Who’s causing congestion on I-66? Hybrids or scofflaws?

RoadsBy Steve Offutt (Guest Contributor) February 20, 2011

Photo by afagen on Flickr.

An informal count of cars on I-66 in Arlington just east of Sycamore Street seems to indicate that the clean-fuel exemption may be a factor in slowing traffic down without providing significant transportation value.

Hybrids and other clean-fuel cars are exempted from the HOV requirement on I-66. All vehicles inside the Beltway on I-66 must be HOV-2, be a motorcycle or have a clean fuel exemption license plate.

Dr. Gridlock has suggested removing this exemption to help the HOV lanes function better. I believed instead that the problem was caused more by scofflaws—single occupancy drivers illegally on the road—than the hybrid cars, based on times I drove on I-66 and observed the cars around me.

Recently I had a chance to test this presumption. I was crossing over I-66 on the pedestrian overpass near Madison Manor Park (map).

I tried to count how many cars were scofflaws but soon realized that it was difficult to tell if there was a child in the back seat or not. Also, many of the cars had the clean-fuel license plates

From this vantage point, the license plates could be read fairly easily, so I chose to count the plates instead. Traffic seemed to be traveling about 30-40 miles per hour, slower than free-flowing.

Plates with CF, CX, CY or CZ are exempt from the HOV requirement. (One can have a vanity plate exempted, too, but there were few of these; but how should one count the car with the plate GOVEGAN?) The following cars passed between 8:04 and 8:10 AM:

Of the cars, 67 had clean-fuel license plates, approximately 20%.

Here’s some quick math extrapolating to an hour and making some assumptions:

The clean-fuel cars represent less than 5% of the people being transported along this corridor but represent 20% of the cars. Would removing that 20% increase flow enough or more than enough to make up the difference? Would increased flow entice more HOV cars onto I-66?