How Much Congestion does the Clean Fuel Exemption Cause on I-66 Inside the Beltway?

Hybrids and other clean-fuel cars are exempted from the HOV requirement on I-66.  All traffic inside the beltway on I-66 must be HOV-2, be a motorcycle or have a clean fuel exemption license plate.  Dr. Gridlock at the Washington Post has argued to remove this exemption to help the HOV lanes function better.  It has been my opinion that the problem was caused more by scofflaws--that is, single occupancy drivers illegally on the road--than the hybrid cars.

Today I had a chance to test my presumption.  I was crossing over I-66 on the pedestrian overpass near Madison Manor park (map).  I tried to notice 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-car license plates.  Then I realized I could read the license plates easily.  Traffic seemed to be traveling about 30-40 miles per hour, slower than free-flowing.  So one could argue that there was congestion, since the HOV lanes were not moving at 55 miles per hour.  So I started counting.

Plates with CF, CX, CY or CZ are exempt from the HOV requirement.  (One can have their vanity plate exempted, too, but there were few of these).  I pulled out a piece of paper and a pen.  Here's what I observed in six minutes from 8:04 to 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 car license plates, approximately 20%. 

Here's some quick math extrapolating to an hour and making some assumptions:
  • 2700 HOV cars with 2.2 riders = 6000 people
  • 670 clean-fuel cars with 1.1 riders = 750 people
  • 20 buses with 40 people = 2000 people
  • 10 metro trains with 800 riders = 8000 people
So 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.  My intuition says yes, but perhaps a transportation planner can weigh in on the comments.

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