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Consultant: VDOT used faulty data for bypass study
by Sean Tubbs | Thursday, October 18, 2012 at 3:59 p.m.

Traffic forecasts used in the Virginia Department of Transportation’s Environmental Assessment of the Western Bypass of U.S. 29 were based on data the agency knew to be flawed, according to a consultant hired by the Southern Environmental Law Center.

The area’s Metropolitan Planning Organization was made aware of an error in its traffic model in January and corrected the analysis. VDOT, however, has not yet utilized the updated information.
 
“It is imperative that a new, reliable traffic analysis be completed, and that this new analysis take a fresh look at alternatives,” wrote Norm Marshall of the Vermont-based firm Smart Mobility.
 
In 1990, the Commonwealth Transportation Board selected the current alignment for the bypass. That decision was informed by a traffic analysis completed that same year.
 
“To this day, that 1990 Traffic and Transportation Analysis technical memorandum is to the best of my knowledge the only traffic study that VDOT has ever completed for this project that provides a comparison of the proposed bypass to alternatives,” Marshall said.
 
To comply with the National Environmental Policy Act, VDOT is preparing the EA in order to convince the Federal Highway Administration that its previous approvals in 1993 and 2003 are still appropriate and will result in no significant impact on the environment.
 
As part of the process, VDOT used the MPO’s regional travel demand mode to provide an update to its traffic forecasts.
 
“The travel demand model provides a mathematical method for analyzing the effect on the area’s transportation facilities of changes in the region’s population, employment or transportation systems,” said Stephen Williams, executive director of the Thomas Jefferson Planning District Commission, which houses the MPO. “You could think of the model as addressing the supply and demand for transportation in the region.”
 
The draft Environmental Assessment released in August by VDOT references information generated by the traffic model on Jan. 12, 2012.
 
“The new parallel road is forecast to carry just under 28,000 vehicles per day in design year 2040,” reads the EA. “Much of this volume would shift from existing Route 29, resulting in reductions of daily traffic volumes on existing Route 29 of up to 28 percent.”
 
However, Marshall had been hired by the SELC to review the model and discovered that it overestimated the amount of trips that would be generated by people driving through the region.
 
The MPO later acknowledged the issue and corrected the model in early February.
 
“One issue that existed in the model which we did not catch in our process was the approach the model used to assign trips between the region and the area outside the region,” Williams said. “The model … assumed that the same number of external trips would go between the region and areas outside no matter where they started or ended within the region.
 
The SELC urged VDOT to use the new model, but the agency declined. Marshall said that means the efficacy of the bypass, as claimed in the EA, is over-inflated.
 
“Using the corrected version of the model that the MPO provided to us on February 7, the amount of traffic it projects would use the proposed 29 bypass in the year 2040 dropped 14%, from 27,798 vehicles per day to 23,918 vehicles per day,” Marshall wrote. “This is a significant decrease for purposes of evaluating the effectiveness of the proposed bypass in addressing traffic congestion.”
 
However, Williams denied that the error results in over-inflated numbers.
 
“The traffic forecast on the Western Bypass in the January version of the model was about 4,000 vehicles per day higher than in the February version of the model,
Williams said. “That might seem like quite a bit, but in reality that is only six additional trips per minute between 6 a.m. and 6 p.m.”
 
VDOT did not respond to a request for clarification on the data for this story.
 
Neil Williamson, president of the Free Enterprise Forum, a group that supports the bypass, dismissed Marshall’s analysis.
 
“Based on our discussion with traffic engineers, we believe such modeling is both an art and a science,” Williamson said in an email to Charlottesville Tomorrow. “No model is perfect and the minute you publish a model it is out of date. While we are sure that VDOT will reexamine their numbers, we firmly believe the empirical data will continue to support the construction of this long awaited roadway.”
 
In his study, Marshall also claimed that VDOT did not significantly address the impact the bypass would have on congestion north of the northern terminus.
 
“The bypass would generate additional traffic volume in the area north of the proposed northern terminus that would have inadequate capacity in 2040,” Marshall wrote. “The additional congestion the bypass would generate north of its northern terminus would undercut what little effectiveness it might offer in facilitating intrastate and interstate trips passing through central Virginia because the most severe bottleneck would become even more congested.”
 
However, Williamson said Marshall’s report is part of a campaign to stop the bypass at all costs.
 
“The bypass never claimed to relieve congestion north of its northern terminus,” Williamson said. “The opponents’ goal is to wallpaper the room with studies and lawsuits to the point of delay and hope for a change in the political landscape.” 
 
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