A municipal judge in Phoenix ruled in 2006 (Snopes, NBC) that a woman was properly ticketed for using the carpool lane, despite her argument that she was in fact riding with a "passenger" because she was pregnant.
A good question for Alito, if it's not too late. [UPDATE: nobody asked him.]
Candace Dickinson was stopped on November 8, 2005, for using the carpool lane without a passenger. The officer who stopped her said she pointed to her "obvious pregnancy" in hopes of avoiding a ticket, and after being fined $367, she contested the ticket in court on the grounds that her unborn fetus was a "passenger." Judge Dennis Freemen rejected that argument, saying that the common sense meaning of the term, and the intent of the law, required the passenger to be one who occupied a "separate and distinct . . . empty space in a vehicle." Dickinson's womb did not qualify.
Besides, as the officer argued in court, this theory would require officers to carry pregnancy testers to defeat sham excuses, "and I don't think we want to go there."