"Signage is the least expensive, yet most effective form of advertising for independent and national retail businesses"
according to U.S. Small Business Administration (SBA)
ARE SIGNS EXPENSIVE?
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HERE
HOW DOES SIGNAGE COSTS COMPARE TO OTHER MEDIA?
Cost per 1000 Impressions (CPM) of a $10,000 sign on Tecumseh Rd would be approx 8.3 cents.  
Monthly sign costs of $100 divided by monthly impressions (40,000 cars x 30 days x 1.33 occupants) divided by 1000 = $.0833  
To find out how this compares with other media click
HERE
WHY DO I NEED A SIGN?
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HERE
ARE THERE STUDIES ON THE VALUE OF SIGNS?
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SIGN VALUE STUDIES