There are a number of critical values that need to be considered when designing for outdoors. These include letter size, colour visibility & distance comprehension.

• Upper & lower case type combined is easier to read than all capital letters
• Too little spacing between letters makes them appear to merge together.
• At long distance, very heavy letters become blobs, & very thin letters become invisible.
• Ornate script faces & excessive contrast between thick & thin reduce legibility.

Complementary colours such as red & green are not readily legible. Any combination of colours of similar value, even without vibrating, will have low visibility. However, complementary colours that have strong contrast in value, & therefore little vibration, provide maximum visibility.
Try the 6 Metre Hall Test. Print out your design at 3.28% of the final size. Take the artwork & place it in a hallway 6 metres from where you're standing & see if the type can be easily read. Why 3.28%? Because one centimetre on a proof is equivalent to one foot in real size - ie. 1cm = 30.48cm.