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Would I-66 widening increase accidents? VDOT doesn't know

Commenter Geof Gee posted a summary of last night's meeting on widening I-66 in three spots by adding lanes between entry and exit ramps. Geof describes himself as guided by "practical considerations" and is "not inherently against increasing auto capacity," but nonetheless came to the conclusion that the project is a bad idea, even by the standards of VDOT's own engineers. The problem, Geof thinks, is that traffic modeling technology is too primitive:

Las Vegas widening gone wrong. Photo by Roadsidepictures on Flickr.
There is nothing preventing drivers from leaving the through lane to move up in the queue in the third/auxillary lane and re-entering the through lane later in the process. My understanding of queuing theory suggests that this type of interaction would increase travel times through the corridor as well as increase the variability of travel speeds.

From a conversation with the traffic operations researcher at the meeting, the simulations still predict that the net effect of giving drivers for space for merging is still positive... [but] there are a few serious problems. These models fail to consider a change in an accident rate due to the increased number of merges/aggressive driving. ...

Moreover, a greater number of accidents increases the variance of travel times primarily by increasing its skewness. Consequently, even if the net average travel times decrease, it appears to me that a commuter is more likely to be screwed with a really long commute. If you ask, "What is the probability—with its 95% confidence interval—that net travel times decrease?" the answer is that there is none and that the science has not progressed to that point.

Long story short, I think that calling the project—at least for Phase 1 and 3—an "improvement" is an overstatement of simulation's veracity.

So far we have three major problems with this project:
  • Induced demand: There is a good chance that the added capacity will create new auto trips, adding more traffic. We don't know for sure, but this is the usual effect of freeway projects.
  • Environmental laws: As Michael P wrote, VDOT is saying this will have no impact because it's "just" a weave lane. However, this is an extraordinarily long such lane, and the law requires changes which could create induced demand to go through the full analysis.
  • Modeling limitations: As Geof explained, the lane might not even accomplish its purpose of moving more cars.
Sounds like a good set of reasons not to spend a lot of money widening this freeway. As Geof concludes, "Given the local opposition to the project and less expensive alternatives that are better understood—this was the traffic operations guy's language—it appears to me that the project is a bad bet and almost certainly not a huge improvement."
David Alpert is the founder of Greater Greater Washington and its board president. He worked as a Product Manager for Google for six years and has lived in the Boston, San Francisco, and New York metro areas in addition to Washington, DC. He now lives with his wife and two children in Dupont Circle. 


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Well, it looks like major problem #3 solves major problem #1.

by Tom on Oct 28, 2008 3:22 pm • linkreport

Why the focus on 'induced demand' without acknowledgment of the arguably induced development- aka the townless townhouse developments (16 dwellings per acre) erected along and near I-270 following its early 1990s widening?

by Douglas Willinger on Oct 28, 2008 7:45 pm • linkreport

VDOT's lame excuses that it can't properly model demand indicate either incompetence or an attempt to increase its budget. The state of knowledge in traffic engineering is far enough along to create sophisticated models that account for induced demand and dynamically model accident rates.

As I stated before in, VDOT is full of beans regarding its inability to calculate statistics when it has the necessary data staring them in the face.

Until VDOT proves its case, and that means using best practices in traffic analysis and basic statistics, I can't support this proposal, nor should anyone else. The burden of proof is on VDOT. Maybe they're right and traffic flow on I-66 will improve, but they have not proven their case. Moreover, significant effects may only be felt for, say half an hour. Would construction be worth an improvement that can only be felt for less than 180 hours per year. Finally, they have done no cost/benefit analysis. This requires quantifying both costs and benefits (not done) and comparing them to other projects to determine first, whether there is a net gain from construction, and, second, how to prioritize this project. It may come out very low. Ideally, mass transit improvements would also be considered as alternatives, but this is not how a roads-and-cars agency works.

by Chuck Coleman on Oct 29, 2008 7:57 pm • linkreport

Correction about the link:

by Chuck Coleman on Oct 29, 2008 10:13 pm • linkreport

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