ECNALUBMA is actually a mirror image of AMBULANCE. This is specially made to ensure that drivers who riding just before the ambulance can read in their rearview mirror the word AMBULANCE in a normal way.
You can experiment with a tool for mirrored letters on the website: www.typemirror.com. The text you enter can be displayed in a mirrored. A good tool if you want to fool around with your friends with a MIRROR-Writhed text.