Should left turns be banned?

Tom Vanderbilt, the traffic guru, has authored a piece in Slate today that advocates getting rid of the left turn.

Forty percent of accidents happen at intersections, he said, most of them involving crossing into the path of traffic.

What’s a better solution. I give you — er, he gives you — the diverging diamond interchange:


As the eastbound driver approaches the highway interchange (whose lanes run north-south), traffic lanes “criss cross” at a traffic signal. The driver will now find himself on the “left” side of the road, where he can either make an unimpeded left turn onto the highway ramp, or cross over again to the right once he has gone under the highway overpass.

What makes the DDI work is that it reduces the number of “conflict points” where traffic streams cross each other. There would usually be 26 such points in an intersection like this, but the DDI has only 14 (because, for example, drivers turning onto ramps no longer have to turn across oncoming traffic). But… not having those left-turn movements adds another advantage. In a standard “diamond” interchange, where traffic entering the highway has to turn across traffic, the two sets of traffic signals, because they have to account for the left-turn phase, are difficult to synchronize–which means cars wait in longer queues. But with the DDI, Chlewicki told me, “each signal in the interchange is only two phases, not three. And each of these two phases have some unique characteristics. The left turn from either ramp gets the same green phase as the arterial thru movement that does not conflict with that turn.

Guess what state is — literally — heading in this direction? Minnesota. Specifically Pine Island. The Highway 52 interchange there is a particular nightmare.

Vanderbilt points out, however, that the underlying problem in any traffic design is congestion. As long as we keep building houses with three-car garages, he says, traffic problems are likely to get worse (and more dangerous) no matter what the design.

Vanderbilt will discuss his idea during a segment of today’s Talk of the Nation from NPR.