TABLE 2—

Snowy Day Crash Rates, Adjusted for Potential Confounders

	Fatal Crashes, IRR (95% CI)	Nonfatal-Injury Crashes, IRR (95% CI)	Property-Damage-Only Crashes, IRR (95% CI)
Nonfirst snow days relative to dry days	0.93 (0.90, 0.97)	1.23 (1.18, 1.29)	1.45 (1.38, 1.52)
First snow days relative to nonfirst snow days	1.14 (1.08, 1.21)	1.04 (0.98, 1.11)	1.01 (0.95, 1.08)

Note. IRR = incidence rate ratio; CI = confidence interval. Potential confounding factors that were adjusted for (in negative binomial regressions) include the following: (1) time since last precipitation; (2) rainfall on “snow days”; (3) state, year, and month fixed effects.