Google says it has turned a corner in its pursuit of a car that can drive itself.
The tech giant's self-driving cars already can navigate freeways comfortably, albeit with a driver ready to take control. But city driving - with its obstacle course of jaywalkers, bicyclists and blind corners - has been a far greater challenge for the cars' computers.
In a blog entry posted on Monday, the project's leader said test cars now can handle thousands of urban situations that would have stumped them a year or two ago.
"We're growing more optimistic that we're heading toward an achievable goal - a vehicle that operates fully without human intervention," project director Chris Urmson wrote.
Urmson's post was the company's first official update since 2012 on progress toward a driverless car, a project within the company's secretive Google X lab.
The company has said its goal is to get the technology to the public by 2017. In initial iterations, human drivers would be expected to take control if the computer fails. The promise is that, eventually, there would be no need for a driver. Passengers could read, daydream, even sleep - or work - while the car drives.
Google maintains that computers will one day drive far more safely than humans, and part of the company's pitch is that robot cars can substantially reduce traffic fatalities.
The basics already are in place. The task for Google - and traditional carmakers, which also are testing driverless cars - is perfecting technology strapped onto its fleet of about two dozen Lexus RX450H SUVs.
Sensors including radar and lasers create 3D maps of a self-driving car's surroundings in real time, while Google's software sorts objects into four categories: moving vehicles, pedestrians, cyclists and static things such as signs, curbs and parked cars.
Initially, those plots were fairly crude. A gaggle of pedestrians on a street corner registered as a single person. Now, the technology can distinguish individuals, according to Google spokeswoman Courtney Hohne, as well as solve other riddles such as construction zones and the likely movements of people riding bicycles.
To deal with cyclists, engineers initially programed the software to look for hand gestures that indicate an upcoming turn. Then they realised that most cyclists don't use standard gestures - and still others weave down a road the wrong way.
So engineers have taught the software to predict the behaviour of cyclists based on thousands of encounters during the approximately 10,000 miles the cars have driven autonomously on city streets, Hohne said. The software projects a cyclist's likely movements and plots the car's path accordingly - then reacts if something unexpected happens.
"A mile of city driving is much more complex than a mile of freeway driving, with hundreds of different objects moving according to different rules of the road in a small area," Urmson wrote.
Before recent breakthroughs, Google had contemplated mapping all the world's stop signs.
Now the technology can read stop signs, including those held in the hands of school crossing guards, Hohne said.
While the car knows to stop, just when to start again is still a challenge, partly because the cars are programed to drive defensively. At a four-way stop, Google's cars have been known to wait in place as people driving in other directions edge out into the intersection - or roll through.
The cars still need work on other predictably common tasks. Among them, understanding the gestures that drivers give one another to signal it's OK to merge or change lanes, turning right on red and driving in rain or fog (which requires more sophisticated sensors).
And when will these and other problems be solved?
"You can count on one hand the number of years until people, ordinary people, can experience this," company co-founder Sergey Brin said in September 2012. He made the remarks at a ceremony where California Governor Jerry Brown signed legislation legalising the cars on public roads in the state.
To date, Google's cars have done more than a million kilometres in self-driving mode, the vast majority on freeways, the company said.
Google has not said how it plans to market the technology. Options include collaborating with major carmakers or giving away the software, as the company did with its Android operating system. While Google has the balance sheet to invest in making cars, that likelihood is remote.
Traditional automakers also are developing driverless cars. Renault-Nissan CEO Carlos Ghosn said he hopes to deliver a model to the public by 2020.
Five things to know about Google's cars, and their future.
MEAN STREETS
Google says its cars have now driven more than a million accident-free kilometres on freeways in "autonomous mode" - with the car in control, though a safety driver sits behind the wheel. That's the equivalent of about 120 San Francisco-to-Manhattan-to-San Francisco road trips.
With that success, Google has been focusing on city driving for about the past year. Freeways are relatively simple for the cars - no blind corners, no cyclists and no pedestrians. City streets have all that and more, including intersections and complex interactions with other drivers, such as who goes first at a four-way stop sign.
TO-DO LIST
Google says that in the past year, the Lexus RX450H SUVs it has retrofitted with lasers, radar and cameras rapidly learned how to handle thousands of urban driving situations. The robot's vision can now "read" stop signs (rather than rely on a map to plot them out) and differentiate between hundreds of objects in real time. It also can negotiate construction zones much more reliably.
But the technology is far from perfect. Improvements are needed in merging and lane changes, turning right on red and handling bad weather.
COMING TO A NEIGHBOURHOOD NEAR YOU?
Not in the near future - unless you live in Mountain View, California, where Google is located. So far, the tech giant has focused street driving in its hometown, which it has mapped parts of in tremendous detail. The mapping helps the car's computer make sense of its environment and focus on moving parts - other cars, cyclists and pedestrians.
Just four US states - California, Nevada, Florida and Michigan - and Washington, DC, have formally opened public streets to testing of self-driving cars, though testing is probably legal nearly everywhere (because it is not expressly banned).
THE FUTURE IS (NOT QUITE) HERE
In 2012, Google co-founder Sergey Brin predicted that the public would be able to get hold of the technology within five years. Google isn't revising that date. Initially, drivers would be expected to take control if the computer fails. Eventually, the vision goes, there would be no need for a person in the driver's seat - or at least not a driver who has to watch the road.
GOOGLE, THE CARMAKER?
While Google has enough money to invest in making cars, that likelihood is remote. More likely options include collaborating with major carmakers or giving away the software, as Google did with its Android operating system.
Meanwhile, traditional automakers are developing driverless cars of their own. Renault-Nissan's CEO said he hopes to deliver a model to the public by 2020.
