Toyota’s cars alone, he figured, log perhaps one trillion miles of annual travel around the globe. Making a robocar perform in controlled demonstrations is easy, Mr. Pratt says, such as having it effortlessly avoid hay bales tossed in front of it. Making a robocar so foolproof that consumers and automakers can trust it with their lives, including in one-in-a-billion situations, is very different.

“Ever since, we’ve tried to turn down the hype and make people understand how hard this is,” he said.

That’s not preventing companies from trying. Toyota’s Chauffeur technology fully intends to create autonomous cars for corporate fleets. But using 80 to 90 percent of the same software, its Guardian concept blends inputs from man and machine.

General Motors’ Cadillac is also working to keep humans in the driving loop — even if it requires an occasional slap on the wrist, via the driver-monitoring system developed by an Australian company, Seeing Machines.

Consider Cadillac’s Super Cruise the digital disciplinarian that makes drivers sit straight and keep eyes up front. It is G.M.’s consumer answer to Tesla’s Autopilot, but its approach illustrates the divergent philosophies of traditional automakers and the Valley rebels.

Many experts say Super Cruise, or a system like it, might have prevented the highly publicized fatal crashes of some Tesla Autopilot users, or Uber’s robotic Volvo that struck and killed an Arizona pedestrian in March last year. In the Uber case, police investigators said the human backup driver had been streaming Hulu before the accident. In some Tesla crashes, driver overconfidence in Autopilot’s abilities, leading to inattention, appears to have played a role.