Apple keeps adding autonomous vehicles to its test fleet in California, boosting its ranks 27 percent since May, according to records from the California Department of Motor Vehicles.

The company now has 70 autonomous vehicles permitted to test on public roads, Mac Reports first reported. The permits, which are issued by CA DMV, require a safety driver to be behind the wheel.

Over the past 18 months, Apple has gone from just three autonomous vehicles to 27 by January, 55 by May and now 70. GM Cruise has the most permitted autonomous test vehicles at 175, followed by Waymo with 88. Apple has the third-largest fleet.

The number of permitted test vehicles is one of the only ways to track what Apple is up to. The company doesn’t talk about its self-driving vehicle program.

The tech company’s permit with the CA DMV, the agency responsible for monitoring AVs in the state, is the only official acknowledgment that it even has a program. Apple’s self-driving program has been considered an open secret in Silicon Valley. CEO Tim Cook has more recently made references to the company’s interest in autonomous systems.

Last month, the company disclosed its first accident, according to a report filed with the CA DMV. The low-speed accident occurred August 24. The number of accidents involving autonomous vehicles have become more common as companies put more of these self-driving cars on public roads. The vast majority are minor, low-speed incidents.

There was just one accident involving a self-driving vehicle (that one was owned by Delphi) reported to the DMV in 2014. So far this year, there have been more than 40 accidents involving self-driving cars reported to CA DMV.