Android Auto developers now have way to try out their apps without spending lots on an actual head unit or entire car. The Android Auto Desktop Unit (DHU) runs on your Windows, OS X or Linux desktop and works in conjunction with the companion app on Android 5.0 (Lollipop) or higher. Installing it is a bit of a rigmarole and you'll need to connect your Android device via a USB cable, but you probably already know that if you're a developer. Once you've compiled and installed your app, it'll "behave as if it's connected to a car," according to Google.
Android Auto apps naturally need to be minimally distracting, so porting a regular app straight across is out of the question. Google provided some rough simulation through its SDK before, but more or less expected you to "work from your car," according to its blog. However, using the emulator is now tantamount to using an actual Android Auto device. And since you can use it on a PC, why not test it with a road simulator (like the one below) to make sure it's truly driver-friendly?