So looking at my county, and assuming I’ve got the arithmetic right, we’ve got:
429,597 total commuters, and
275,220 commuting by car (drive alone)
74,562 by transit
9,038 by walking
and 1,264 by bike.
That’s a total of 360,084, which leaves a remainder 69,513 who commute either by car (not alone) or multi-modal?
I’d guess carpooling in Prince George’s county is a relatively small share of the remainder, but I don’t know. Perhaps bike/transit and walk/transit would be the remainder of the remainder? Or maybe the survey just got lots of non-responses to the question?