The following of almost all rules requires taking your self interest to a more meta-level. In this case, yes, it's one less car sharing your lanes. However, it

The following of almost all rules requires taking your self interest to a more meta-level. In this case, yes, it's one less car sharing your lanes. However, it's reducing the incentive for other people to carpool. That erosion of rules can substantially increase the number of cars sharing your lanes.