# Traffic Light Pattern Efficiency

**URL:** <https://boards.straightdope.com/t/traffic-light-pattern-efficiency/346661>\
**Category:** Factual Questions\
**Created:** [March 1, 2006, 8:22pm UTC](https://boards.straightdope.com/t/traffic-light-pattern-efficiency/346661 "2006-03-01T20:22:45Z")\
**Posts on this page:** 2\
**Page:** 2

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**Author:** ![RaftPeople](https://avatars.discourse-cdn.com/v4/letter/r/6f9a4e/32.png) [@RaftPeople](https://boards.straightdope.com/u/RaftPeople)\
**Post date:** [March 2, 2006, 8:53pm UTC](https://boards.straightdope.com/t/traffic-light-pattern-efficiency/346661/21 "2006-03-02T20:53:29Z")

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Pattern B is and has been used for as long as I can remember in the suburbs of seattle on any road with a left turn lane.

And yes, I agree, it’s more efficient than an all stop between each change, can’t see why anyone would program that.

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**Author:** ![Kilvert\_s\_Pagan](https://avatars.discourse-cdn.com/v4/letter/k/c37758/32.png) [@Kilvert\_s\_Pagan](https://boards.straightdope.com/u/Kilvert_s_Pagan)\
**Post date:** [March 3, 2006, 3:04am UTC](https://boards.straightdope.com/t/traffic-light-pattern-efficiency/346661/22 "2006-03-03T03:04:21Z")

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> [@Balthisar](#):
>
> … on the streets that are big enough to contain boulevards but for which traffic circles have not yet been invented, there is the [“Michigan left turn.”](http://www.michiganhighways.org/indepth/michigan_left.html). Any time I’m at a busy place outside of the area, it irritates me that they didn’t do something so simple.

We had an [In Praise of Michigan Left Turns](http://boards.straightdope.com/sdmb/showthread.php?t=278391&highlight=praise+michigan+left+turn) thread a few years ago. One of the many dissenting posters attempted to claim that having all four directions of traffic stopped while left turns were being made was somehow more efficient than having All Through Traffic, All The Time (at least E-W or N-S).

We have a few Case B’s here, but not many.

I always thought there should be such a thing as “contingent synchronized” intersections with sensors. If a car is detected by one of those metal-detector thingies in the road, the signal would cycle _ **during its next scheduled time slot** _ as determined by the light timing scheme. Otherwise, when it’s time for that intersection to cycle, if there’s no car triggering the sensor the light doesn’t change. That way all the timings still work OK. Do some intersections work like this?

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