Advertising recall is a measure of return on investment (ROI) and ad effectiveness. This PJSC study examined the 5 primary ad channels (television, radio, online, out of home, and print) to discover which had the best recall.
They exposed various ads to people then asked them to recall it later (both aided and unaided). Out of home advertising scored the highest in both the unaided and aided ranges. With OOH's massive reach, it is not surprising that it scored so well. It cannot be skipped or ad-blocked like most other advertising channels. Source: https://oaaa.org/StayConnected/OAAAOutlookNewsletter/tabid/867/id/4872/Default.aspx