Today was the dedication ceremony for the Ashford Dunwoody diverging diamond interchange. I’ve had a few posts about this before, surrounding the opening of the crossovers, and I’m happy to say its doing well.
Lieutenant Governor Casey Cagle gave a speech along with various other dignitaries.
I’m proud to have been involved with this project and I’m looking forward to seeing how it operates in years to come.
Yesterday I had two people post links on a Facebook discussion regarding the Ashford Dunwoody diverging diamond. Apparently this drove a lot of traffic. In fact, it upped my site traffic by 1.5 orders of magnitude.
So, welcome everyone! Here are the aggregated posts on the diverging diamond, or you can just peruse the blog.
Update and Correction:
Whoops. From an analysis of the traffic I received, I see that only about 0.5 orders of that magnitude came from facebook (still, Welcome!), the rest came from the hacking that had occurred yesterday.
Update no. 2!:
Welcome to everyone who’s coming in from Applachaintrail.com. You’re presence is welcome, too!
One of the things that is striking about the Ashford Dunwoody diverging diamond is how you’ll be walking across the road, and from one end of the bridge to the other.
We’re putting you in the middle.
“What?” you ask, confident that you must have misheard.
No. It’s true. As a pedestrian you will be walking down the middle of the bridge, between the lines of traffic. And you’ll feel safer than you ever did on Ashford Dunwoody Road, or on any other bridge in the Atlanta metro area.
This is an image of the signing and marking plan sheet from the construction plans. You’ll see, from the highlights I’ve added, the path that pedestrians are intended to follow. In fact, the only path that pedestrians can follow, because the rest of the bridge deck is occupied by fast moving vehicles and a fence. This may seem a bit unsafe, walking in the middle of the road, but we’re also building two concrete barriers to protect you.
This is a picture of the barrier in an early stage of construction. At the end of the process, it will look something like this.
This image taken in Springfield, Missouri during our field visit to the first DDI in America.
The design team thought long and hard about how to deal with the pedestrians on this project. It was not a foregone conclusion that we would do it like Missouri did. We obviously did go with pedestrians in the median, and I will tell you why. There are several reasons why this was a good idea for Ashford Dunwoody
The bridge over I-285 was not going to be replaced or widened. This was a default assumption of the project when we started design. Because of that assumption, and because of the need to achieve as high an angle of intersection as possible where the cars cross over, it was necessary to have the cars “swing wide” at the ends of the bridge. This both gave us additional room in the middle of the bridge and removed the space on the sides where there would normally be a sidewalk.
I’m proud to say we designed and constructed the Ashford Dunwoody diverging diamond without buying a single square foot of land from anyone. All work is being done within the existing right of way that the Georgia Department of Transportation or DeKalb County already owned. We could have put pedestrians on the outside on sidewalks, but to get around the “swing wide” I mentioned in the last point, we would have had to push the crossover points farther from each other, and farther from the ends of the bridge. This would have necessitated buying right of way, driving up cost and the length of time required to get the project moving.
Most sidewalks on bridges are just that: a side walk. They’re a slightly raised concrete surface, on the side of the bridge, exposed to traffic. It’s certainly unlikely that a car or truck would run up on the sidewalk, but it’s possible. It’s also possible to trip and fall onto the road, or drop your phone and have it clatter out into traffic, or whatever. With the median “side” walk, we’re building a 3.5 foot high concrete barrier on each side that will deflect a car, give you a good place to rest if you need, and prevent those unfortunate cell phone clattering-crunching moments I just alluded to.
One of the challenges that signal timing engineers wrestle with is providing time for a pedestrian to walk the crosswalk. Signals are normally optimized to handle the vehicular flow along a roadway and doubly so when you have a corridor as highly coordinated as Ashford Dunwoody Road. Throw a pedestrian signal cycle in, and suddenly all sorts of things go out of whack! ((In no way should you take this statement to mean that I do not support pedestrians within and around the roadway. I’m merely telling you how it is with signals optimized for vehicles.)) The engineer has to give time for a pedestrian phase which might double, or more, the green time of the adjacent signal head. That screws up the timing plan and you end up having to steal the time from somewhere else, or you screw up the cycle length and that’s a bad idea if you’re trying to coordinate with signals upstream and downstream.
With this particular diverging diamond, we provided protected pedestrian phases across all of the ramps, because they’re two-lane crossings. If we’d kept the pedestrians to the outside, we’d have had to provide protected phases at all the crossings shown here with red circles.
Now, we already are providing protected pedestrian crossings on the four “outside” ramp crossings (i.e. the ones that are farthest away from the center of the interchange). What’s the big deal about the ones on the inside?
The big deal is that half of these crossings, the ones where vehicles are turning off of the bridge and on to the on-ramps, are designed to be free-flowing. To have a protected pedestrian crossing, we’d have to erect a signal head, and stop the flow of traffic just as it’s about to leave the interchange and head off onto the freeway. This would have serious consequences for traffic congestion if it happened during the peak periods.
So, instead, we put pedestrians in the middle.
The image here shows the four pedestrian crossings that are necessary to access the center median from each of the four corners. The reason why this works out better from a signal timing perspective is that we can give a full pedestrian movement for each crossing during every green vehicular phase! The vehicle green time for the parallel movement exceeds what you need for the pedestrian to walk across, so there’s no need even to put up a pedestrian pushbutton. You just wait for the vehicles to get a red light, then you cross with your WALK symbol. If you’re trying to cross the road, instead of walking along the bridge, you’ll have to wait in the center median, but again, no button pushing necessarily required.
