Eh. I'm more interested in questions of how these effects interact with each other. For example, I find it much easier to be engaged as a driver when I'm driving 5-20 mph faster than other cars on the road (or, if there are no cars or only fast cars, at somewhere between 80 and 100 mph, assuming clear skies and a straight highway), and so I'm less likely to get into an accident because of the increased attention, but I'm much more likely to die in an accident if it does happen.

Similarly, the increased fatalities due to driving at night are probably primarily due to fatigue (with reduced visibility likely as the secondary cause). Notice the difference in traffic accidents due to daylight savings time- picture study (notice the scale on the picture- it looks like it doubles and halves, but it's really just an increase or decrease of a few hundred accidents). If you're not fatigued, there are many less other drivers on the road- which suggests that it's safer, so long as you're extra careful when someone else shows up.

I'm also skeptical about the value of defensive driving courses (at least, the sort that you can take to dismiss speeding tickets), though in-car tutoring by skilled drivers seems effective. (If there are driving courses that offer that, I'd suspect they're worth trying.)