Even as automated vehicles such as robotaxis grow more prevalent in the U.S., almost every mass transit system still requires a driver or operator on board. Two major cities are seeking to change that.

Officials in Washington, D.C., recently advanced a plan that could let computers fully control one of its subways, and Los Angeles is seeking to build a new, completely automated line. Honolulu already has one.

The Washington and LA projects, which would take years to implement and still face financial and technical hurdles, come amid an already fierce debate over the safety risks and benefits of automated trains and whether the substantial upfront expense is worth the long-term savings in labor costs.