I noticed during Monday’s online chat that many travelers were writing to ask practical questions about the uses of the 495 Express Lanes, the high occupancy vehicle lanes scheduled to open by the end of the year. I’ll try to post several of your concerns this week, starting with this question about whether the northern part of the system is useful at all.

Beltway bottleneck: Could you please explain what will be done to prevent the northbound (towards Maryland) Capital Beltway express lanes from creating a bottleneck where they end north of the Dulles Toll Road? From what I can tell, there’s no additional capacity being added to the Beltway north of where the lanes are ending.

Backups from Tysons Corner all the way up into Maryland will still be occurring even after the construction is completed. Where the Express Lanes are merging is basically at the northernmost point where Beltway backups begin on average.

Those express lanes are guaranteed to backup down to at least the Toll Dulles Road. Obviously, the amount of the toll will limit the number of vehicles using the lanes, but what good will it do a driver to use the lanes if they’re just going to bypass less than a mile of backup (assuming most traffic going into Maryland is coming from Tysons) just to sit in bumper to bumper traffic from the lane merge all the way into Maryland?

DG: The Virginia Department of Transportation and Transurban, the company that will operate the 495 Express Lanes, will be monitoring what happens at the northern point where the lanes end around Old Dominion Drive. It’s is a potential trouble spot because cars will be entering the inner loop’s regular lanes from the left side. (I hate left-side merges.)

But at this point, it’s unclear whether this will prove to be a problem. One thing Transurban officials point out about this and other travel issues through the 14 miles of the express lanes is that they don’t expect to add traffic to the Beltway.

In other words, they think that by adding the four lanes in the middle, they’re going to shift traffic around, not draw extra vehicles. On one day, a driver might decide that things are so bad in the regular lanes that it’s worth the money to shift to the toll lanes. Other days, the driver would be back in the general purpose lanes.

They think their market will mostly consist of Virginia drivers who want to escape traffic for a few miles. The number of Maryland customers will be limited. And they believe it would be a rare thing for a long-distance driver — let’s say, someone driving from Florida to New England — to use the toll lanes.

So, as bad as the Beltway congestion is south of the American Legion Bridge, the express lanes project isn’t likely to add a lot more vehicles to that mess.

Also, as the questioner noted, it’s in the interest of the express lane operators to keep the traffic in those lanes moving and not let the speeds drop south of the merge point. One way to do that is to raise the toll and reduce the traffic volume in the express lanes before it feeds into the regular lanes.

But to address another of the questioner’s concerns, there’s no plan to do anything with the Beltway north of the merge point, on either side of the Potomac River.

The Maryland State Highway Administration has some ideas for extending the merge lanes on its side of the river to create spot widenings of the Beltway, but that’s about it.