Driverless Cars

The first driverless cars are being tested, and a number of new cars are automating some things that people find difficult or dangerous, like parallel parking and emergency stops.  The potential is enormous--for safety.  But as long as there are human drivers on the roads, they provide little improvement in the capacity of our roads.

The safety advantage is enormous.  The cars use a radar or other sensor to detect a foreign object, including pedestrians and other cars, and figure out if a collision is imminent, and if it is, apply the brakes to avoid it.  This is actually fairly straightforward to do, although it is a big engineering challenge, and would have been impossible with the computers of just a few years ago.

But what they do not do is improve road capacity.    While there are human drivers on a road, all cars must obey the same separation rules, speed rules, traffic signals, etc.  Unless there are special, reserved roads for robocars, they can't go any faster, drive any closer together, ignore traffic lights because they know the crossing traffic is also a robocar.   There are, however, a few places they might give a small capacity advantage.  For example, today's short term rental systems, such as zipcar and car2go, require that user go to where the car is being stored. A driverless rental car could come when called, just like a taxi, obviating the need for a large number of parking places.  Or instead of being used like a taxi, it could be used as automated valet parking.   A driverless car is potentially enough cheaper than a human-driven taxi or parking valet that it would increase their use significantly--getting a lot of cars off of our dense urban streets and making the best use of parking space.   But it doesn't help capacity at all in places where parking is already easy to come by: the suburbs.

The transit term for reserving a road so that all vehicles on it can obey special rules is called "grade separation".  This is commonly used for heavy rail vehicles, such as subways and elevated trains.  If small, independently scheduled and routed vehicles are used on such a road, it is called "Personal Rapid Transit".  It is more efficient to use special vehicles and guideways for such a system.  Vehicles that can be driven on roads shared with human drivers are much more complex and heavier than dedicated PRT vehicles.

Where driverless cars have a big advantage is when we can give over a part of the road completely to them.  For example, a dedicated lane on long highways where driverless cars are allowed to draft (including bump-draft)  and go at much higher speeds than would be safe for humans.  This is not a big capacity advantage, but it can be a big throughput advantage, and the passengers are safer and can rest or recreate during their journey.