They obey the law all the time, as in, without exception. This may sound like the right way to program a robot to drive a car, but good luck trying to merge onto a chaotic, jam-packed highway with traffic flying along well above the speed limit. It tends not to work out well. As the accidents have piled up — all minor scrape-ups for now — the arguments among programmers at places like Google Inc. and Carnegie Mellon University are heating up: Should they teach the cars how to commit infractions from time to time to stay out of trouble?

“It’s a constant debate inside our group,” said Raj Rajkumar, co-director of the General Motors-Carnegie Mellon Autonomous Driving Collaborative Research Lab in Pittsburgh. “And we have basically decided to stick to the speed limit. But when you go out and drive the speed limit on the highway, pretty much everybody on the road is just zipping past you. And I would be one of those people.”

Last year, Rajkumar offered test drives to members of Congress in his lab’s self-driving Cadillac SRX sport utility vehicle. The Caddy performed perfectly, except when it had to merge onto I-395 South and swing across three lanes of traffic in 150 yards (137 meters) to head toward the Pentagon. The car’s cameras and laser sensors detected traffic in a 360-degree view but didn’t know how to trust that drivers would make room in the ceaseless flow, so the human minder had to take control to complete the maneuver.