FHWA's vehicle-miles-traveled (VMT) tables, broken out by state and road type. This is exposure data — the denominator that turns raw counts into meaningful rates. A death toll means little without the miles driven behind it; VMT lets analysts express fatalities per 100 million miles traveled rather than as raw totals.
A tip: change the year in the URL to pull other years.
Best for: normalizing crash and fatality figures into rates, and any comparison that has to account for how much driving actually happens.