Review of the Loudoun Countywide Transportation Plan

Review of the Loudoun County Transportation Plan Draft, 2010

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Smart Mobility, Inc. has been retained by the Citizens for a Countywide Transportation Plan to providean independent review of the proposed Loudoun County Transportation Plan (CTP). This review encompasses both a preliminary critique of the Loudoun County Transportation Model (LCTM)which is the basis for the plans recommendations, and on the plan’s effectiveness in supporting the county’s overall goals of more sustainable and compact land use forms as stated in the Loudoun Comprehensive Plan. More information about Smart Mobility, Inc. and the staff qualifications to provide this review are attached to this report. The following summarizes our key findings of this review.

Outdated Approach to Transportation Planning:

  • The approach is not “industry accepted” for planning of multimodal transportation systems, and does not reflect the goals for compact, mixed use development with highly connected street networks for the suburban communities, as well as reduced development in the transition area, as articulated in the Loudoun County Comprehensive Plan.
  • The approach used in recent analysis for the Tysons Corner redevelopment illustrates the state of the practice in the transportation modeling for compact, mixed use development.


Flawed Model Underlying the Analysis

  • The TPB model is known to have serious structural flaws, most of which have not yet been addressed. It falls far short of federal guidelines for travel forecasting. It is simply not a good foundation on which to base decisions for hundreds of millions of dollars of infrastructure investments.
  • The LCTM is based on the Transportation Planning Board (TPB )model, and has some modest improvement in calibration, but it still is a very poor model, falling far short of the FHWA model guidelines. (Model correlation is on a scale of 0 to 1.0, where 1.0 would be perfect. FHWA guidelines allow a minimum correlation of 0.88, or 12% below perfect. The LCTM is 0.74, or 26% below perfect – more than twice the deviation allowed by FHWA.)
  • Specifically, the LCTM substantially overestimates circumferential traffic, leading to erroneous conclusions about how much more capacity is needed. A poor model will lead to poor decisions, with millions of dollars of public investment at stake.
  • The LCTM includes very high external‐to‐external commuting volumes (i.e West Virginia or Maryland to Fairfax County). These unrealistic results are due to a combination of the model’s underlying structural problems, and in the assumption that fuel prices will be very low into the future, making long distance commuting more attractive than is likely. This substantially increases traffic projections for major corridors, especially in the rural and transition areas, leading to erroneous conclusions about the need for road improvements.


Application of the Model considered only a narrow range of traffic congestion solutions.

  • The model was used in a primitive way that only sought to address peak hour levels of service, and not system‐wide operations and efficiency.
  • The model evaluates traffic congestion only in terms of “links” rather than “nodes” (i.e. segments of roadway rather than intersections). This leads to misleading or erroneous conclusions about options to address traffic congestion.
  • The identification of a long list of roadway expansion projects in the CTP is a lost opportunity to consider more compatible and sophisticated approaches to traffic congestion, such as those outlined in the consultants’ Traffic Abatement report