Study: Just Six Tracks Carry 30% of People Across the Potomac (57 Highway Bridge Lanes Carry the Rest)

July 29th, 2015

A new Virginia study (PDF) finds that Metro and other transit operators carry a major portion of all cross-Potomac travel in a just a few crossings, using far less space than the 57 highway bridge lanes that carry the rest. If built, an expanded bridge crossing will need transit to maximize its ability to move people across the river.

PotomacRiverStudy Map

Transit’s Role is Critical

While some media outlets focused on the study’s highway expansion recommendation, the presentation acknowledged that Metro, VRE, and other bus operators plays a major role in the movement of people across the river from Virginia to DC in the core of the region. Seeing that, we thought we we’d drill down further to estimate how many people are actually crossing the river, using which bridge, and by what mode. Supplementing the study with available transit ridership data and vehicle occupancy data (PDF), we arrived at the following estimates:

Potomac Charts

Estimated AM Peak Period Trips (* includes buses on highway bridges)

This is all the more impressive when one considers the numbers of highway bridge lanes (57!) needed to move the other two-thirds of people (as opposed to just six rail tracks). Plus local and express buses also share the load across the Key, Roosevelt, 14th Street Bridges, perhaps carrying 6 to 16% of the travel on those bridges in the morning. Notably, in the morning rush, the Rosslyn tunnel carries more people across the Potomac River than any other crossing to or from Virginia, including the highly congested Legion, 14th Street, and Wilson Bridges, which are all major multi-lane interstate routes.

New Crossings Will Need Transit to Move More People and Foster Sustainable Regional Growth

The study’s recommendations include a possible extension of the 495 Express Lanes across the Legion Bridge into Maryland. The Potomac River is a major choke point for regional travel and the Legion Bridge is effectively the only major crossing in the growing western portion of the Washington Region, increasing the importance of whatever project might be implemented. If a robust level of transit service were added, potentially connecting the growing White Flint area, Tysons Corner and other areas, the capacity of the crossing would be greatly increased.

Legion Bridge Options

For example, if just a single additional managed travel lane were added in each direction, that lane’s ability to move people would be more than doubled with strong HOV usage, and a variety of bus services using the new lanes to bypass congestion. The region was forward in its thinking when it built the Wilson Bridge to accommodate future transit, however today no transit actually crosses it. If the region decides to expand the Legion Bridge while continuing to grow smarter and manage congestion, transit should continue to play a strong role moving people efficiently across the river.

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  1. BTA
    August 3rd, 2015 at 11:11 | #1

    I wonder if something like a BRT loop that just hits Rosslyn, Pentagon and a few locations in DC (Georgetown, Farragut, Metro Center, Union Station, Capitol, L’Enfant) would make sense. Can’t see it operating outside of rush hour, but if you could get 5 minute headways it could be competitive with some Metro trips between 7AM-9AM, 4PM-6PM. Would only make sense with a good portion of transit dedicated lanes.

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  2. Scott Anderson
    August 3rd, 2015 at 16:31 | #2

    One of the interesting things about the Wilson Bridge is that there is NO TRANSIT OPTION for people moving between Virginia and that part of Maryland. To drive from Alexandria to National Harbor (where a good many Alexandrians, many without cars, work) takes ten or fifteen minutes; a transit trip is an expensive, multi-hour trek. The last time bus service was attempted, 10 years ago or so, none of the trip generators of today existed: Carlyle in Alexandria was a construction site; National Harbor was a bunch of paper plans. It shouldn’t cost several dollars and take such a ridiculous amount of time to make such a short trip, especially when it has the possibility of diverting a little bit of traffic from that bridge.

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