This is about 3 years late, but I've found a way to create a 100% automatic text-reply and it's been saving my life as of late, so if anyone else has had this problem, here you go.
First off, your phone needs to be recent enough/updated enough to have the "Do Not Disturb While Driving" setting. Go to settings and then go to Do Not Disturb. Where it says "DO NOT DISTURB WHILE DRIVING" click activate, and select manually. Go back to the Do Not Disturb settings page, scroll all the way to the bottom, and where it says "Auto-reply to", select all contacts. Where it says "Auto-reply", you can personalize the automatic message that anyone who texts you will receive. I have it set so it says something like "I'm not checking my messages right now - I'll get back to you as soon as I can."
Next, go to Control Center in your settings. Click customize controls, and add "Do Not Disturb While Driving" to your control center. Make sure you can access your control center when your phone is locked.
Now, lock your phone. Open your control center and the button for "Do Not Disturb While Driving" should look like a little car. Press that and it should turn on. Test your auto-reply by having a friend text you and see what comes up.
Now, there are a few conditions to this working.
-Your phone can't die. If it dies, it'll stop sending the automatic reply and you'll have to turn it on when you charge your phone and it turns on again. If your phone isn't on you for a long period of time, this can be a problem.
-Your phone will send an automatic message that you can't change saying that if the message is urgent, the person can reply "urgent" and the message will come through. If you know you can't respond for an extended period of time, make sure to say that in your customizable auto-reply message.
Besides these few conditions, I've found this to be pretty foolproof.
