Why are messages on the road printed in reverse?

After reading this question, something occurred to me that I've always wondered - but never thought to ask.

Why are road messages/signs printed in reverse?

(source https://xkcd.com/781/)

Anytime I encounter them, I read them reversed in natural order, and it provides a slight distraction as I flip them around in my head and parse the message. Road signs/messages are short and concise, and always fit into the normal human field of view (e.g. "Llama crossing" instead of "Watch out for the llamas that sometimes wander into the road in this area"). So why would road messages need to be printed in reverse order?