I'm not certain that I agree that overall risk decreases with an increase in speed. Can you point me to an analysis that demonstrates this? I suspect you are mistaken on this point.
Even the NHTSA agrees that the average driver on a highway is driving over the speed limit and that your crash risk increases if you driver slower than other vehicles.
http://casr.adelaide.edu.au/speed/vol-1.htmlThis study suggests that your crash risk is minimized when you are going faster even than the average vehicle on the highway. (See figure 2.2) And this is all *before* you correct for higher speeds meaning less time on the road.
The newer studies are even more dramatic (because vehicles got safer), but I can't seem to find them online.