In the ambulance van, why the word AMBULANCE is written in reverse?

In the ambulance, the word AMBULANCE is written in reverse order so that when the word is viewed through the rearview mirror (having convex mirror which provides a wide view of the field) of the vehicle then the driver could easily get to know about the ambulance and give way to it. 

It happens due to the property of the mirror termed as "lateral inversion".

It is a phenomenon in which when an object is placed in front of the mirror the right side of the object becomes left side of the image and the left side of the object becomes the right side of the image.

Therefore the already written inverse AMBULANCE when viewed through the rearview mirror of the vehicle then the driver will see it in the correct order.