Drivers like these on I-66 near Vienna know where they’re likely to encounter chronic congestion, but they may know know why. (Karen Bleier/AFP-Getty Images)

There’s a consistency in the D.C. region’s traffic congestion: Commuters know where it’s most likely to be bad. What many don’t know is why.

We’d like your help in identifying a few of the highway zones that are chronically congested, then we’ll get some help from the INRIX traffic data service in figuring out the causes.

The areas that will be the most interesting are the ones that you find the most mysterious.

Those might not turn out to be the places where traffic is the worst. For example, one of the worst spots in the region is Interstate 395 northbound to the 14th Street Bridge over the Potomac River into D.C. While this is a waking nightmare for morning commuters many of them know what’s going on: An already enormous volume of northbound traffic encounters even more traffic entering the Interstate via ramps a relatively short distance apart. Meanwhile, many drivers on the bridge are trying to change lanes so they can line up for 14th Street NW on the left or I-395 on the right. The major mystery is why nothing has been done to ease this long-standing situation.

Another notorious morning slowdown occurs on the Capital Beltway’s outer loop between College Park and Georgia Avenue. Here again, I think most drivers sense what’s going on: The morning traffic flow in the D.C. region is generally east to west. That big volume of traffic swinging west around the Beltway meets the heavy traffic coming south on I-95, then it takes a while to sort itself out. Driving experience and observation on the traffic maps and cameras tells me that this heavy congestion breaks up by Georgia Avenue, but is there really that much traffic exiting there, or is it the combined effect of exits at New Hampshire Avenue, University Boulevard and Colesville Road?

My colleague Bonnie Berkowitz, a Post graphics reporter, wonders about the slowdown between the spot on the Beltway where the 495 Express Lanes end and the American Legion Bridge. This one generates debate among some drivers. Some folks blame the express lanes merge, but I don’t see that — not yet, anyway. There’s just not enough traffic in the express lanes. Also, that slowdown didn’t start with the opening of the express lanes. I say it’s the traffic from the Tysons area combining with the drivers from the Dulles Toll Road and the George Washington Parkway, all facing the bottleneck at the Legion Bridge and the lane shifting north of there.

There are other slowdowns with more subtle causes. The highway may bend several times in a relatively short stretch. Or drivers may find themselves staring into the sun at the height of rush hour.

Here’s one I just don’t understand: The morning slowdown on the Beltway’s inner loop through Oxon Hill in Prince George’s County. The problem seems to disappear as drivers pass northbound I-295, but it doesn’t look like there’s a heavy volume of traffic heading north on I-295.

What’s your highway mystery? We’ll collect examples and submit them to INRIX for study. You can place your nominations in the comments field or e-mail them to