[beat]

Maybe not 50%, but there's certainly a strong bias in that data toward friday/saturday nights. Since the data resets at midnight rather than on bar hours, look at the difference in midnight-4am data on saturday and sunday mornings, vs the rest of the week.

[throwaway0a5e]

Those are also the only nights when people are out at all. People who have to be at work on weekday mornings aren't driving home from visiting their parents late on a Tuesday night, they're doing it late on a Saturday night. Yeah, it probably is alcohol but there's a lot of confounding factors.

[throwaway201103]

Indeed. A drunk driver alone on the road isn't really a danger to anyone but himself.