Left turns at intersections are the bane of traffic. They impede traffic flow (a “protected” green arrow holds up the majority of the intersection for a minority of drivers), UPS drivers’ routes are programmed to avoid them, and they’re dangerous for drivers (who often have trouble judging the speed of oncoming drivers, or have their view obstructed in determining whether it’s safe to cross) — not to mention pedestrians, who typically have the “Walk” signal when cars have a left-turn signal (and, because of the aforementioned problems, cars may only be paying attention to pedestrians at the very last moment).

Yesterday, John J. Miller asked me about a regional specialty in the world of traffic, the so-called “Michigan left.” Having spent some time in the environs of the Motor City, I had a passing familiarity with the system, but had sort of filed it away in my brain — and didn’t bring it up in the book. But he raised a very good point: Why isn’t it used elsewhere?

The Michigan left (not to be confused with the “Pittsburgh left”), or the “median U-turn crossover,” is what the Federal Highway Administration, in engineering logo, would call an “alternative intersection treatment.” It originated a few decades ago in response to rising traffic volumes, particularly on suburban arterials. Basically, it seeks to remove that dangerous, traffic-slowing left-turn from the main intersection by having drivers who wish to turn left as they approach an intersection first make a right turn, drive a short ways, then make a u-turn via a short bay, and then head in their desired direction, back through the original intersection.

It looks a little something like this (illustration courtesy of Michiganhighways.org):

If you’re still confused, there’s a nice Flash animation by the Michigan DOT here. MDOT goes on to say that the treatments provide “20 to 50 percent greater capacity than direct left-turns” and, on roads with Michigan lefts, crashes have been reduced “30 to 60 percent overall.”

The only problems I can see with the Michigan left is 1.) You need adequate space for dividing medians (which have a further benefit in reducing head-on crashes) and 2.) Drivers tend to find them a bit confusing and counterintuitive — why should I have to turn right to turn left?

The FHWA also opines that they should only be used in places where left-turning traffic is a relative minority of traffic: “Locations with high left-turning volumes may not be good candidates because the out-of-direction travel incurred and the potential for queue spill back at the median U-turn location could outweigh the benefits associated with removing left-turns from the main intersection.”

The Michigan left is not limited to Michigan. North Carolina has been giving them a spin, while New Jersey-ians of course have the related “Jersey Jughandle.” (I’ll save its intricacies for another post). Then there’s CFIs (ditto). The strangest approach to the left-turn, over-saturated intersection problem I’ve seen recently was on the approach to Sanibel Island, Fla., from Fort Myers, at the intersection of Summerlin and San Carlos: A huge, ghastly, looming “flyover,” which seemed horribly out of place amidst the flat sprawl (a few businesses have been literally lost in its shadows).

But, from what I can tell, the “Michigan left” essentially remains a regional specialty, the traffic equivalent of the state’s sour cherries. What do you say, America, are you ready for the Michigan left at an intersection near you?

ADDENDUM: The reader in the comments below rightfully asked about roundabouts, which are of course superior in both safety and flow to conventional signalized intersections — up to a certain traffic flow (after which they lose effectiveness) and when space permits. I should have qualified the whole discussion by positing Michigan lefts as a superior alternative to “conventional signalized intersections.”

This entry was posted on Wednesday, July 9th, 2008 at 2:20 pm and is filed under Drivers, Traffic Culture, Traffic Engineering, Traffic Wonkery. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.