Once behind the wheel of the modified Lexus SUVs, the drivers quickly started rummaging through their bags, fiddling with their phones and taking their hands off the wheel — all while traveling on a freeway at 60 mph.

“Within about five minutes, everybody thought the car worked well, and after that, they just trusted it to work,” Chris Urmson, the head of Google’s self-driving car program, said on a panel this year. “It got to the point where people were doing ridiculous things in the car.”

After seeing how people misused its technology despite warnings to pay attention to the road, Google has opted to tinker with its algorithms until they are human-proof. The Mountain View, Calif., firm is focusing on fully autonomous vehicles — cars that drive on their own without any human intervention and, for now, operate only under the oversight of Google experts…

“I’ve had people say, ‘Look, my Windows laptop crashes every day — what if that’s my car?’ ” Urmson said at a conference held by The Times on transportation issues. “How do you make sure you don’t have a ‘blue screen of death,’ so to speak?”