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?


closed as too broad by GentlePurpleRain, Roland, Engineer Toast, AJL, xnor Sep 18 '15 at 20:52

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This is assuming that upon reaching the bend, a car would be required to brake immediately to stop for a red light.
If while approaching the bend he sees a car at the bend and it is braking, he can assume that the light is red, if they do not brake it is green.

Why it doesn't always work:

Some people always brake going around bends.
There happen not to be any cars ahead of him that day.

  • $\begingroup$ I was sort of hoping my first riddle would last longer than that before being solved. Good job! $\endgroup$ – Mason Wheeler Sep 18 '15 at 14:50
  • $\begingroup$ It was a good first riddle! Hope you stick around and post some more! $\endgroup$ – alexmc Sep 18 '15 at 14:57
  • $\begingroup$ I gave the same answer first, and have a higher upvote count $\endgroup$ – dfperry Sep 18 '15 at 17:24
  • 1
    $\begingroup$ Your first answer (the intended solution) doesn't include a response as to why it wouldn't always work. $\endgroup$ – alexmc Sep 18 '15 at 17:30
  • $\begingroup$ I said "if someone is driving in front of him". if there was nobody else, it wouldn't work $\endgroup$ – dfperry Sep 18 '15 at 17:32

If someone is driving ahead of him, he'd see them slowing down as they go through the bend and are able to see the light at the intersection.


if there is traffic coming the other direction, their speed determines if the light is green. If the other traffic is moving fast, they came through a green light. If they're moving slower, they are traffic coming from the intersecting road and haven't had time to come up to speed. that would mean his light is red. It wouldn't always work, because slower traffic could be oncoming traffic coming out of a newly-green light, and Bob will have slowed down for a green light.


Those new LED stoplights can get really bright, so I imagine that on (semi-)foggy days, the light can be seen reflected off of the fog around the corner.

  • $\begingroup$ Good thought, but no, he's driving home in the afternoon when fog is almost unheard of. $\endgroup$ – Mason Wheeler Sep 18 '15 at 14:33

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