Apple has 55 self-driving cars that are licensed to be tested on California roads.

That makes it the second largest fleet in the state, behind only GM's Cruise.

Apple has more self-driving cars in California than Tesla, Waymo, Uber, Toyota, BMW, and Mercedes-Benz.

Apple's fleet of self-driving testing vehicles continues to grow rapidly, with 55 self-driving cars registered with California as of May 9, according to DMV figures.

Apple's fleet is up from 27 cars in January. The company first received a permit to test autonomous vehicles in the state last April, starting with only three Lexus Rx450h SUVs. 51 companies have registered 409 autonomous test vehicles in the state, according to the DMV.

Apple's 55-vehicle fleet is the second largest in California, behind only Cruise, a self-driving company owned by GM, which sports a 104-strong fleet. Apple actually has more self-driving cars in testing than self-driving pioneers such as Waymo (51) and Tesla (39), as well as big automotive companies like BMW (12), Toyota (11), and Mercedes Benz (5).

Apple's object-detection software. Apple These permits enable autonomous vehicles to legally drive on California roads as long as they have a driver behind the wheel ready to take over at the first sign of trouble.

It's important to note that companies like Waymo and Uber used to have more self-driving cars in California. Waymo, for example, used to have over 100 self-driving cars in the state before moving part of its fleet to Arizona and Michigan, where additional testing is currently taking place.

Uber left California in late 2016, taking its 16 vehicles with it, after the California DMV revoked its vehicle registrations over safety issues. Uber received a new permit for mapping vehicles as well as some autonomous vehicles with a driver, but its testing permit expired this past March, and Uber chose not to renew it. It does not appear on the May 9 list seen by Business Insider.

MacReports first reported some of the new details of Apple's fleet.

Apple's quickly growing fleet of cars suggests that it's certainly taking self-driving development seriously. Apple CEO Tim Cook has called self-driving "a core technology that we view as very important." Apple's self-driving car program is still a mystery, but it appears to be centered around testing and collecting data for self-driving software, instead of a complete vehicle.

"We sort of see it as the mother of all AI projects. It's probably one of the most difficult AI projects actually to work on," Cook said last summer.

Apple, however, has not yet received a permit to test self-driving cars without a driver behind the wheel. Waymo applied for that permit.

Here's the top self-driving car companies in California, by size of fleet: