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Some Relief For Chain Bridge Drivers

The District Department of Transportation and the contractor working on the rehab of Chain Bridge have been trying to figure out ways of easing the traffic congestion that resulted from the closing of one of its three lanes for the duration of the project. They plan to try something new on July 27.

This effort deals with a complaint I've heard from some of you: Virginia-bound traffic wanting to make a right turn and go up Route 123 gets caught behind traffic wanting to go straight up Glebe Road.

So the plan is to adjust the concrete barriers and add some lane striping to create separate through and turn lanes at the traffic signal on the Virginia side. That should at least create enough space for several cars of turning traffic to get by, says John Lisle, spokesman for the District Department of Transportation.

By Robert Thomson  |  July 16, 2009; 3:29 PM ET
Categories:  Commuting , Congestion , Construction , Driving  | Tags: Chain Bridge, Dr. Gridlock, delays, rehabilitation  
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Comments

This should not have been hard to figure out; don't these construction guys do this all the time, and observe what the effect is what they do? By the way, while they are at it, if they would also adjust the concrete barriers on the DC side, so that two lanes are created before the traffic light (even for just several cars), more cars would get through the light, and I'll bet congestion would go way down.

Posted by: mfm1 | July 16, 2009 4:00 PM | Report abuse

The construction guys do this a lot. But (unless it is a design-build project) the construction guys don't develop the MOT (maintenance of traffic) plan, the engineers do. And I can say without a doubt that there is not another bridge like Chain Bridge in DC. So the engineers came up with an MOT plan that they thought would work well. But it is really difficult to know how well it will work until you put it into effect, AND wait a few days for traffic patterns to sort themselves out. There is always a little pain the first few days.

I give major kudos to DC for being willing to be flexible on the MOT plan for this project. They've re-timed traffic lights several times (including a signal that isn't even theirs...it's Arlington County's), and they are willing to try to squeeze an extra lane in now. I have seen several agencies have awful MOT plans and refuse to make even the simplest changes to fix traffic flow (I'm thinking Rock Creek Parkway back in 2007).

It is quite possible that they waited until work was completed at the Arlington end of the bridge before announcing and implementing this change, and that could be the reason for the delay.

I doubt having a second lane on the DC side would help much. First, except for a couple of hours during morning rush, there is only one lane to turn into on Canal Road. And then the second lane would only be as good as the number of cars that can fit in the extra lane.

The capacity of a signalized intersection is directly related to how many cars fit through at saturation. Traffic flows at saturation when it comes off a queue. If you have a single lane feeding two lanes at the signal, but the 2 lane portion can only fit three cars, then the first three cars in each lane get through at the maximum saturation flow rate. After that, the cars are coming off the queue in the single lane portion, which means the saturation flow is equivalent to only one lane's worth, but inefficiently distributed across 2 seperate lanes. And there is also no guarantee that people would use both lanes....everyone might try to use the left lane so they could turn left at Arizona. So adding a lane a couple cars long on the DC side would not significantly reduce congestion, and is more trouble than its worth.

Posted by: thetan | July 16, 2009 9:53 PM | Report abuse

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