EDIT :
What this app does (Drive Safely) is you turn it on but you've also signed up for their service. When you get a text it is communicating via WiFi to their servers that handle processing and sending out the text messages. The app itself isn't sending one out.
I would love for Apple to build some of the features into the default app that does this. Basically a "Driving" button that sends back a text message response that you are driving and will reply to them later. Or something like that. Actually even Do not Disturb I would like something like this as well. Lots of cool features could be created such that it detects when you're in the car and automatically turns on. Or you can geo fence your office so when you're in your office it turns on DND, etc...