"Diverging Diamonds": The Solution To Onramp Congestion?

Edward Niedermeyer
by Edward Niedermeyer

Ever been stuck behind acres of traffic waiting for a left turn signal to enter a freeway? If so, you know it’s one of the more annoying traffic scenarios out there. But a crazy scheme called “diverging diamonds” might just be the fix. NPR has a widget that makes it a lot easier to understand, as well as this memorable response:

Some folks say, ‘It’s crazy. Why did they put it in? Wrecks gonna happen. Why did they do that to us?’

[Hat Tip: Richard Chen]

Edward Niedermeyer
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  • Jpcavanaugh Jpcavanaugh on Nov 25, 2009

    The only issue that I see is that you will have traffic slowing and exiting from the left lane that is, in theory, the high speed lane. I know that there are a few left-lane exits out there in interstatedom, but in my area they are rare. Also, it looks like the folks exiting have a very sharp turn to make, and an immediate merging as those leaving the highway from opposite directions begin to share the same exit ramp. It would be interesting to see some traffic studies which would tell us if these are really problems.

    • Highway27 Highway27 on Nov 25, 2009

      The left lane isn't really considered a 'high speed lane' in an area between two close-together signals. The sharp turn you reference isn't really that sharp, it just looks that way on this diagram, and the immediate merge could be a join (two lanes) that merge farther down the ramp.

  • RetardedSparks RetardedSparks on Nov 25, 2009

    Not entirely convinced - you are still sitting at 2 lights to cross the bridge. Two things that would help - make the intersections as close to 90 degrees as possible (like Highway27 said) and make the median on the bridge opaque, to lessen the perception that you are on the "wrong" side of the road - more like a narrow one-way bridge.

  • Sigsworth Sigsworth on Nov 25, 2009

    With all due respect to the traffic engineers out there, most of the changes I've seen made on my local streets don't seem to fix anything. The above proposal, as pointed out, makes the through traffic stop twice, vice the turning traffic, and replaces T-bone collisions with head-on collisions. Yes, it saves real-estate, but only if you are building a brand new interchange. To take an existing interchange and replace it with one of these would certainly cost a lot of money. Would it be worth the expense? Who makes that kind of decision, and what factors into the decision? Recently they widened my local freeway on-ramp (westbound to southbound) from 1 to 2 lanes. Did it fix the morning traffic problem? No, because the bottleneck is on the single lane road that is residential on my side of the freeway, but commercial once it crosses the freeway and widens into three lanes. The back up happens because of all the people getting off the freeway from both directions and heading west. If I can figure that out, why can't the engineers? What am I missing? On the other hand, a few years ago at the same intersection they widened the freeway offramp (northbound to eastbound) from 1 to 2 right turn lanes. This seems to have helped a lot in reducing the back up onto the freeway itself, but of course doesn't do anything to help the traffic on the road as I try to get home...

    • Highway27 Highway27 on Nov 25, 2009

      Sigs, as I said above, don't believe the diagram so much. The benefit of this kind of alignment is that there wouldn't be head on accidents, there would be the same likelihood of t-bone accident, but because you've simplified the intersection - by having 2 simple movements with no turns - you've really helped with people able to pay attention to the signal itself. In fact, going through the signal, you only have opposing traffic from a single direction. The NPR story shows more of it, and it does seem that they've used the existing bridge, which is pretty wide on Google maps. Basically, they turned the left turn lanes on the bridge into the wide median, and use that space to effect the needed alignment changes. As for 'stopping twice', that happened already: There was a signal at either end of the bridge for the ramps on and off I-44. So they've taken 2 5-phase signals and made them 2 2-phase signals. As for your roadway, unfortunately not every problem is fixable given the funding available to roadway projects. Sometimes the funding required would be astronomical to reconstruct a roadway / buy right-of-way / provide other capacity. Maybe what they could do was prevent backups onto the freeway, so that's what they did. There are a lot of times where you know something else is an issue, but it's outside of the scope of what you can do.

  • Kkt Kkt on Nov 25, 2009

    This could be a significant improvement for many interchanges. The cloverleaf sets up a conflicting movement where the exiting traffic must merge with the entering traffic in a short distance. It's also relatively expensive, using quite a bit of real estate as well as a big structure. Yes, there are two traffic lights in the crisscrossed diamond, but they don't have a phase for left turns, and the two lights could be coordinated so that you'd never (well, hardly ever) have to wait for more than one of them.