Casual carpooling — or “slugging” as some of its partisans like to call it — is a perfectly rational response to very congested freeways with High Occupancy Vehicle (HOV) lanes. At informal queues located near an onramp, motorists who want to use the HOV lanes meet up with other commuters who want to ride the lanes as passengers. These passengers fill the empty seats in the motorist’s car so that they can all travel in the HOV lane. The phenomenon appears to happen where and when an HOV lane offers quite dramatic travel time savings, as it does on certain Washington DC freeways and on the San Francisco Bay Bridge. It happens only in intensive commute periods, because those are the time when the HOV lane’s advantage is substantial.
You could say that these meeting points arise spontaneously, but in my experience they’re usually a safe spot just upstream of a freeway onramp, so it’s not as though they’re hard to invent or find. And while it’s fun to think of it the phenomenon as cutting edge, it appears to be as old as the HOV lane.