The United States Sign Council (USSC) has regularly collaborated with the Larson Transportation Institute at Penn State University. As one component of a sign-visibility study, PSU used computer-generated, simulation software and analytical algorithms to determine how often a motorist's view of a 5-foot-tall monument sign would be blocked on a four-lane road. Signs were placed on each side of the road at 10- and 20-foot setbacks. The motorist was in the curb lane. Other variable were the speed of traffic and the "flow rate," or the number of vehicles that would pass a specific point in an hour. The flow rate ranged from 200 to 1200 cars.
In a best-case scenario, when the sign was set back 20 feet from the same side of the road, the flow rate was 200 cars per hour and the traffic speed was 35 mph, the sign would only be blocked from view 6% of the time.
Conversely, however, if the sign was located 10 feet away from the road on the other side of four lanes, and the traffic flow was 1200 cars per hour at 35 mph, the sign would be blocked from view 79% of the time.
For all other combinations, the percentages ranged between 6% and 79%.
This information is outlined in much greater detail in USSC's "Effects of Traffic Characteristics and Mounting Height" study.