OOH reports their audience in the ACTUAL number of people who looked at your ad, unlike radio and TV who report the number of people who are tuned in, but not necessarily seeing your ad. This is possible because OOH impressions are not just counts of people who pass a billboard every week. That raw number is adjusted based on how visible that billboard is to a passerby. This way, you get a true, qualified measurement.
These adjustments are calculated based on the results of a series of eye tracking tests that were done to determine what factors make a billboard more or less visible. Test subjects' eye movement was tracked in a simulated driving environment using real roadways and billboards across the US. These eye-tracking studies determined that the size, distance from the road and side of the road were the most common factors effecting billboard visibility. Geopath Ratings adjusts each traffic count based on these billboard characteristics.