The march for fully autonomous driverless cars marches on. In May, Google announced the Next Phase in Driverless Cars: No Steering Wheel or Brake Pedals.

Google's prototype for its new cars will limit them to a top speed of 25 miles per hour. The cars are intended for driving in urban and suburban settings, not on highways. The low speed will probably keep the cars out of more restrictive regulatory categories for vehicles, giving them more design flexibility.

Google is having 100 cars built by a manufacturer in the Detroit area, which it declined to name. Nor would it say how much the prototype vehicles cost. They will have a range of about 100 miles, powered by an electric motor that is roughly equivalent to the one used by Fiat's 500e, Dr. Urmson said.