A Virginia Department of Transportation study recommends that the state work with Maryland to address traffic congestion on the American Legion Bridge, but its numbers also make a good case for relieving the Rosslyn tunnel transit jam.

The VDOT study of the traffic on 11 Potomac River crossings, underway since 2013, will be presented to Virginia’s Commonwealth Transportation Board on Wednesday. The only recommendation is that high-occupancy toll lanes across the Legion Bridge to Interstate 270 be the top priority for addressing crowding on the western Potomac crossings: “With concurrence of the board, staff will begin outreach to Maryland to determine interest in examining options for extending HOT lanes to the 270 spur.”

So far, the Maryland state government has shown no interest in a HOT lanes project involving the Legion Bridge. It was not part of the highway spending program outlined by Gov. Larry Hogan (R) in June.

The VDOT staff noted that the study does not rule out the possibility of creating a new river crossing farther to the west at some point.

While the study notes that the Legion Bridge suffers from the worst highway congestion among the crossings, the VDOT chart shows the crucial role that the Rosslyn tunnel plays in getting commuters across the river on weekday mornings. The chart below expresses the travel volumes in numbers and percentages for the morning crossings from Virginia to the District.

The study notes that the volumes at the Rosslyn tunnel exceed those on the Interstates at peak periods. About 35 percent of the morning peak crossings from Virginia are made via Metrorail, as are about 28 percent of the crossings to Virginia during the afternoon peak.

Some notes on evaluating the charts: It’s difficult to compare the movement of vehicles and people. Some of the vehicles are commuter buses. But at the Legion Bridge, very little of the rush hour traffic is made up of buses and the vast majority of travelers are solo drivers. (A HOT lanes program could increase the share of carpoolers and commuter buses, if supported by a network of carpool lots and commuter bus facilities.)

The charts reflect data from the region’s transportation departments if less than two years old, or from new counts if recent data was unavailable, as well as 2012 passenger surveys done for Metro.

The VDOT report notes that “significant investment is required in the future to further address core capacity issues on WMATA,” which include the capacity of the river crossings, particularly the Rosslyn tunnel. However, any significant increase in transit capacity is years away. The Metro staff recently proposed a cutback in the frequency of trains sent through the Rosslyn tunnel and over the Fenwick Bridge.

The transit staff said that an average of 24.5 trains per hour are getting through the Rosslyn tunnel, rather than the 26 scheduled for peak periods. While decreasing the train frequency, the proposal also calls for increasing the number of rail cars on some of the trains. However, this proposal would still result in more crowding on the rush-hour trains that cross the Potomac.

No one fix — whether it’s at the Rosslyn tunnel or the Legion Bridge — is going to be sufficient to address the needs of the region’s commuters. The study notes that the bridges serve different markets between the Nice Bridge downstream and the Point of Rocks Bridge upstream. I’ll have more about the origins and destinations of the bridge users in an upcoming posting.