About one out of four vehicles using the I-395 HOV lanes in 1999 carried a slug, said Valerie Pardo, a senior transportation engineer for the Virginia Department of Transportation.
"It's a big factor in the success of those HOV lanes out there," Pardo
takes vehicles off the road by allowing people to share rides without having to commit to the strict meeting times of most carpools, Pardo
But unlike I-395, where HOV lanes require vehicles with three or more people, those in Maryland require only two people.
That's probably a key reason slugging hasn't caught on in Maryland or along I-66 in Northern Virginia, where the carpool lanes also require only two people, Pardo
HOV-2 lanes also are available on the Dulles Toll Road.
It's not as difficult to find one neighbor or co-worker to carpool with as it is to find two, Pardo
said, many people don't feel comfortable getting into a car alone with a stranger, while they might feel fine in a group of three travelers.
"If there's someone else getting in there with you, it's a shared risk," Pardo
Engineers determine how many people per car should be required for the carpool lanes by measuring how the road space can be used most efficiently.
They require enough people per vehicle to limit use of the lane while still attracting enough vehicles to avoid wasting the road space.
For example, when VDOT officials in 1999 considered making the I-395 carpool lanes HOV-2 instead of HOV-3, Pardo
said, they found that it would allow so many additional vehicles into the carpool lanes that they would back up from the Pentagon to the Beltway.