Google Self-Driving Cars: What We Learned From Latest Report

— -- Google's fleet of self-driving cars still have some learning to do.

A new report issued by Google Auto LLC to the California Department of Motor Vehicles shows that over a period of 14 months, there have been 341 instances when cars in the company's fleet have needed human intervention -- or as the report calls it, "disengagements."

"Disengagements are a critical part of the testing process that allows our engineers to expand the software’s capabilities and identify areas of improvement. Our objective is not to minimize disengagements; rather, it is to gather, while operating safely, as much data as possible to enable us to improve our self-driving system," Google explained in the report.

Spanning between Sept. 24, 2014, and Nov. 30, 2015, the report focuses on how Google's self-driving cars performed on California public roads. Here's a look at what else we learned from the report.

Reasons for Human Intervention

The top three reasons for a human driver stepping in include perception and software discrepancies or after an unwanted maneuver of the vehicle. While technology may top the reasons Google's cars needed a human driver's touch, there were 23 instances when someone had to step in due to another driver on the road behaving recklessly, according to the report.

"As part of testing, our cars switch in and out of autonomous mode many times a day. These disengagements number in the many thousands on an annual basis though the vast majority are considered routine and not related to safety," Google noted in the report.

Technology Is Improving

Google has some good news: The number of times a human driver has needed to intervene continues to decline. During the fourth quarter of 2014, there was one intervention recorded for every 785 miles driven. This drastically dropped in the fourth quarter of 2015, with just one intervention per 5,318 miles driven, according to the report.

Monthly Mileage

Google's fleet of self-driving cars log a total of 30,000 to 40,000 miles per month. It would take the typical adult driver in the United States two to four years to rack up that kind of mileage, according to the report.

Google's autonomous vehicles have more than 1.3 million miles driving experience, with about one-third of those miles coming from drives on public roads.