Apple Inc. appears to be revving up its efforts to develop self-driving car technology.

According to a regulatory filing with the California Department of Motor Vehicles reported by Bloomberg today, the company is expanding its test fleet to 27 vehicles.

The iPhone maker was long rumored to be developing its own car, not just self-driving technology, before apparently abandoning those plans. The first Apple-powered self-driving car was spotted in Silicon Valley in April after the company received permission to test three 2015 Lexus RX450h hybrids earlier the same month.

Dubbed “Project Titan” inside Apple, the project remains somewhat of a mystery. Chief Executive Officer Tim Cook confirmed the project for the first time in June, telling Bloomberg that Apple is “focusing on autonomous systems” and that it’s “the mother of all AI projects … probably one of the most difficult AI projects to work on.” What exactly that means still isn’t clear.

Although Apple signed a deal with Hertz Global Holdings Inc. to supply vehicles for testing in June, the company Steve Jobs built was then claimed to be laying off workers and winding down its autonomous vehicle project in September.

Jump forward one month and reports emerged that Apple was now simply working on an “operating system for a self-driving car with the help of a new set of Canadian engineers.” Then in November, Apple published research about how machine learning can be used to get more out of LiDAR, a remote sensing method used by self-driving cars. However, the research referenced existing LiDAR units, not any developed by Apple.

Whatever Apple is building and testing, it still has a long way to go to catch up to competitors. Waymo, the company formerly known as the Google self-driving car project, passed 4 million miles tested on public roads in November and is already doing a trial of a ride-hailing service in Arizona. Uber Technologies Inc., despite numerous setbacks, is claiming that its self-driving vehicles will be picking up passengers within 18 months.

Image: automobileitalia/Flickr