Stand out in a sea of visual clutter by designing a car top sign, for example a quad sign, which conveys your message in 5-10 seconds. Good, legible car signs are a combination of smart color choices, the right font and an appealing layout.
Like sound waves, light rays have varying wavelengths, or frequencies: the lighter the color, the higher the frequency. These wavelengths determine perception of color. Some pigments absorb certain light frequencies, while others reflect. We see the reflected frequencies as color. Complementary colors such as red and green, are not readily legible. They have similar black and white values, so their wavelengths set up a vibration. Any color combinations of similar value, even without vibrating, will have low visibility. Although yellow and purple are complementary colors, they have strong contrast in value and therefore, little vibration and provide maximum visibility. Bold, highly contrasting colors help get you noticed. Soft, mellow pastels may work fine in slick magazines but they don’t do well in outdoor advertising.
Forget fancy font styles. Thin and/or elaborate script fonts are hard to read (and often invisible) at long distances. Use thick strokes and simple styles to increase legibility at distances. Keep ample space between individual letters to avoid blurring. Avoid ALL CAPS, as it's hard to read as well. “Can you tell if that is a capital D or a lowercase P from 300 feet away?” Does the font have a unique style that makes it hard to recognize? If so, you may want to reconsider your options.
Keep it SIMPLE. Viewers have only 5-10 seconds to notice, become engaged by and process your message. Don’t make it difficult by cramming your car sign with too much stuff. You’ve only got seconds to convey your message, no time to list multiple features. Make one point with IMPACT. Finally, show your design layout to someone for 5-10 seconds. Are they able to read the entire message? Can you recall the business name? Simulating a drive-by-viewing will quickly point out simple mistakes that could clobber your response rate.