In this day and age of really great machine learning I find that most people refuse to use voice services unless they are forced to do so (usually because of injury). Trying to tap out a message at 80km/h on a 5" screen is pretty fucking futile. But most distracted drivers will attempt to do it anyway.
Here's a neat trick, just say, "Hey Siri* Send a message to $firstname $lastname's $phone. Tell them I'm [on my way, 5 minutes away, picking up milk, buying pizza]"
* Or whatever platform you use. I'm an iOS user.