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
No Bridge Widening
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.
No Right of Way Purchase
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.
Simplify Signal Phasing
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!1 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.
Safety and Expectations
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.
- 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. [↩]