On a Virginia Department of Transportation computer model, cars traveling south on the Capital Beltway were approaching the ramp to westbound Interstate 66. The one-dimensional vehicles, displayed on a desktop screen, reacted in varying ways because the programmers can adjust for different conditions and driving habits.

Sometimes, the traffic in the model moved fairly well. Sometimes, the cars bunched up where ramp lanes merged. Sometimes they slowed farther back, on the right lane of the Beltway’s outer loop. Sometimes, the cars to their left attempted to cut in front of those already waiting to use Exit 49.

This model was one of the things engineers studied before making the recent change in the traffic pattern at the Beltway/I-66 interchange, part of the 495 Express Lanes work zone.

Sometimes, traffic will behave as predicted by such models and other research. But to the dismay of the VDOT planners, this didn’t turn out to be one of those times.

When VDOT narrowed the ramp two weeks ago to ease the adjacent bottleneck on westbound I-66, the planners thought they could avoid creating a new traffic jam on the ramp. Instead, southbound rush-hour traffic has backed up badly, sometimes to the American Legion Bridge, according to drivers.

“It didn’t behave like we thought it would behave,” said John Lynch, VDOT’s regional transportation program director.

Drivers’ experience

After VDOT narrowed the ramp, outer-loop drivers began complaining. It was the main topic in last Sunday’s Dr. Gridlock column and again during my online chat Monday.

Here’s a sampler of some of the comments submitted for the chat.

66W & the Beltway: The traffic has been horrendous since they made this switch [on the ramp] from two lanes to one. ... I work in Rockville and live in Fairfax, and the commute home has been awful compared to normal. The backup to enter I-66W extends for miles!

●I-66/I-495 merge: Now that they’ve fixed the backups on I-66 West by taking away a lane from the ramp from I-495 South [outer loop] to I-66 West, I-495 is backed up onto the entrance ramp from Route 7 East. One day last week, it was backed up on Route 7 past the mall. And this was at 2:30 p.m.! I can’t imagine what it’s like at 4:30 or 5.

●Toll Road @ Beltway — different pattern? For a while, traffic from the main toll plaza on the Dulles Toll Road to the Beltway was moving nicely. As of about a week ago, however, we seem to be experiencing a lot of traffic in the two left lanes heading toward I-66.

●I-66W/I-495 Interchange: I was always taught that the needs of the many outweigh the needs of the few. You’ve stated that the outer loop to I-66 West ramp was reconfigured to take pressure off the traffic coming onto I-66 West from the inner loop and I-66 West inside the Beltway. While certainly the changes have done just that, the result is now an absurd traffic jam. ...

So could you please explain how easing congestion for those getting onto I-66 West from I-66 inside the Beltway and the inner loop of the Beltway, resulting in 15 miles of backup, is better than what we had before?

I-66 Interchange: The crux of the problem is that the ramp from I-495 South to I-66 West starts out as two lanes ... [then] about 100 yards after the ramp separates from the Beltway, the left lane abruptly ends, creating a pinch point. That singular pinch point is what backs up traffic 10-plus miles on the Beltway.

The work zone

Project managers heard similar complaints, and now are trying to fix the fix. Among the possibilities VDOT is discussing with the construction team:

●Modify the ramp. Driver indecision at the new merge point is part of the problem. If VDOT reduced the ramp to one lane back to the Beltway, drivers wouldn’t have to figure out how to merge on the ramp.

But that also would reduce the capacity of the ramp, which might just lengthen the spill-back of cars onto the Beltway. Seeing that long line, some drivers would try to cut in near the front, always a nasty business.

●Modify I-66. It might be possible to adjust the work zone around where the ramp meets the westbound interstate to ease traffic in that merge area.

But it’s a tough work environment that includes an old bridge ramp from the Beltway that must be demolished and the very active Orange Line tracks underneath it, right by the Dunn Loring Metrorail station.

●Restore the former pattern. That would mean not only restoring the second lane on the outer loop ramp — a good thing — but also squeezing down the space just opened up for traffic on I-66 — a bad thing. This is not what VDOT wants to do, said Senior Project Manager Larry Cloyed. It’s demoralizing to drivers to offer an improvement, then snatch it back.

Changing patterns

A temporary adjustment now would ease the pain for thousands of rush-hour commuters, but the ultimate fix will come during the summer when this phase of construction ends. The Express Lanes, four high-occupancy toll lanes along the Beltway, are scheduled to be open by the end of the year.

During the interchange reconstruction, the pain has rotated counterclockwise. Several years ago, drivers on eastbound I-66 faced a jam created by rebuilding the ramp to the Beltway’s inner loop. That’s been significantly eased by the opening of a new flyover ramp.

Last year, crews permanently shut the left exit from the inner loop to westbound I-66. All exiting traffic now uses the ramp on the right side, which means it enters I-66 farther east than before, joining all the other traffic coming from D.C. and clogging I-66 in the middle of the interchange.

What’s next

Nothing remaining is likely to be as disruptive as what commuters already have experienced with these ramps. But before construction is done, these things must happen:

●March/April: That old bridge ramp from the left side of the inner loop will be demolished.

●April: Traffic will shift to a new ramp from westbound I-66 to the outer loop.

●Summer: All lanes from the outer loop to I-66 West are scheduled to be restored.

●Fall: All original lanes will be back in service on the inner loop at I-66, and a fifth lane will be added in that area.