"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 programmed the software to look for hand gestures that indicate an upcoming turn. Then they realized 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 behavior 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 programmed 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 Gov. Jerry Brown signed legislation legalizing the cars on public roads in the state.
To date, Google's cars have gone about 700,000 miles in self-driving mode, the vast majority on freeways, the company said.
California's Department of Motor Vehicles is in the process of writing regulations to implement that law. Nevada, Florida, Michigan and Washington, D.C., also have written driverless car laws.
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.
Contact Justin Pritchard at https://twitter.com/lalanewsman
5 things to know about Google's self-driving cars
The director of Google's self-driving car project wrote in a blog post Monday that development of the technology has entered a new stage: trying to master driving on city streets. Many times more complex than freeways, which the cars can now reliably navigate, city streets represent a huge challenge.
Here are five things to know about the cars, and their future.
Google says its cars have now driven about 700,000 accident-free miles 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.
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 NEIGHBORHOOD NEAR YOU?
Not in the near future unless you live in Mountain View, Calif., 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 states California, Nevada, Florida and Michigan and Washington, D.C., 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 ahold 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.