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Comment by fedd | Hacker News Reader

you mean it's already invented? than it should. (i just never met this function in programs i used)

Just take Dijkstra's Algorithm (or possibly A* if you want to try to optimize the runtime performance) and give a higher cost to edges in your map that correspond to left turns. Now it's just a matter of tuning the additional weight added to suit your preference. You won't get the shortest distance any more, just the "cheapest" path in terms of the distance/left-turn weight; if you find you have to go too far out of your way to avoid left turns, you may want to reduce the added weight so you can make a few more left turns without going so far away. For example, you probably don't want to go 10 miles down the freeway and another 10 back to avoid a single left turn on what would otherwise be a 5 minute drive, but maybe you'd instead be willing to drive a 1/2 mile to avoid a left turn that you'd otherwise have had routed -- so route the left turn as whatever distance it would be plus 1/2 mile additional cost.

UPS uses this in the direction software their delivery vans use. minimization of left turns means the vans get everywhere faster and more packages are delivered in a given day.

I wish the US/state governments would do the same thing. I hate how the green left-turn arrow is the first thing to light at a stoplight. This penalizes efficiency and rewards a behavior that costs us all time and energy.

Why is it like this? The only valid reason I can think of is that turn lanes would tend to back up at most lights in busy cities. I would think correcting this behavior would help reduce the number of vehicles turning, to offset the backup.

You should check out the "Michigan left" which is being increasingly implemented around the country. A Michigan left prevents backup by having people turn right and then do a U-turn further down the road at a designated area.

Thanks for the concept epikur! This is better, but it seems this design would not work on very busy roads. You would never get a chance to cross oncoming traffic due to the volume.

This does seem perfect for most Midwestern highways where space is not really a concern and traffic is relatively light.

They installed one of these a few years ago where I live. Each U-turn area gets its own traffic light in addition to the light at the grade crossing; I don't drive through there often enough to know how the patterns are synchronized for the different lights.

For one highway and one non-highway, check out SPUIs: http://www.youtube.com/watch?v=mwpoPQ1SPJU

UPS considers it highly proprietary, so I'm not surprised that you haven't come across their implementation.

I had some customers who used it (Anheuser-Busch has some weird licensing partnership that their distributors get it for free-ish). So you CAN get it, if you have the money.

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