How can I decline carpooling?

I've had this very same situation at work - my solution was to fill my empty seats with office equipment and tennis gear! It seems that you are already going in that direction, so how about this:

Beat them to the punch by volunteering somebody else;

Cleverly talk about how your vehicle is brimming with stuff.

Best luck, man.

Just turn the tables on whoever volunteers you for these services, and say," no I can't do it but I'm sure you can, since it is your idea". I would think that would shut them up. If that doesn't work just come out and say my rear seat is missing, my jeep is full of gear, I have no room, sorry 'bout that.

I guess they haven't invented public transit yet in America. In UK and European cities such as Paris, Berlin, Rome etc. the majority of people travel to/from work on public transit

Just say you prefer not to as your schedule varies greatly and you don't want anyone left in the lurch because you are unable to fulfill your obligation. If they continue to press you then say again that you just do not have the time.

Sorry find someone else. I'm not a taxi service. I know it sound rude. But they will get the point.

Simply say that because of your schedule, you may have to leave early, so you can't take any passengers.

Don't beat around the bush. Just tell them your not up for it

Charge em by the mile