What you probably really want to compare flying and driving is fatalities per trip--for the same trip. As others have noted, a longer trip in a plane is almost certainly safer per mile than a shorter trip given where accidents happen. (And the effect may or may not be further magnified by the fact that I assume the accident rate is higher on small regional jets. Though, on the other hand, they carry fewer people.) On the other hand, driving is probably much more proportional to distance.

Fatalities per hour seems like a reasonable statistic if you want to talk about fear - how likely am I to die in the next 5mn?

If my goal is "I wish to spend X hours in a form of transit", sure. If I need to get from LA to NYC, though... there's a lot more of those 5 minute periods if I'm driving.