The Virginia Department of Transportation has scheduled an information session Tuesday night on its plan to allow rush hour traffic on the left shoulder of the Capital Beltway’s inner loop south of the American Legion Bridge.

The project would open the shoulder for a mile and a half north of where the 495 Express Lanes end. This plan is one of the reasons I think it’s meaningless to judge after one year whether the express lanes are a success. The Northern Virginia transportation network keeps evolving. The major changes will be the opening of the Metro Silver Line and the 95 Express Lanes. But the opening of the shoulder is another small piece of the jigsaw puzzle.

Virginia Gov. Robert F. McDonnell (R) announced the plan in June. It still must be approved by the Federal Highway Administration, but VDOT hopes to begin construction of the $20 million project during the summer of 2014 and complete it by the end of the year. Drivers between Old Dominion Drive and the George Washington Parkway interchange would see lane markings similar to those on Interstate 66 indicating whether the shoulder was open to traffic.

VDOT offers these reasons for opening the shoulder: It gives rush hour drives five lanes through a very congested area north of Tysons Corner, it lengthens the merge distance for express lanes drivers, it doesn’t require taking land on the sides of the Beltway, and it could decrease the number of drivers using local streets to get around the Beltway traffic.

These are worthy goals, though drivers will recognize that they don’t solve the fundamental problem of too much traffic in too little space on the Beltway between Tysons and Bethesda.

Lengthening the distance for merging will help the express lanes drivers, who must rejoin the regular lanes from the left side, something that’s always tricky. I find that merge more difficult at off-peak periods, when drivers are moving faster and making more lane changes, but the shoulder would be shut then.

And of course, the shoulder opening won’t diminish the traffic flowing in from the GW Parkway or widen the lanes over the Legion Bridge and north to the Interstate 270 split.

One thing some travelers worried about before the express lanes opened last November was whether the additional lanes would increase traffic going to and from Maryland. That may prove to be a long-term issue, but so far, I haven’t seen any signs that the express lanes have added traffic to the Beltway. Commuters who use some other mode besides driving to get between Maryland and Tysons, or other points in Northern Virginia, would need to switch to driving to make that area more congested than it was before.

Given the lack of transportation options in that cross-Potomac corridor, what other transportation modes could they have been using besides driving? And how could they possibly find the Beltway between the end of the express lanes and Bethesda more appealing now than it was a year ago?

I think it’s up to Maryland to become more engaged in easing one of the region’s worst Beltway bottlenecks.

The VDOT session on opening the shoulder is scheduled for 7 to 9 o’clock tonight at Cooper Middle School, 977 Balls Hill Rd., McLean, near the Beltway’s Georgetown Pike interchange. This is an open-house style of meeting, with VDOT staffers available to discuss the project and answer questions. People also can provide their written comments at the meeting.