Map shows the I-66 corridor under study inside the Capital Beltway. (VDOT image)

A traffic study to support Virginia’s plan to create high-occupancy toll lanes on Interstate 66 inside the Capital Beltway shows just how difficult it’s going to be to fix this highway.

In fact, it doesn’t even sound like one highway as much as a cluster of different roads stitched together. How do you smooth out the traffic flow and give commuters a trip of reliable length when there’s no one particular peak during the morning or evening rush? The high-occupancy vehicle rules are much less restrictive in practice than in theory. Traffic volumes are uneven along the nine-mile route. And traffic volume turns out to be just one factor slowing trips.

Three public meetings are scheduled for next week to review the design for the inside-the-Beltway HOT lanes, which the state government hopes to open in summer 2017. But let’s look first at the study for the Virginia Department of Transportation that lays out what problems the plan hopes to ease and why this effort is so challenging.

Where traffic comes from
I-66 isn’t a steady stream of east-west traffic. Big clusters of traffic approach the highway from several different directions with several prime destinations along this shortish route. During the 5:30 to 9:30 a.m. rush, nearly 38 percent of all eastbound traffic comes from the Dulles Connector Road (Route 267), while 29 percent comes from I-66 west of the Beltway and another 29 percent comes from the Beltway south of I-66. While I-66 crosses the Potomac River into the District, many commuters are going to get off before that for destinations in Arlington County.

During the 3 to 7 p.m. rush, 42 percent of the westbound traffic comes from Arlington County and 40 percent starts from the District. Their primary destination is I-66 west of the Beltway; next is the Dulles Connector Road for targets that include Tysons Corner and Dulles International Airport. Another big cluster is bound for the Arlington area.

Those varied origins and destinations mean that even within the short path of the highway, the traffic is unevenly distributed. The traffic volumes are lightest between the Beltway and the Dulles Connector Road, a stretch a bit over two miles on the west side. Then the heaviest volumes occur between the Dulles Connector Road and North Westmoreland Street, a stretch a bit over a mile long.

How they travel
I-66 inside the Beltway is HOV2 in the eastbound direction from 6:30 to 9 a.m. weekdays and in the westbound direction from 4 to 6:30 p.m., but the carpoolers have a lot of company. Exemptions apply to approved hybrid vehicles, law enforcement and traffic coming from or going to Dulles Airport.

Chart shows how people travel during peak periods on I-66 inside the Beltway. "SOV" is single-occupant vehicle. (VDOT image)
Chart shows how people travel during peak periods on I-66 inside the Beltway. “SOV” is single-occupant vehicle. (VDOT image)

During the a.m. peak, carpoolers dominate in the eastbound lanes, but single-occupant vehicles still make up nearly 36 percent of the traffic. There’s no breakdown of how many are hybrids or airport travelers, or cheaters breaking the rules.

On the unrestricted westbound side, 89 percent of the morning peak traffic consists of single-occupant vehicles. When the HOV restrictions come into play during the afternoon peak westbound, most of the traffic consists of carpoolers, yet 43 percent of the vehicles are single-occupant. Again, that’s an unknown combination of hybrid drivers, airport traffic and cheaters.

When they travel
I-66 traffic doesn’t mount to a peak and then decline. The pattern is much more complicated, largely because of the HOV restriction. In the morning, the eastbound traffic reaches a peak just before the HOV restrictions are imposed, then the volume plunges when the HOV rules take effect before building up again to an even higher peak at about 7:45 a.m. The traffic volume tends to go into a decline after that, before reaching a third peak about 9 a.m., as the HOV restrictions end.


Eastbound traffic on I-66 peaks at several points during the morning and afternoon rushes. The shade area covers the time when HOV restrictions are in effect. (VDOT image)

The afternoon pattern during the westbound rush is more muddled. The peak of the peak traffic volume tends to occur just before the HOV restrictions take effect at 4 p.m. A second peak shortly after 5 p.m. is lower than the first one. Then there’s another surge just after the HOV rules are lifted.

One thing evident in the traffic volume study is that the HOV rules have a big impact. The highest overall traffic volumes recorded at rush hours were in the off-peak directions, which are not subject to the HOV rules.


Westbound traffic volume also shows peaks and valleys. The shaded area covers HOV hours. (VDOT image)

Travel speeds
The average travel time for the full-length trip eastbound during the morning peak is 21 minutes. The average is 20 minutes during the afternoon peak eastbound. Westbound, it’s 14 minutes for the morning peak and 17 minutes for the afternoon peak.

But the average travel time fluctuates not only with the hour at which you travel but also with your location within the corridor. For example, the average westbound travel time between Route 7 and Gallows Road is the longest in the entire corridor. The traffic there is slowing behind congestion outside the Beltway.


Color code and grades show the variety of average speeds at the peak travel hour for segments of I-66 inside the Beltway. “LOS” is level of service. (VDOT image)

There are other congestion problems for I-66 drivers heading east. The weave area between Route 29 near Rosslyn and Route 110, the merging traffic from Route 50 onto the Roosevelt Bridge into the District, and the exit movements onto E Street, Constitution Avenue and Independence Avenue into the District, which can affect traffic flow a lot farther back in the line.

Other factors complicating travel times include the heavy volume of merging traffic heading eastbound at the Dulles Connector Road, and the reduction in the number of lanes.

A third eastbound lane continues to Route 29 where it becomes an exit-only lane and the drivers who aren’t exiting have to merge their way back into the two remaining lanes. In the westbound side, there’s congestion between Fairfax Drive and North Sycamore Street because of the lane drop at North Sycamore.

Up for review
This traffic report is part of a package of information up for review at three public meetings that VDOT has scheduled for next week. You can see the reports in greater detail on this page. I’ll be writing next about the state’s HOT lanes proposals, which are meant to ease the congestion and make the travel times more reliable.

Each of next week’s sessions is from 6 to 8 p.m., with a presentation at 6:30 p.m. Here are the dates and locations.

Monday: Washington-Lee High School cafeteria, 1301 N. Stafford St., Arlington. (The snow date is Jan. 28, from 7:30 to 9:30 p.m., with the presentation at 8 p.m.)

Jan. 26: Stone Bridge High School cafeteria, 43100 Hay Rd., Ashburn. (The snow date is Feb. 5, from 6 to 8 p.m., with the presentation at 6:30 p.m.)

Jan. 27: VDOT Northern Virginia District Office, 4975 Alliance Dr., Fairfax. This one will be available for viewing on a live stream from Transform66.org at 6:30 p.m. (The snow date is Feb. 3, from 6 to 8 p.m., with the presentation at 6:30 p.m.)