Minh’s Notes

Human-readable chicken scratch

Minh Nguyễn
December 2nd, 2011



Traffic lights in Northern California are horribly inefficient. Whenever you want to make a left turn at an intersection, you typically wait up to a few minutes for the protected green light to come on for your lane. Protected greens are great, except they only come after protected reds, where the opposing lanes have no traffic and only your car is anywhere near the intersection. This arrangement is only a big deal because the area’s roads are so overarchitected: absolutely every turn lane has a separate, protected traffic light – or three – and there are turn lanes at every intersection large enough to require a traffic light.

Things were better in Ohio, where I learned to drive. There, traffic engineers used the protective/permissive left turn pattern that combines the straight and turn lane signals into a single, five-light “doghouse” signal. They make a lot of sense, especially at T-intersections. When you want to turn left, either there’s a protected green, or there’s a normal green in which you pretend there wasn’t ever a turn signal to begin with.

Doghouse signal at a T-intersection
The various states of a doghouse signal at a T-intersection. (Animation drawn in PowerPoint:mac and composed using ImageMagick.)

Doghouse signals also have their drawbacks: depending how they’re programmed, they can lead to the dangerous yellow trap. Basically, the light turns yellow while you’re in the intersection, blocked from turning left by oncoming traffic, which continues to have a green light. That’s actually an issue with arrow-less, permissive traffic signals, too. But I don’t recall ever using any intersection in southwestern Ohio that allowed for a yellow trap: protected greens were always followed by normal green or red lights in both directions.

Doghouse signals have already been adopted in Merced. Hopefully someday they’ll make it out to the Bay Area.