But city driving -- with its obstacle course of stray walkers, bicyclists and blind corners -- has been a far greater challenge for the cars' computers.
In a blog entry posted today, 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.