Waymo is about to launch a pilot program to explore how self-driving cars can be used in combination with public transit, the Google self-driving car unit announced on Tuesday. Starting in August, employees of Valley Metro, the agency that operates the Phoenix area's bus and light rail systems, will be able to get Waymo rides to their nearest bus or light rail stop.

Eventually, Waymo hopes to open this service up to the general public, providing first- and last-mile service for customers who want to use transit but aren't quite close enough to walk to the nearest stop.

Two visions for transit in a driverless future

An important question about the rise of self-driving cars is whether the technology will complement conventional public transit or replace it. Back in May, we talked to Thomas Bamonte, an official at the North Central Texas Council of Governments. The council has been involved in self-driving transit projects in Texas, including the startup Drive.ai's first shuttle service in the Dallas suburb of Frisco.

As a transit official working on self-driving car projects, Bamonte has thought a lot about how to integrate public transport and self-driving cars. And he told me that there are two basic models for transit in a self-driving future. One possibility is that conventional bus routes will gradually be replaced by fleets of self-driving shared vans that operate along flexible routes, eventually rendering conventional buses—and perhaps even conventional subways, commuter rail lines, and street cars—obsolete.

But the other possibility, Bamonte told me, is a "first-mile, last mile" model. In this model, self-driving cars could complement transit by helping more people get to conventional transit stops. If you're a suburbanite who works downtown but lives a mile or more from the nearest bus or subway stop, taking transit may not be a very appealing option. Theoretically, you could take a conventional taxi to the nearest transit stop, but taxis today are far too slow and expensive for this to be practical.

Self-driving cars could change the economic calculation. With no driver to pay, a self-driving taxi ride could be much cheaper than a conventional taxi. And with no driver, a self-driving car might be more willing to show up a few minutes early, ready to go as soon as the customer is. Hence, self-driving cars could actually expand transit use.

Waymo's latest announcement is a sign that the company is interested in this last-mile approach. The announcement is an attractive early application for Waymo because Waymo's technology requires the company to construct detailed maps of the areas where its service will operate. Offering a last-mile transit service allows Waymo to expand its service map in a controlled way, adding support for more transit stops as it collects more map data.

Eventually, this kind of first- and last-mile service could become a standard part of transit services, with customers buying a single multi-mode ticket that covers both the short taxi rides at either and of the trip and the bus or train ride in between. Self-driving taxis could have realtime data on transit schedules and let passengers know when they need to leave to catch the next train or bus. By offering service to Valley Metro employees first, Waymo may be hoping to build relationships that could eventually lead to this kind of collaboration.

The announcement is also the latest sign that Waymo is thinking hard about practical use cases for its technology. Last week, the company announced another pilot program to ferry customers to and from their nearest Walmart store. Initially, that pilot program will be limited to the same group of about 400 "early riders" who have had access to Waymo's fleet for months. But it will presumably be available to the general public once Waymo launches its commercial service—something Waymo says will happen before the end of the year.