The contrast between the letters and the background renders it highly legible, the illumination stresses the importance of the message, and the color is evocative of both fire and fire-safety devices (fire extinguishers, fire engines, fire alarms, and the like).
But in other parts of the world, pictograms rule. The "running man" sign was designed by Yukio Ota and adopted internationally for exits a quarter century ago!
The sign's wordlessness means it can be understood even by people who don't speak the local language. And the green color, they argue, just makes sense. Green is the color of safety, a color that means go the world over. Red, on the other hand, most often means danger, alert, halt, please don't touch. Why confuse panicked evacuees with a sign that means right this way in a color that means stop?
Slate lays out the arguments for both and a history of exit signs in one chapter of a six-part series on signs. Links to all the chapters are found at the top of each. Link -via Simply Left Behind