To make AMBULANCE easier read for vehicles in front of the ambulance. Ambulance are typically in greater speed than surrounding vehicles which make the ambulance front visible in other cars rear view mirror. At least those cars that need to get away to give room for the ambulance passing through traffic.
The most common modern usage of mirror writing can be found on the front of ambulances, where the word “AMBULANCE” is often written in very large mirrored text, so that drivers see the word the right way around in their rear-view mirror.
Reference: Mirror writing
Nowadays, it’s done more out of tradition and convention than for functional reasons
While the original purpose of the reversed ‘AMBULANCE’ was better readability, advances in lighting, safety design, and understanding of visual cognition have rendered this lettering of secondary importance.
Some issues with the reverse AMBULANCE signage:
- The word is long and difficult to read in rear-view mirrors.
- It presents problems for multicultural/multilingual cities/towns.
- There are far more effective ways to visually communicate an emergency vehicle.
- When ambulances need to be noticed, flashing lights, sound, and sign coloring are more effective than lettering.
Here are some more updated practices on ambulance lettering:
- FEMA studied Emergency Vehicle Visibility and Conspicuity here and concluded that flourescent materials, vehicle outline, colors, lighting, and emblems are the most impactful factors in vehicle visibility.
- The 2014 FEMA Emergency Vehicle Safety Initiative has a section on Visibility and Conspicuity which focuses on striping, lighting, and color rather than lettering/fonts as primary mechanisms for vehicle visibility.
- The National Fire Protection Association published a Standard for Automotive Ambulances which specifies reflective striping, flashing lights and placement rather than lettering for visibility.
- The Federal Specification for Star-of-life Ambulances has very specific requirements for lighting, placement, and reflective materials but is open on lettering and emblems.
There is a good argument that the front of an ambulance would be much more communicative if used with an emblem or (as in this Australian study on ambulance visual safety) a simple reflective band.
For example, the following symbols are all far easier to read at a glance and understandable in multi-cultural cities than the word
AMBULANCE. They are also mirror-friendly, in the sense that they are symmetrical enough so that the meaning is clear whether you’re looking straight on or through a mirror: