After several years of testing self-driving cars on freeways, where driving conditions are more predictable, Google in the past year shifted its focus to city street driving, the company said in a post on its official blog on Monday.

Google said it has driven thousands of miles on the streets of Mountain View, California, a small suburban community where the company maintains its headquarters roughly 35 miles South of San Francisco. Google's driverless cars rely on video cameras, radar sensors, lasers and a database of information collected from manually driven cars to help navigation, according to the company.

"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," wrote Chris Urmson, the director of Google's self-driving car project in the blog post on Monday.

"We’ve improved our software so it can detect hundreds of distinct objects simultaneously - pedestrians, buses, a stop sign held up by a crossing guard, or a cyclist making gestures that indicate a possible turn," Urmson said.

Google is one of several companies, including automakers Nissan Motor Co, Volkswagen AG's Audi and Toyota Motor Corp, testing self-driving car technology. Both Nissan and Mercedes-Benz parent Daimler AG say they plan to start selling self-driving cars by 2020.

It is unclear whether Google, the world's No 1 Internet search engine, intends to partner with other companies or develop its own self-driving vehicles.

The company posted a video that depicted how a self-driving car views the world as it navigates.

Google's test cars have logged more than 700,000 miles in self-driving mode since 2009. Google said its cars have not caused any accidents while operating in self-drive mode.

Google said it still has many "problems to solve," including teaching the car to drive more streets in Mountain View, before testing on the streets of another town.