I was driving in moderate-to-heavy traffic, maybe less than 20 mph, when I came upon an instance of backwards-ordered lane markings. Because of the traffic pattern, each word was revealed and then covered, for a couple of seconds each. I forget what the message was, but taking "Only Bike Lane" as the example, I really did read it as "Bike......Lane.....Only...." over about a five-second period. If it had been the other way -- which I agree makes more sense on a higher-speed, clear road -- it would have been "Only....[what? only what?]......Lane.....[only lane? lane only? huh?].....Bike [only...lane...bike... aha! Got it. Bike Lane Only.]" And I would have been confused and slightly distracted for all of that time. (In fact, this is a little bit of the phone number problem now, because it involves holding several words in your head for a few seconds.) But at high speed, although the message is indeed reversed, you see it all at once, and so it's much easier and faster to figure out the reversal.

So maybe the logic is that you trade off an easily-reinterpreted message, at higher speed, to avoid a disproportionately more confusing message in the low-speed case.

(And on the phone numbers -- have you ever noticed how many people leave their phone numbers on your answering machine as if they're trying to prove how quickly they can say the numbers?)