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Did safety experiment work?

In the spring, the Virginia Department of Transportation will complete an assessment of its effort to improve safety and traffic flow through the Wilson Bridge construction zone. The effort, called the Variable Speed Limit program, was an attempt to slow down traffic so that drivers would get where they were going more quickly.

vsl sign outer loop.jpg Changeable speed limit sign. (Thomson)

It sounded like something against nature. In fact, it probably was. During my travels through the Capital Beltway work zone near Telegraph Road, it was not apparent that drivers were observing the electronic signs that displayed the current speed limit.

The maximum speed limit in the work zone was 50 mph. The Variable Speed Limit zone extended beyond that, covering most of the Beltway between the Springfield interchange and the Wilson Bridge. Along that stretch, the speed limit could be reduced to as low as 35 mph, depending on traffic conditions. If the monitors could see that traffic was severely congested around Telegraph Road, they could reduce the speed limit for drivers approaching the area.

If those drivers actually slowed down, they might not have to jam on the brakes when they encountered the congestion. Braking for congestion causes more congestion, and sometimes, it causes crashes. Think of sand pouring through a funnel. The slower the pour, the more sand gets through. Pour too fast and you can block the funnel.

The experiment is over. The speed limit throughout that portion of the Beltway has been restored to 55 mph, something drivers haven't seen since 2005, when the construction period intensified.

The contract for the speed limit program has expired, but the Wilson Bridge project also says that the continuing construction impact on the Beltway -- the reason for the program -- will no longer be as intense as what we saw during the earlier years of the building program.

I'll be interested in the results of the VDOT review of the program. If it worked, the Variable Speed Limit system could be used elsewhere. But I'd also like to hear from drivers who have traveled the Beltway since the program went full time in May 2009.

My take: Unlike grains of sand, drivers react in different ways to efforts to control their flow. Many -- probably most -- ignored signs intended to slow them down before they reached the serious congestion. Enforcement was always a key and always a problem. Anytime you set up a very visible enforcement zone, you create a new source of traffic congestion. Also, the Beltway is much more than a route for D.C. area commuters. It's also the East Coast Main Street. Many long-distance drivers never got familiar with the speed zone or its purpose.

By Robert Thomson  |  February 23, 2010; 10:22 AM ET
Categories:  Congestion , Construction , Driving , Safety , highways  | Tags: Dr. Gridlock  
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Doing this sort of one-time test may yield imperfect results. For a lot of us, we encounter these variable speed signs and it is the first time we've seen them in decades of driving and we only see them in this one place. Also, I'm on the friggin' beltway where there is almost no speed enforcement during rush hour (any hour?). What do I do? I go as fast as the bumper in front of me, same as always.

"Many -- probably most -- ignored signs intended to slow them down before they reached the serious congestion." Yup.

Posted by: KS100H | February 23, 2010 12:55 PM | Report abuse

I thought it was a nice idea in theory that didn't work in practice. I don't ever recall seeing any drivers obeying the reduced speed limits except when forced to do so by traffic. I think there are a couple of reasons for this:

(1) The prevalence of artificially low speed limits on most roads in Virginia and Maryland has people conditioned to ignore speed limits. To me this is the single biggest issue. People simply do not care what the number on the sign is--they're going to drive the speed they think is appropriate. In most (certainly not all, but most) cases, it's perfectly fine that they do this. But it causes problems when there's a real reason for a lower limit.

(2) The work zone with the variable speed limit extended well beyond the actual work zone on the Beltway, at least on the Outer Loop. I think many drivers resent the way work zones are often designated in areas where no work is going on, viewing such areas as a revenue grab because of increased fines. The variable speed limits were not lowered to 35 mph in the areas between Van Dorn and Springfield, at least not that I can recall, but I think once people conclude that "work zone" signs are posted for areas that they do not consider work zones, they tend to tune out other signs.

(3) Following on point 2, I think it seems to be routine for lower speed limits to be posted in work zones even though the average driver sees no real need for the lower limit. Then there are other work zones where lower limits would be appropriate but aren't posted (a good chunk of the HOT construction zone on the Beltway could well be posted at a lower limit). Once people become conditioned to the idea that "work zone speed limits are lower because that's what Virginia does," they'll become accustomed to ignoring the work zone speed limits, and I think that happened with the Wilson Bridge project.

(more in a minute due to the Post limiting comment length)

Posted by: 1995hoo | February 23, 2010 1:04 PM | Report abuse

(continued from previous comment)

(4) I think your average driver did not know of the "keep traffic flowing" rationale behind the variable speed limit. I also think most of the drivers on the road are not intelligent enough to understand the theory and that most of them would reject its validity even if they understood it. Problem is, there is no good way to explain the theory to people, as the vast majority of drivers don't read this blog or the Commuter Page in the Sunday Post.

(5) I think people are simply not willing to accept a 35-mph speed limit on an Interstate highway such as the Beltway. I kind of understand that attitude, although it also seems to me that if you’re on the Beltway and you see a 35-mph sign it ought to cause you to think something out of the ordinary is going on.

I think a variable speed limit makes a lot of sense, not just in work zones. The New Jersey Turnpike has had one for years, although their signs are so old that they don't work well. Too many people view whatever number is posted on the sign as the minimum speed you’re supposed to go, regardless of conditions, and that’s not how speed limits are supposed to work. The recent snowstorms are one occasion where I’d submit that driving at the posted speed limit was tantamount to reckless driving because it constituted going way too fast for conditions. In that type of situation, I think the idea of having a variable speed limit to underscore the conditions isn’t a bad idea at all. I also think that on roads plagued by fog or other particular conditions (I-75 northeast of Chattanooga comes to mind, as does I-64 over Afton Mountain near Charlottesville) the variable speed limit would be useful.

But I also understand why many people might be suspicious of variable speed limits because changeable signs immediately bring to mind Rosco P. Coltrane on the Dukes of Hazzard flipping the number over as the General Lee approaches a speed limit sign so as to write a "gotcha" ticket. Obviously that’s not going to happen in real life, but I definitely remember comments here on the Get There blog when this experiment began in 2008 where people were concerned that the cops would use changes in the variable speed limit as a way to meet their ticket quotas.

Posted by: 1995hoo | February 23, 2010 1:05 PM | Report abuse

There were many times on the inner loop going over the bridge around 4:00 PM the signs would still read 50 but we'd be slowed or stopped and other times the sign would read 40 and we'd be going through at 60/65.

Posted by: wlevey | February 23, 2010 3:07 PM | Report abuse

No enforcement. What is the ticket quota by the way? 2?

Posted by: member5 | February 23, 2010 3:50 PM | Report abuse

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