The fleet of autonomous test vehicles is now only requiring human intervention an average of once per 5,000 miles.

Google's Waymo has reportedly a significant improvement in its autonomous drive system from 2015 to 2016.

The company's head of self-driving technology, Dmitri Dolgov, suggests the rate of safety-related disengages -- when human drivers intervene to take control of the vehicle -- has dropped from 0.80 incidents per 1000 miles to 0.20 per 1000 miles. The numbers equate to an average of just one human disengagement for every 5,000 miles driven.

"Even as we increased our driving by 50% in the state — racking up a total of 635,868 miles — our total number of reportable disengages fell from 341 in 2015 to 124 for this reporting period," he said in a blog post.

Importantly, the company claims almost all its miles have been spent on complex urban and suburban streets. Other automakers have initially focused on semi-autonomous systems that can operate on highways without human control, but Waymo is creating software specifically for a door-to-door ride-hailing service.

"This has given us valuable experience sharing the road safely with pedestrians and cyclists, and practicing advanced maneuvers such as making unprotected left turns and traversing multi-lane intersections," Dolgov added.

Each time a Waymo test driver chooses to intervene, the scenario is analyzed and put through a simulator to test hundreds or thousands of similar situations. The process allows software engineers to refine and virtually test the software before the changes are activated on the prototype fleet. In 2016, the company claims to have drove "over a billion miles in simulation."

"With eight years of careful development and testing, we're optimistic that we're closer to the day where fully self-driving technology can begin to make a difference," Dolgov said.