The Guardian has uncovered documented proof of the existence of Apple's Car project. According to those documents, Apple has been in touch with GoMentum Station, a testing and research facility built on the bones of the Concord naval weapons station.
This facility has more than 20 miles of paved roads, overpasses, city streets, and highways, all of which is not only closed off to the public, it's protected by the U.S. military. GoMentum Station has been setup as an R&D facility with dedicated programs for Connected Vehicles and Autonomous Vehicles.
GoMentum Station Promo Photo
Here's a video Honda published to YouTube testing its own self-driving system specifically at GoMentum Station:
In other word, it's an ideal place for a secrecy-driven company like Apple to test a car. The Guardian obtained documentation of Apple inquiring about using the facility through a public records request. According to that documentation, an Apple engineer named Frank Fearon wrote:
"We would [...] like to get an understanding of timing and availability for the space, and how we would need to coordinate around other parties who would be using [it]."
He also wrote that, "We are hoping to see a presentation on the [...] testing grounds with a layout, photos, and a description of how the various areas of the grounds could be used."
GoMentum Station Grounds
GoMentum Station is owned by the Contra Costa Transportation Authority (CCTA), a public agency. The CCTA did sign a non-disclosure agreement with Apple, but as a public agency, it didn't just say no comment. Randy Iwasaki, executive director of the CCTA, told The Guardian, "We had to sign a non-disclosure agreement with Apple. We can’t tell you anything other than they’ve come in and they’re interested."
What does a yellow light mean? Slooooooow down!
The Guardian's conclusion is that Apple is further along in developing its car than most folks think. I would caution against such thinking, however; developing a car and developing a self-driving system are two different things. Apple might well be at or near a point where it wants to test a self-driving system, but that has nothing to do with how far along development of the actual car might be.
To wit, look at Honda's promo video above. The self driving system that company is working on was (literally) bolted onto an ordinary car. You can be sure Apple will be testing in the same manner, and that it will do so alongside the separate development of the car itself.