Yeah, I'd bet that

If we assume that 1 of every 140 of those miles are walked drunk--the same proportion of miles that are driven drunk

underestimates the fraction of miles walked drunk by several times; if it's underestimated by a factor of 8, then driving drunk is as dangerous as walking drunk. (But I might be overestimating it, due significant differences between my country and the US, e.g. here in Italy, whereas you don't get arrested if you're caught driving moderately drunk for the first time,¹ I'm pretty sure the probability of getting caught is much larger than 1 in 27,000 miles, given that I know at least four people to whom that happened. Therefore I guess people are much more reluctant to drive when drunk where I am than over there. Also, Italian towns are less car-friendly than I guess US towns are.)

1. Though they temporarily revoke your driver's licence, and they make it a pain in the ass to get it back (lots of psychological tests, drug tests once a year for the first three years then once every five years -- paid by you, etc.).