Bob works in a rural area, so when Bob drives home from work each day, he takes a back-country highway. However, because Bob works in a rural area in Pennsylvania, where civil engineers are crazy, there are several intersections controlled by stoplights along this (60 MPH) highway.
One of these intersections is particularly aggravating, since it's located less than a quarter mile after a bend in the road. Being a rural area, the area off to the side of the road is heavily wooded, such that it's impossible to see the stoplight before coming around the bend.
However, on most days, Bob is able to accurately predict whether the light he's coming up on is green, or whether he'll need to slow down for it, well before reaching the bend in the road. But on some days, he has no idea whatsoever.
What's his secret, and why doesn't it always work?