Alistair Barr & Mike Ramsey:

The cars are “a little more cautious than they need to be,” Chris Urmson, who leads Google’s effort to develop driverless cars, told a conference in July. “We are trying to make them drive more humanistically.”

Google is moving closer to commercializing self-driving cars, with the hiring earlier this month of auto-industry veteran John Krafcik as chief executive of its car project. One big remaining challenge is to make the cars, which have run more than a million miles on public roads, move more seamlessly among human drivers.

Since 2009, the cars have been involved in 16 minor accidents. In 12 of those mishaps, the vehicles were rear-ended. That’s a higher accident rate than the national average, but Google says national statistics exclude many minor accidents similar to those its cars have experienced.

Google said it wasn’t at fault in any of the crashes. But some allies say the vehicles’ habit of braking to avoid real, but marginal, risks may play a role.

“Why is it getting rear-ended? It drives like a computer,” said Jen-Hsun Huang, chief executive of Nvidia Corp., which designs powerful graphics processors that help Google’s cars recognize objects.