Las Vegas widening gone wrong. Photo by Roadsidepictures on Flickr.

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:

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.”

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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 lives with his wife and two children in Dupont Circle.