A letter from Tesla to the California Department of Motor Vehicles has outlined details of its autonomous vehicle testing in the area, conducted between November 30, 2016, and December 1, 2017.

The letter first states how Tesla develops its vehicles using simulation in laboratories, on test tracks, and on public roads in various locations around the world.

The letter continues, “Additionally, because Tesla is the only participant in the program that has a fleet of hundreds of thousands of customer-owned vehicles that test autonomous technology in ‘shadow mode’ during their normal operation (these are not autonomous vehicles nor have they been driven in autonomous mode as defined by California law), Tesla is able to use billions of miles of real-world driving data to develop its autonomous technology.

“In ‘shadow mode’, features run in the background without actuating vehicle controls in order to provide data on how the features would perform in real-world and real-time conditions. This data allows Tesla to safely compare self-driving features not only to our existing Autopilot advanced driver assistance system, but also to how drivers actually drive in a wide variety of road conditions and situations.

“For Reporting Year 2017, Tesla did not test any vehicles on public roads in California in autonomous mode, as defined by California law. As such, the company did not experience any autonomous mode disengagements as part of the Autonomous Vehicle Tester Program in California.

“As described above, Tesla analyzes data from billions of miles of driving received from our customer fleet via over-the-air (OTA) transmissions. We supplement this with data collected from testing of our engineering fleet in non-autonomous mode, and from autonomous testing that is done in other settings, including on public roads in various other locations around the world.

“Through all of this data we are able to develop our self-driving system more efficiently than only by accumulating data from a limited number of autonomous vehicles tested in limited locations.”