Driving Mode was introduced by Microsoft in Windows Phone 8.0 Update 3 a few months ago. It carries over into Windows Phone 8.1 update, and it remains one of the few truly unique features of Microsoft's platform. Heck, it even showed up on NBC Today.
The Windows Phone Driving Mode is designed to automatically start when you connect to a Bluetooth device and it can be set to ignore calls and text messages. In a nutshell, the Driving Mode removes aspects of your Windows Phone that may cause a distraction while you are cruising down the road. It can even auto-respond via text message to tell people you're driving - now that's smart!