What effect will robot cars have on traffic congestion?

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.

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.