As my family and I traveled through the states of Georgia and Florida on I-75, I saw that there are many areas where billboards can be seen for a good 15-20 seconds even at 70 mph. Not sure what the price per month of renting a billboard is in that area, but if you there is money available, a billboard would be seen by many thousands per day even in spots where major cities are 20 or 30 miles away.