Traffic Light Sensors
When you drive or bike the same route daily, you quickly get a feel for the rhythm of the traffic lights. You memorize their timings, you know the long lights, you know the lights that are usually green.
Something I've come to appreciate are traffic light sensors. I am impressed at how carefully they have been programmed for the different possible configurations:
 One pair of directions (N-S or E-W) will have green lights for going straight (and red left-turn lights). This pair of directions is usually for the road that's more major. This pair of directions will have perpetual green lights until a vehicle approaches the intersection in a perpendicular direction. |
  For all of these configurations, both top and bottom will have a green light for going straight and a red left-turn light. |
 Top and bottom will both have green left-turn lights. Once they time out, both top and bottom will have green lights for going straight only. |
  For the first of these intersections, you could green both the top and bottom left-turn lights. However, I find that sometimes the first of these intersections will behave the same as the second: green both the straight and left-turn lights on top, and only show red lights on the bottom. |
  These are interesting intersections. You could green the left-turn lights on top and bottom, let them time-out, and then green both straight lights. However, what I find usually happens is: (1) Top gets green straight AND left-turn lights, bottom gets only red lights. (2) Once the top left-turn sensor stops detecting vehicles, it will time-out, and the top left-turn light will turn red. (3) Then the bottom straight light will turn green. Note that the top straight light has remained green during this whole process. |
 Another interesting intersection. Typical approach is to green both left-turn lights, let them time out, then green both straight lights. However, you could also do this: (1) On top, green both straight and left-turn lights; on bottom, all red. (2) Time out the straight light on top. (3) Green the left-turn light on bottom. Note that the top left-turn light has remained green the whole time. Edit (10/9/13): Actually, I think they always do left-turns first, then expire the left-turn lights when no more turning cars are detected, and then green the straight lights. Expiring only the straight light is confusing because the expected pattern is always left turn light first, then straight light, and the left turn light always expires first or at the same time as the straight light. |
 A typical full intersection during rush hours. Green left-turn lights first, time them out, then green straight lights. Sometimes, if the traffic from one direction is a lot heavier, they'll do this slightly differently: (1) Green left-turn lights first. (2) Expire the left-turn light on ONE side only and then green the straight light on the other side. (3) Expire the other left-turn light and then green the straight light on the other side. |
The timings on these lights sometimes varies throughout the day. Also, as a biker, sometimes I have a tough time triggering the sensors in the ground. When I can't trigger a light, I either hit the pedestrian walk button, or I just run the red light, after first checking the cross direction.
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