Eliminating the Dreaded Left Turn

The FHWA says that 40% of all traffic accidents happen at intersections, and many of those are caused by left turns. A new type of intersection called the "diverging diamond" takes left turns out of the picture.

Slate's Tom Vanderbuilt looks at the left-turn problem, and the solution proposed by Gilbert Chlewicki. Here's how it works:

"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."

Full Story: Don't Turn Left!



Irvin Dawid's picture

FHWA recommends agains left turns....

Just had to note that it's FHWA, not the Federal Housing Admin, making the recommendation....
Here's link to Slate article...
Irvin Dawid, Palo Alto,

Why not just build PRTs?

Sounds like a nice idea, but I'm still voting for PRTs. With the recent 1 billion dollar 1 lane 405 expansion here in LA we could have built about 62 miles of PRT network to get us a really great start. That would have reduced traffic much more than that one lane will do, not to mention we could build a PRT in half the time it takes to expand this freeway.

Note that Armitsar India is building their complete PRT system for 50 million USD.


Another idea is roundabouts: no left turns, no stopping, a more regular flow of traffic (therefore fewer emissions) and less impermeable surface!

No Left Turns??

This might work as an alternative to a conventional freeway interchange. It would not improve safety, because freeway interchanges already have no left turns, but it might save some land compared with a conventional cloverleaf intersection.

This would not work at all as an alternative to left turns at typical suburban intersections - if you care at all about accommodating pedestrians as well as cars. Look at how much longer pedestrian crossing distances would be. Notice that the visualization shows only cars, no one walking.

The way to improve suburban intersections is to add sidewalks if they are not already there, and construct new buildings that are oriented to the sidewalk. Then you get something more like an urban intersection, which works for pedestrians as wall as cars.

Charles Siegel

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