Well let's attack this from a different perspective. So long as ANYBODY regardless of race, creed, sociopolitical standing, etc. etc. etc. can use the carpool lane as long as s/he has one or more passengers, how does that violate the 14th amendment? Seems to me equal protection means that everybody is treated the same under the law. So the law restricts a lane to two or more people in a vehicle.
Same as laws that say you can't be married to more than one person or you can only have so many dogs or cats on your property or you can't have the big party booth if its just you and your date in the restaurant.
Same principle. Except the party booth, the restaurant is private property.
Treating any two groups differently whether it's on the basis of hair color or how many people in a car is a form of discrimination, but it's not unconstitutional if it passes the appropriate level of review. For all but the big ones (race, religion, etc.) that's rational basis. The law can be really stupid, but the 14th doesn't care about whether a policy is smart. It leaves that to the legislatures and hopes for the best. It only cares whether the discriminatory law is rationally related to acheiving a legitimate interest or policy objective.
