The problem is less about trusting the autonomous bus to safely drive it route as it is about trusting the machine to properly abide by the Americans with Disabilities act. "You're going to need a human being there to take care of people," Proterra CEO Ryan Popple told SFGate. "I'm not comfortable yet with the idea that an ADA passenger could get on an autonomous vehicle, and that we could code for every possible disability." There simply needs to be someone around to make sure riders board okay, settle any possible disputes or, if it's a School bus, take care of the children.

Before Proterra sorts out the human element, however, it does need to lock down the actual autonomous driving. That process will be deliberately slow -- starting by outfitting the company's electric buses with sensors that can scan the street during normal human-driven routes and test how well the bus can detect traffic, pedestrians and passengers. The program will slowly add more complexity, developing and testing self-driving algorithms slowly until the team is confident to run a real life test.