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Google has shed some light on why one of its driverless cars was stopped by police for driving too slow last month, an incident that attracted widespread attention as the vehicles continue to capture the public's imagination.
The firm does not send its self-driving car on roads with a speed limit above 35 mph. In its monthly report on its autonomous vehicle project, Google explained why.
"First, slower speeds were easier for our development process. A simpler vehicle enabled us to focus on the things we really wanted to study, like the placement of our sensors and the performance of our self-driving software," Google's report said.
"Secondly, we cared a lot about the approachability of the vehicle; slow speeds are generally safer (the kinetic energy of a vehicle moving at 35 mph is twice that of one moving at 25 mph) and help the vehicles feel at home on neighborhood streets."
Google also revealed that its cars have built up a "library of various sirens," and taught its software to identify them, so when an emergency vehicle approaches, the cars will "drive more conservatively until it has a better sense of where the sirens are coming from." The autonomous cars are also equipped with cameras that can detect flashing lights so if an emergency vehicle is coming through an intersection, the car can stop and will resume when it is safe.
After two months of driving without an accident, Google's self-driving car was involved in an incident in November. The autonomous vehicle was signaling a right turn. It came to a complete stop at a red light, then began to slowly move forward to get a better view of traffic from the left to determine if it was safe to make a right turn. A vehicle from behind rolled into the Google car at a speed of 4 mph. There were no injuries and the Google car sustained "minor damage" to the rear bumper.
In the six years of the project, Google said it has been involved in 17 minor incidents, and completed more than 2 million miles of autonomous and manual driving combined. Google added that the driverless car has never been the cause of an accident.
Google has been working on ways to make its vehicle fit into current road conditions safely. It was granted a patent last week in which it outlined how a driverless car could interact with a pedestrian through a flashing sign on its doors and bumper as well as a robotic hand to make gestures.