cpzilliacus notes the difficulty the Dulles exemption causes on I-66. Another factor is that according to VDOT, the exemption only applies if you're going to or from the airport to depart on a flight or to pick up someone who's flying in. VDOT claims the exemption doesn't apply if you're putting gas in the car, buying coffee, etc.–that is, all the loopholes people use to get onto the Dulles Access Road.
I have no clue to what extent or how well this part of it is enforced.
It's odd that the way it's stated below doesn't allow for an exemption for returning from the airport after dropping off a departing passenger or for travel after you yourself arrive on an inbound flight.
This is from the "Exceptions" section at the bottom of http://www.virginiadot.org/travel/hov-rulesfaq.asp:
Motorists traveling to and from Dulles International Airport to go to the airport to board a flight or to pick someone up at the airport are permitted to use I-66 inside the Beltway (I-495) during HOV hours. Motorists traveling to or from Dulles International Airport are not exempt from HOV restrictions on I-66 outside the Beltway (I-495). You are not permitted to use I-66 inside the beltway during HOV hours if you are going to the airport to eat, get coffee, get gas or any other reason other than boarding a plane or picking someone up at the airport.
That seems not to be reflected by reality. Of course, once the tolling on I-66 between I-495 and the Rosslyn Tunnel starts, this becomes mostly irrelevant.
Consider the "captive"
Washington Flyer Taxi fleet, which has a
contractual right (granted by the Metropolitan Washington Airports Authority (MWAA) and enforced by the MWAA Police) to pick-up most persons (arriving passengers or otherwise) desiring a taxi ride
from Dulles Airport.
On more than one weekday afternoon during the westbound HOV restricted period, I have observed dozens of these cabs headed back to Dulles via I-66, the Dulles Connector Road and the Dulles Access Road, with only the driver in the vehicle (nearly all of the Washington Flyer cabs pick-up fares at the airport, but
very seldom carry a paying customer
to the airport). Yes, I suppose they can be considered to be headed to the airport to get another fare (is that construed as "picking someone up at the airport"?).
In the mornings, there are other cabs (usually licensed by Arlington County (Va.); City of Alexandria (Va.); the District of Columbia; or (rarely) Prince George's County (Md.)) who run empty eastbound from the airport on the Dulles Airport Access Road, then onto the Dulles Connector and eastbound I-66. Since they are not involved directly in someone "boarding a plane" (and can presumably not prove it if they are stopped), are they in violation?
Personally, I have had business at Dulles associated with my job (but not boarding a plane) where I needed to get from D.C. to the airport in the afternoon, and have used I-66 and the Dulles Connector/Access Road with a clear conscience.
The deadheading cabs do not appear to meet the conditions quoted above, nor did I.
As an aside, I have seldom seen drivers of cabs purchase fuel at the Dulles gas station (previously Exxon, now Sunoco). Presumably they purchase fuel at off-airport locations at lower cost.