Emily Badger has a useful article on casual carpools, though it would be a little more useful if she -- or her editors at Miller McCune -- didn't keep implying that public transit is somehow the enemy.
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, usually 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 that's when the HOV lane's advantage is substantial.
For many, it's fun to think of casual carpooling as some sort of revolt against conventional transit. The term slugging, Badger explains, arose as an insult uttered by "bitter bus drivers" who saw their waiting passengers disappearing into private cars. Miller McCune's headline describes slugging as "the people's transit," as though conventional transit is something else.
In fact, casual carpooling or "slugging" is largely compatible with conventional transit. Really, the two are mutually beneficial. The casual carpool markets in San Francisco and Washington are both parallel to rapid transit lines, but the trains are still full. As for competition with peak bus services, the long one-way commuter bus run is one of the most expensive services a transit agency can operate. Often, each bus can be used for only one run during each peak, so all the costs of owning and maintaining the bus must be justified by a single trip. Drivers for these peak buses are also expensive, because there are costs associated with the short shifts that peak-only service requires, and because drivers must usually be paid to get back to where the shift began before clocking out.
Long commuter bus runs can still make sense, but they are very expensive compared to conventional two-way, all-day transit. If casual carpooling reduces the demand for them, the effect on transit is to flatten the overall peak that transit has to serve, increasing its potential cost-effectiveness and improving the utilization of fleet. It's especially helpful on the AM peak, which is usually the sharper of the two.
So slug away, if you need to feel that you're attacking something. I prefer to call it a casual carpool, because that term describes what it really is. And I see no reason not to welcome them. In fact, when new HOV lanes are developed, the casual carpool phenomenon should be planned for, both by ensuring that there are safe and logical pickup points and also by counting casual carpool trips in the mobility benefits of the lane.
Of course, such planning would contradict the libertarian fantasy -- heavily stressed in the Miller McCune piece -- that casual carpooling is a "government-free" form of spontaneous social organization, a kind of Tahrir Square for the cul-de-sac set. In fact, "slugging" is a freely chosen response to the design of the government-funded transport infrastructure -- just like everybody else's commute.