Google announced that its self-driving car test project surpassed the two million mile mark in total miles driven on public roads.

Launched in 2009, the autonomous fleet continues to log about 25,000 miles of test drives per week, Google said. Putting that many miles on public roads has given the company much data on complex situation like cars going the wrong way to bicycles darting in front of traffic, according to Reuters.

As shown in Google’s monthly reporting chart above, the test fleet consist of 34 Google prototype self-driving cars and 24 Lexus RX450h SUVs fitted by Google with self-driving technologies. Google riders are going along on test runs in Mountain View, Calif., Kirkland, Wash., Phoenix, and Austin, Texas.

Google has taken a tough stand on government allowing for fully autonomous vehicles, versus semi-autonomous or vehicles that gradually evolve into autonomous. Google believes that fully automated vehicles will drastically reduce traffic fatalities, and will provide mobility lacking for the disabled and aged. Last year, the company said it could have autonomous vehicles ready for production by 2020.

While Ford Motor Co. would agree with Google’s end goal, most other automakers would prefer to see a more gradual and evolving plan adopted. Tesla’s Autopilot crashes have given weight to the argument that autonomous systems will need a good deal of testing and monitoring. Some would argue that the fatalities reinforce the argument that humans should always have the ability to take back control of their vehicle.

While Tesla said it’s logged over 100 million miles since last October with Model S and Model X drivers using its partially autonomous Autopilot system, Google said that its experience is much different. Miles driven on predictable highways, typical to Autopilot use, are easier than navigating busy city streets, said Dmitri Dolgov, head of Google’s self-driving technology effort.

“You have to have a deeper understanding of what’s on the road and to the side,” Dolgov told Reuters. “Every time you drive it’s different.”

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Reuters’ time in a self-driving Lexus near Google’s Mountain View campus illustrated these recurring risks. The Lexus reacted to individuals in wheelchairs, people pushing strollers, and a car making a U-turn. The vehicle’s driving system calculates the probability of fast, unpredictable movements and uses the results to determining how the vehicle will react.

These types of complicated social interactions with pedestrians, bicyclists, and other vehicles are the last, most difficult element of autonomous vehicle technology, Dolgov said.

“You get to 90 percent autonomy in 10 percent of the time and then spend 90 percent of your time on the last 10 percent,” he said.

Delphi Automotive is number two in most miles driven on California roads during autonomous vehicle testing. The automotive supplier had 16,662 autonomous miles, according to a January report filed with state regulators.