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:

  • 333 cars: 67 with clean-car license plates; 266 without.
  • 1 motorcycle
  • 2 buses
  • 1 18-wheeler! (which are illegal on this highway)
  • 1 eastbound 6-car Metro train

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

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

  • 2700 HOV cars with 2.2 riders = 6,000 people
  • 670 clean-fuel cars with 1.1 riders = 750 people
  • 20 buses with 40 people = 2,000 people
  • 10 Metro trains with 800 riders = 8,000 people

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?

Steve Offutt has been working at the confluence of business and environment for almost 20 years, with experience in climate change solutions, green building, business-government partnerships, transportation demand management, and more. He lives in Arlington with his wife and two children and is a cyclist, pedestrian, transit rider and driver.