

They've 700,000 miles, but mostly the same few thousand miles, over and over again — because the cars only work if every single light, piece of street furniture, and other detail is mapped and verified by armies of human and computer analysts, and when anything changes, the mapping needs to be re-created.

Another problem with maps is that once you make them, you have to keep them up to date, a challenge Google says it hasn't yet started working on. Considering all the traffic signals, stop signs, lane markings, and crosswalks that get added or removed every day throughout the country, keeping a gigantic database of maps current is vastly difficult. Safety is at stake here; Chris Urmson, director of the Google car team, told me that if the car came across a traffic signal not on its map, it could potentially run a red light, simply because it wouldn't know to look for the signal. Urmson added, however, that an unmapped traffic signal would be "very unlikely," because during the "time and construction" needed to build a traffic signal, there would be adequate opportunity to add it to the map.

But not always. Scott Heydt, director of marketing at Horizon Signal Technologies, says his company routinely sets up its portable traffic signals at road construction sites. Frequently, they are simply towed to a site and turned on. "We just set one up like that in New Jersey," said Heydt. "You can be driving to work and everything is normal, but on your way home, discover a new traffic light." (Of this possibility, a Google spokesperson said, "We will have to be ready for that.")

Noting that the Google car might not be able to handle an unmapped traffic light might sound like a cynical game of "gotcha." But MIT roboticist John Leonard says it goes to the heart of why the Google car project is so daunting. "While the probability of a single driver encountering a newly installed traffic light is very low, the probability of at least one driver encountering one on a given day is very high," Leonard says. The list of these "rare" events is practically endless, said Leonard, who does not expect a full self-driving car in his lifetime (he's 49).