clustering by rainy date widens the overall interval (SE × 1.4) but narrows most category intervals. The results identify where, when and for whom crash frequency rises most, as targets for site-specific analysis.