With the passage of time, cars have become far and away the No. 1 means of getting us to our daily jobs; three-quarters of the people who drive to work drive alone, and that number is going up. But those same ever-lengthening commutes have become a time sink that threatens to put the brakes on our economies and our productivity. Ask mayors of big cities what keeps them up at night. Chances are, it’s not the threat of terrorism. It’s gridlock.