Self-driving cars are expected to usher in a new era of mobility, safety and convenience. The problem, say transportation researchers, is that people will use them too much.
Researchers believe the number of miles driven will skyrocket. It's less certain whether that will mean a corresponding surge in traffic congestion, but it's a clear possibility.

A study last year by the International Transport Forum, a transportation policy think tank, simulated the impact on traffic in Lisbon, Portugal, if conventional cars were replaced with driverless cars that take either a single passenger at a time or several passengers together.