How Many of Our Vehicles Could Be Electric? How Does 87% Strike You? - Bloomberg

87% would be a good start; for another timely transport trend, see Uber and Lyft Want to Replace Public Buses (Bloomberg)

"With all the limits on electric vehicles—battery life, cost, the availability of charging stations—you might expect that at most 50 percent of the vehicles on U.S. roads could be replaced by more-sustainable cars.
Buckle up: It’s 87 percent, MIT reckons, in a study published Monday in the journal Nature Energy. Such a proportion, if it were the case today, would lead to a 60 percent reduction in total U.S. gasoline consumption and a 30 percent decrease in the 1.8 billion metric tons of greenhouse gas emissions emitted by all American transportation in 2014.1 Transport represents 26 percent of America’s total greenhouse gas emissions.
That's huge. It's also a thought experiment. Currently 0.7 percent of vehicles in the U.S. are electric, and plug-in electric vehicle sales declined 17 percent from 2014 to 2015."

How Many of Our Vehicles Could Be Electric? How Does 87% Strike You? - Bloomberg