One of many unknowns with driverless cars is what their impact on traffic will be. Boosters have predicted that driverless cars will reduce traffic by reducing the number of vehicles needed to serve travel demand and reducing following distances between cars. Detractors have predicted that these effects will be more than offset by latent demand for travel that will come out of the shadows when it gets easier to travel in a car, by being cheaper or less stressful, which will encourage people to travel more and longer distances.
We may be getting ahead of ourselves since we don’t know when true driverless cars will become available and achieve a large enough market share to have a big difference. However, it is worth considering what we can learn from differences in commuting time by travel mode.
The graph below presents the mean commute time for various travel modes for ten relatively large metropolitan statistical areas (MSAs) in the US. The data was compiled by Governing Magazine.
Just about all cities have mean drive alone commute times of 25-30 minutes, with Portland being the low at 24 minutes and DC the high at 32 minutes. Transit commute times are about 1.5 to 1.8 times as long as drive alone commute times, and commuter rail commute times are about 2.0 to 2.5 times as long. Commute times are remarkably consistent across MSAs. Interestingly, Riverside has the longest commuter rail commute time, which reflects its dual role as its own MSA and as LA/OC’s more distant suburbs.
Since many transit riders are captive riders, especially in places like LA and Riverside, perhaps it is best to focus on the comparison between commuter rail and drive alone, since many commuter rail riders are choice riders. The data strongly suggests that people are willing to put up with much longer commutes when they don’t have to drive the vehicle themselves, which would support the idea that people will increase their commute length if they can use driverless cars.
This data, from a 2009 Census report, aligns with that conclusion. Note that carpoolers, who have to drive some of the time but not all the time, are willing to put up with longer commutes than drive alone, but not as long as public transportation.
Driverless cars may be even more enticing than commuter rail, as there’s no inconvenience of having other passengers. On the other hand, unreliability caused by traffic may be a deterrent. The appeal of driverless cars will also be uneven, as certain types of work do not lend themselves very well to being done by oneself in a moving car. In addition, many people will not have the schedule flexibility to increase their commute; for example, if you have kids, you may not be able to or want to leave them in child care for a longer time every day. On the balance, though, it seems likely that if driverless cars become widespread, commuting times will get longer.