Peter L said, "Over here at the weekend people drive into ditches, houses, telephone poles, bridges or just forget to take the bends in the road all the time. There's more going on than just fatal accidents. And I don't think it's the non drinkers doing it either."

But do they often do that sort of thing after having had one or two beers?

And Good Soldier Schweik is right that totally sober, non-drug-using drivers do that sort of thing sometimes too.

Several years ago in Oregon a cop had pulled off the interstate to help a driver with a disabled vehicle. They were well off of the shoulder, with one car parked between the highway and the other car, and the people standing on the far side of the two cars, when another car left the road and hit them, killing everyone but the driver of the wayward vehicle.
That driver had not been drinking at all. He was a full-time student and working full time on top of that. Sleep deprivation, pure and simple.

There are lots of things that can and do contribute to inattentive or erratic driving or slowed reflexes: emotions (e.g., anger), in-car conversations, looking at scenery, fatigue, sneezing, eating or smoking in the car, fiddling with the CD changer or radio, speeding, etc.