Refers to the smoking of marijuana or any other packable substance in a moving vehicle without stopping or parking. The passenger will usually pack the bowl, and once packed, the bowl is smoked by the driver and is passed around the car until it is spent, all while driving.
"Riding a bowl can be dangerous, if you don't know how to drive in the first place."
"There was no safe place to park, so we had to ride the bowl."
"Last time we rode a bowl, we ended up in Newark, from Princeton. When we came to, the driver's excuse was that he wanted to watch the planes."