Yesterday Matt Yglesias wrote about his like of 'proof of payment' public transit systems like the one he used in Strasbourg:
It starts with buying tickets. You can buy one...[t]hen when you actually want to ride the tram (or bus), you go to the stop and while you're waiting for the tram to arrive you "validate" one of your tickets by inserting into a little machine that puts a time stamp on it. Then when the tram arrives you simply step on. Nine times out of ten, that'll be it. But once in a while your tram will have an inspector on it who'll ask to see people's tickets. If your ticket has a recent time stamp...then you're golden. But if you don't have a recently validated ticket, then you'll have to pay a steep fine
The great virtue of this system is that people can board the vehicle very quickly because they simply step on board. The transactional hassles of payment and validation are handled during otherwise wasted waiting time.
Matt uses a generic "nine times out of ten" chance of being checked. When I used Cologne's public transit in 2007, however, I was checked only twice out of the 120 or so rides I took. That's 98 times out of 100 where "that's it", or an order of magnitude difference.
On account of my odds, my youth, and my youth-induced poverty, my thinking about it leapt straight to the rational crime angle rather than Matt's efficiency argument. Many of his commenters also focus on whether an honor-based system would work in the US, a nation in which honor is a concept relegated mainly to internet discussions of Eddard Stark's naïveté.
The question isn't whether it'd work, though, it's whether the tradeoffs required to make it work are sensible. The marginal cost of a transit passenger is negligible, so some free-riding isn't a big deal.
Who would wish a turnstile upon these poor people: