Ashford Dunwoody Divering Diamond: Day 1

Looking north toward Hammond drive from the median

As of noon o’clock on the first real day of operation, the Ashford Dunwoody Diverging Diamond Interchange is operating well. There are some hiccups, illustrated above. These were expected and are quite normal for this sort of change in traffic control. Why? It has to do with the signals.

When the interchange signal at the westbound off/on ramps was converted to the DDI configuration, it went from a three-phase signal to a two-phase. A three phase signal has a left turn phase for the main line of travel, a through phase for the main line, and a through/right/left for the ramp. Now that we have a DDI, our two-phase signal is for through travel south (with ramp turning north) and through travel north (with ramp turning south). Just by eliminating that phase for the left turn, we’re saving time every signal cycle by removing the necessity for a yellow, then an all-red signal. However, it comes with complications.

In order to make the signal play well with the others on Ashford Dunwoody, the signal engineer has to model the traffic and come up with a timing plan for the phasing. Once that is installed, it the has to be tweaked to match the field conditions. The congestion you see pictured above, from this morning’s AM commute1 shows some of that tweaking going on. The signal engineer noticed that the Hammond Drive left turners were backing into the intersection and reduced some of the through time for the northbound movement accordingly. This takes time however; it’s not instant. The tweaking will continue for several days and weeks.

Another complication is that all of the other signals along Ashford Dunwoody are three- or four- (or eight-) phase signals. Getting a two-phase signal to coordinate well with the others is something of an art, which I won’t try to describe here.

I was watching today’s AM commute for the majority of the “thick” period and there were numerous tweaks to the timing. Tomorrow will be even better.

This picture is a great illustration of why you need to have a signal engineer in the field, watching the traffic, and adjusting things on the fly.2

The morning commuting period worked pretty well according to all the press reports and my own humble observations. I’ll be back out for the evening commute to see how that goes.

More pictures and commentary later.

  1. I deliberately picked a very congested moment; the whole morning peak hour was not like this. []
  2. That signal engineer is not me, by the way. When someone opens a signal cabinet, I put my hands in my pockets and step back. []
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3 Responses to Ashford Dunwoody Divering Diamond: Day 1

  1. Pingback: Diverging Diamond: Lots of Press First Day | The Evil Eyebrow

  2. JG says:

    Most of the signal engineers in Atlanta should be fired. It’s one of the main reasons for traffic congestion. Maybe you should keep your hands out of your pockets and step up to show them how to do their job! I live very very close to this area and take Perimeter Center East/West on a daily basis – in the morning to GA 400, and on the way home from GA 400. I don’t know why the signal engineers do not know how to time the lights so that traffic actually FLOWS on this major road that takes people to and from a highway. Same with the lights on Ashford Dunwoody. It is extremely rare that someone will make it past three lights without being stopped by the fourth (if not sooner). There is no timing built into these lights that instead rely on sensors. These should ALWAYS operate on a timing pattern, with the sensors only kicking in between 10pm and 6am. Traffic would flow much better…this is my biggest complaint about Atlanta traffic. OK…end of rant. Thanks for reading!

  3. Bill Ruhsam says:

    JG: I was debating whether or not to respond, however I think I’ll just let xkcd speak for me, and add one or two items.

    One: I don’t know how to do their jobs. If I were in a cabinet, I’d be either electrocuting myself or screwing up the signal entirely. I thought that was evident.

    Two: None of the roads you’re talking about (Ashford Dunwoody or Perimeter Center East/West) operate in an actuated mode during the times you’re referring to. I guarantee that every single green extension and signal call for lefts is being used during every cycle between 4 and 6. All phases are maxing out and there are no gap-outs to speak of. And if you don’t understand what any of that means, then you should maybe tone down your criticism of the people who have tweaked and modeled and sweated over those two corridors. Believe it or not, there are a large number of people who want Ashford Dunwoody to flow, as well as Perimeter Center East and West.

    You seem to be granting me some level of expertise here, so let me lend it back to you and say that these corridors are as finely tuned as possible. There’s just a hell of a lot of vehicles moving and only so much capacity to move them.

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