General Motors Co.’s top executive expects it will take decades for electric vehicles to take over as the dominant form of transportation but sees driverless cars on the road within five years. Chief Executive Officer Mary Barra said American drivers will go electric, but it will take a long time for most of the 250 million vehicles on U.S. roads to be battery powered. “We believe the transition will happen over time,” Barra said on “Leadership Live With David Rubenstein” on Bloomberg Television. When asked if all cars will be electric in 20 years, she said that may be too soon. “It will happen in a little bit longer period, but it will happen.”