The Department of Transportation reports that in 100% of official studies, HOV lanes have positively affected surrounding air quality.
Each car taken off the road gets rid of tons of CO2 emissions. Currently, in the U.S., there are more registered cars than licensed drivers . Transportation is a close 2nd for all Greenhouse Gas Emissions amounting for 27% of all CO2 emissions, with light and medium duty vehicles as the main emitters.
So if you won’t do it for your social life, and you won’t do it for your wallet, just do it for the environment.