Thanks to standardized road signs pretty much universally, text doesn't even need to be printed in order to understand most signs. Red means 'no' or 'be extremely cautious', interstate badges are easy to recognize, services (on American highways anyway) are in blue, yellow means 'hazard', etc.
There is also a standardized text size called for in the printing of most signs (depending on the speed of the roadway). Highways have the largest font in order to be read most easily from the largest distance.
