[logn]

All unlicensed passengers would create more traffic by taking a taxi alone. I think the only ones not allowed in the carpool lane should be a taxi with one passenger (of course, in addition to the lone drivers).

However, you could extend the logic further by saying that a lone taxi driver looking for a better fare causing a second taxi to pick up the unlicensed driver causes more traffic than a taxi with one passenger. So I've neatly saved the current carpool lane rules.

[gte910h]

You want to keep taxi's allowed to use the lanes because then taxis are as fast as renting a car, or owning a car, so you you do not do the above behaviors.

If getting to the airport took 2x as long in a taxi, few would use that.