Human error is the culprit in 93 percent of automobile crashes — including the pileup last weekend that left Tracy Morgan in critical condition, caused, prosecutors say, by a truck driver who had been awake for 24 hours.

Robots, on the other hand, don’t need to sleep. Nor do they get drunk or distracted by cellphones. That is why Marc Andreessen, the venture capitalist, wrote on Twitter about the accident, with his usual bravado, “Self-driving cars and trucks are a moral imperative.”

How much safer would driving be if robots replaced humans on the roads?

It has been hard to estimate because fully autonomous cars are not yet available to test. Google says that its driverless cars have logged more than 700,000 miles without an accident caused by the car, and that its cars do not do unsafe things that people do, like sharply accelerating or braking.