Until today, organizations writing apps for Android Auto had to spend a lot of money buying a vehicle or at least a head unit in order to test their apps. However, Google has released a new Android Auto Desktop Head Unit (DHU) simulator that makes the testing process much easier.
DHU allows developers to run an Android Auto App on Windows, OS X or Linux systems and watch it "behave as if it's connected to a car." To use it, developers will also need a mobile device running Android 5.0 or higher. There are more details on the Android Developers Blog.