Diverging Diamond: Lots of Press First Day

First morning of the Ashford Dunwoody Divering Diamond

It seems that things were successful today, the first full working day of the Ashford Dunwoody Diverging Diamond.

I was out for the evening rush hour, and with all the caveats that come with “it’ll take a few days to tweak the signals” things worked well. The queues on Ashford Dunwoody cleared very quickly and there was little or no confusion that I observed. There was some backup on the eastbound off ramps that affected the interstate, but I have high hopes that the aforementioned signal tweaks will alleviate that in the future. The backup was a transient event that was caused by the high volumes of traffic that use the Ashford Dunwoody off ramp. It’s a well known traffic engineering principle that after a certain volume of vehicles, traffic “quality” becomes unstable, and can be disrupted by very small events. The disruption can take a long time to clear, and can occur very quickly. For example, here is a picture of the congested off ramp, about 20 minutes before it locked up.

The new eastbound off ramp for the Ashford Dunwoody divering diamond

It was working great at 5:05 PM and then it went kerplooie for a while. Unfortunately, that happens, but some very good engineers were on hand to adjust and work with it.

As the title of the post says, there was a lot of press coverage today. Just a sample is:

Atlanta Journal Constitution: Commuters Survive Opening Day at New Interchange
CBS Atlanta
NBC Atlanta

I call your attention specifically to the AJC article. There’s a reference in there that I will quote in full, to emphasize that construction is not complete yet:

Mark Rackin said work on the project added 10 minutes to his commute by eliminating the dedicated right-turn lane onto Lake Hearn Drive where he works. Monday’s opening, he said, cost him another 10 minutes.

“When I heard all the hoopla Sunday afternoon, I knew what was coming and dreaded today’s drive,” he said. “I was right; it was the worst ever.”

The dedicated right turn lane to Lake Hearn Drive is likely to remain closed for a while, but it’s been closed for at least a month before this. What caused the congestion being referred to is that the right turns onto Ashford Dunwoody are now controlled by right turn arrows, which are red while the southbound Ashford Dunwoody traffic is moving.

Right turn arrows for the east to south turning movement off the I-285 off ramp

These arrows indicate that you are not supposed to turn on red1 which is different than the previous movement which allowed for right turns on red.

There is light at the end of that tunnel, though. As soon as the “slip ramp” from the off ramp directly to Lake Hearn Drive is opened, the congestion referred to in the quote will vanish. There is still work to do, though, shown here.

The yet-to-be-completed Lake Hearn Drive slip ramp

It’s looking great and I’m very excited to see it working. I’ll be more excited once the “splitter islands” are in place rather than being outlined by barrels. I’ll be even more excited than that once the median island is constructed and pedestrians can start using it, but that is another post.

  1. Actually, my Georgia Code knowledge is abandoning me here; I’m not sure you cannot turn on a red arrow if there is no attendant “NO RIGHT TURN ON RED” sign. I’ll look that up later. []
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6 Responses to Diverging Diamond: Lots of Press First Day

  1. Mike L says:

    I’ve asked half a dozen cops and two DMV offices whether you can make a right turn on a red right turn arrow after coming to a complete stop (in NC), and none of them could ever give me a straight answer. Even the driver’s manual doesn’t really say, which seems odd given that it shouldn’t be too hard to spell out (it is either a de facto “no turn on red” or it isn’t). Is there some reason why this signal is so ambiguous?

  2. Bill Ruhsam says:

    I went and looked it up (really, I should have known this) and the federal manual on uniform traffic control devices (MUTCD) prohibits a right turn on red arrow unless another traffic control device permits it. Something like “RIGHT ON RED ARROW AFTER STOP” which is the MUTCD R10-17a sign. The relevant MUTCD paragraph (4D.04C.2) says:

    Vehicular traffic facing a steady RED ARROW signal indication shall not enter the intersection to make the movement indicated by the arrow and, unless entering the intersection to make another movement permitted by another signal indication, shall stop at a clearly marked stop line; but if there is no stop line, before entering the crosswalk on the near side of the intersection; or if there is no crosswalk, then before entering the intersection; and shall remain stopped until a signal indication or other traffic control device permitting the movement indicated by such RED ARROW is displayed.

    The Georgia Code says basically the same thing:

    Traffic, except pedestrians, facing a steady RED ARROW signal indication may not enter the intersection to make the movement indicated by such arrow and, unless entering the intersection to make such other movement as is permitted by other indications shown at the same time, shall stop at a clearly marked stop line or, if there is no stop line, before entering the crosswalk on the near side of the intersection or, if there is no crosswalk, before entering the intersection, and shall remain standing until an indication to make the movement indicated by such arrow is shown;

    As for what it is in North Carolina, I don’t know. It should conform to the MUTCD, but I’m not going to stake anything significant on it.

    The ambiguity of the signal is probably due to it’s lack of common use in right-turn situations. In my experience around the east and in Texas, you usually circular red signal indications or “red balls” rather than arrows for these movements. Red arrows are not prevalent enough to enforce the “don’t go through it, dammit!” mindset.

  3. Mike L says:

    Interesting. So if the red right arrow is essentially equivalent to a red light with a “no turn on red” sign, is there some reason why the light/sign combination seems to be used far more often than the red arrow? In some ways, the single package approach seems both more efficient and clearer for a driver (or it would be clearer, if it was more commonplace and/or actually defined explicitly in driver’s manuals). Is it just a quirk, or is there some process behind it (as an offhand guess, is it common for the no right turn restriction to be tacked on after the original installation of the light?)?

  4. scyllacat says:

    (Here through Brian Richardson). Yeah, that’s what I would have thought. If a green (right) arrow means “you can turn right, now” then it makes sense that a red (right) arrow means “you CAN’T turn right now,” in the same way that, if you have a red left arrow, but the forward traffic has a green light, even if you’re clear to make a left turn, you’re not allowed to by the red arrow. …. oh, well, it made sense to me anyway.

  5. Bill Ruhsam says:

    scyllacat: It does make sense but it leads to confusion if it’s not enforced particularly. You’ll see people running red arrows all the time.

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