What is Viewing Angle?

Viewing angle provides the approximate useable angles, relative to straight-on, that the display can reasonably be read in normal daylight conditions. 

A display appears the brightest when the viewer is horizontally and vertically perpendicular to the display face and gradually dims at off-angles. The useable viewing angle increases in low ambient light such as dawn, dusk, and nighttime hours and conversely decreases in very high ambient light or when the sun is low but shines directly on the face.  

Viewing angles are measured in degrees and presented as the total angle from left to right or top to bottom.  For example, a display with 90° horizontal and 40° vertical viewing angle can be read from 45° left to 45° right, to 20° above and below the display.  (need illustration) 

So why not choose a sign with the widest possible viewing angle? Wider angles diminish the peak brightness of the image when viewed straight ahead, such as along a road. Conversely, narrow viewing angles concentrate the images brightness.

Trans-Lux recommends wide viewing angles (140º x 60º) for stationary audiences and slow moving traffic, narrower viewing angles (90º x 40º) for city streets and freeways, and even narrower (70º x 30º) for fast moving interstates.