In the past, OOH impressions were calculated using traffic counts and daily effective circulation to determine the total number of distinct customers within a target population who were exposed to a product or service advertisement, which is known as reach. Daily effective circulation, or DEC, is the measure of traffic that passes by a particular advertisement.

Geopath, originally known as the Traffic Audit Bureau (TAB), recognized the problem faced by advertisers who wanted to know how many individuals their adverts reached. They started working on a new approach to computing impressions and started running tests in which drivers wore goggles that tracked their eyes while driving. They were able to determine the percentage of drivers who saw a certain advertisement using this data. Because Geopath's objective is to independently audit and report OOH advertising ratings across the United States, most outdoor advertising businesses rely on Geopath for all OOH impressions data and other measurements. Advertisers, agencies, and media businesses are all represented on the organization's three-member board, which ensures that all stakeholders are receiving accurate information. To determine reach, outdoor impressions have typically been measured using traffic counts and daily effective circulation. For years, these figures were the only credible means of determining the reach of an out-of-home advertising campaign.