Minimizing the delays caused by left turns — a key to improving traffic flow

A few days ago I posted about Traffic author Tom Vanderbilt’s thoughts about the ineffectiveness of warning signs on streets.

Earlier this month in Slate, Vanderbilt wrote a fascinating piece about diverging diamond interchanges: Don’t Turn Left! A new kind of intersection eliminates dangerous, time-wasting left turns.

Georgia is about to get its first DDI at the intersection of I-285 and Ashford-Dunwoody in the Atlanta metro area. There are still only a handful in the entire country, but there seems to be a lot of optimism out there that DDIs will be relatively inexpensive ways to reduce congestion at some large intersections. Check out the demonstration video here:

Crazy, right? Essentially, you’re looking at the elimination of dedicated left turn lanes that require their own lights when a highway meets a major arterial street at another grade. To achieve that, the main lanes of traffic have to crossover each other and then cross back. Cars from two directions can then go straight or turn left simultaneously, thus eliminating the time spent waiting for dedicated left turn lights.

Here’s another DDI simulation:

And here’s a similar configuration — the continuous flow intersection — for two major arterial roads that meet each other at grade. From the Utah Dept. of Transportation:

Again, the idea is to minimize the amount of time that through drivers have to waste waiting for drivers to make left turns.

As Vanderbilt suggests, the biggest drawback to these configurations probably has to do with the sheer scale of these intersections. You really have to be talking about major traffic congestion — like that of metro Atlanta — before you’re likely to find many intersections that need such a dramatic fix.

The problem of left turns is significant in many places, however.

Here in Savannah, dedicated left turn signals were added not long ago for east-west traffic at Stephenson and White Bluff at the entrance to Hunter. The extra time of the light cycle has significantly increased the odds of north-south drivers getting stopped by the light at that intersection.

Inbound traffic on Middleground Road has two left turn lanes at the new-ish intersection with Abercorn (at Montgomery Cross Road), but it’s pretty common for cars to have sit through an extra cycle before they can turn — that delay makes the Middleground route a much less effective alternative to the streetlight-laden Abercorn.

The exits off the Truman Parkway have always been cluttered by the problem of left turns — at multiple points there seem to be inordinate delays because of the long light cycles.

For what it’s worth, I wish Savannah was looking at more roundabouts for streets that meet at grade. Middleground Road would be a perfect spot for several roundabouts rather than the three traffic lights that were added when the road was widened.

The problematic intersection of Abercorn/204 and King George is one that might eventually demand some form of congestion relief. Traditional thinking would require a hugely expensive bridge, but a continuous flow intersection like the one seen above might work just fine — for a fraction of the cost.

I know people complain all the time about roundabouts, but there’s ample reason to think that Savannahians would adapt to them decently well. Savannah’s squares are essentially rotaries after all. They slow all the traffic down but keep everybody moving.

There are other ways of minimizing left turns, but the simplest is probably in New Orleans. Many significant intersections simply don’t allow left turns; drivers who want to go left go through the intersection, do a U-turn, and then turn right. It sounds cumbersome, but it’s still an effective way of reducing wasted time — and that translates into reduced congestion and fewer demands to expand road capacity.