The last main reason for picking the center median had to do with the expectations of motorists and pedestrians. I already discussed how having a signal stopping vehicles on the bridge for a protected pedestrian crossing was a bad idea from a congestion standpoint, but it’s also a huge problem from a safety standpoint. Drivers have no training to look for a signal in that position, and might not even see it. They might drive right through and possibly strike a person walking in the crosswalk. Similarly, pedestrians are not used to looking back over their shoulder for traffic that might be a danger to them. We don’t want to present a situation where people must do something that is abnormal to them in order to remain safe. We much prefer the safe option to be the obvious one.
So that is how pedestrians are going to be treated by the Ashford Dunwoody diverging diamond. Later, I’ll talk about the barrier and the decorative features going into it.
So, to alleviate that fact, I’m going to give you a grab bag!
I read John Scalzi’s Red Shirts. I was anticipating this with the high expectations you might expect from a crazy-stalker-fan, mostly due to the five chapters that had been pre-released. Unfortunately, I have to rate this one as “less awesome than expected.” While it’s still a very good book, it did not shoot unicorns from its rainbows, and that’s where my expectations were. However, it’s the most meta science fiction book I’ve ever read and it makes such glorious fun of Star Trek that I’d call it a must read if you ever liked Kirk, Spock, and McCoy.
I have a super double top secret surprise for my lovely wife, Dr. Jennifer Bowie on next Tuesday, which I will not tell you about because she occasionally reads my blog. Suffice to say that it’ll be a blast. I’m sure there will be pictures.
Jenn and I are attending the Euphoria burn this weekend. I will not be connected to any sort of internet, so you won’t have to listen to me tweet about it. We’ve never attended a burn before, so it’ll be a new experience for us.
I was highly entertained when I went looking around for an offline blogging tool, only to discover that I already had one on my computer. Windows Live Writer is doing a good job for what I need right now. My only complaint is that it’s clunky for attaching HTML tags like <cite> and <blockquote>, etc. I think there are ways around that, but I’ve only been using it for two days now.
I’m growing back the beard. Or at least the goatee. It’s been a year and a half and I’m interested to see how much grayer it will be.
The cats are still kittens, although they’re kittens who weight 13 pounds and thunder around like rabid elephants. This image is while they’re resting for their next thundering session.
The yard (and all of north Georgia for all I can tell) is growing a bumper crop of poison ivy this year. This is a picture of the third(!) set of ivy to crop up in the middle of our oregano patch. I’ve already RoundUp’d the back yard in glorious defiance of sound ecological practice and have been having fun pulling by hand all the poison ivy in the garden, the front (good) ivy, the juniper, the grass, they flowering shrubs, etc. It’s been a ton of barrels of monkeys. I’ve got a nice outbreak on my arms right now. I’ve talked about this before.
I’ve been yoga-izing myself consistently for the past three weeks. I’ve been not-running for the same amount of time. This is not a bad thing. I need to work on the whole core muscle thing.
I’ve been neglecting my Appalachian Trail section for the past few months. I need to get up there and do the water bar cleanout and whack some weeds. Maybe next weekend.
And that’s all that’s coming to mind right now in this brain dump. Keep it real, folks.
From my sources, Day 2 of the diverging diamond operations are going well. I’ve been on the phone with various people to discuss if there are ways we can change up the striping or signing of the interchange to make it more efficient. After all, this is brand new to all of us and we’ve learned things just in the last couple days.
Here are a before and after photo of the bridge for your enjoyment. Unless some exciting events happen, I don’t plan to keep deluging you with daily updates on the DDI, that’s what twitter is for ((@bruhsam)). I do have some posts coming that will discuss some specific items like the pedestrian walkway and some of the construction challenges. And I’ll keep putting up pictures. Stay tuned.
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.
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
WSB TV
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.
These arrows indicate that you are not supposed to turn on red ((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.)) 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.
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.
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 commute ((I deliberately picked a very congested moment; the whole morning peak hour was not like this.)) 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. ((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.))
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.
In lieu of an actual post, here are some images.
The first (non contractor) vehicle through the interchange was a Honda Civic.
There was a small hiccup during the second signal cycle when a driver misinterpreted an arm raised and went the wrong direction. Other vehicles followed. This was the only incident of its kind.
Compare this image…
…to this image.
Not quite the same location, but close.
The Ashford Dunwoody diverging diamond is open to traffic! Congratulations to the contractors, ER Snell and all their subs, for an efficient and effective weekend.
Remember that the interchange is not done.There will be barrels and cones and workers and form work for the next month plus. Be careful driving through and watch out for construction personnel.
I’m very excited about this! It’s always nice to see something you designed come to be.
As of last night, the construction during the weekend closure of Ashford Dunwoody bridge was ahead of schedule. With the weather almost perfect for what they needed, the contractor ((E.R. Snell of Lawrenceville)) should have the work completed and the bridge reopened ahead of their 5:00 AM Monday morning deadline. In fact, it should be significantly ahead, although I won’t speak for them. A lot of things can happen in 12 hours to delay a project. Let’s just say I’ll be back this evening with hopes.
The image you see above is a striping team putting down water-based paint for one of the stop bars. When you place water-based stripe, you also have to put glass beads on top, for the retroreflectivity. Thus, the hand toss shown.