If you're in the billboard business you'd probably want to know exactly when a billboard becomes visible to drivers. So in 1953 research was conducted at the Iowa State College Experiment Station to get an answer to this question. The study involved having subjects watch miniature billboards slowly approach on a conveyor belt.
Source: Duke University Libraries - Archives of the Outdoor Advertising Association of America.
The Duke University blog
also notes that in 1958 "The OAAA commissioned Jack Prince, a professor of ophthalmology at Ohio State University, to study the visual dynamics of outdoor advertising, resulting in the first legibility studies of ad copy." I'm not sure how these two studies related to each other. They sound suspiciously similar. And the 1958 studies obviously weren't the first given that the pictures below show research labeled as happening in 1953.
The researcher in the photos is probably Dr. A.R. Lauer of Iowa State's Department of Psychology, and he may have been studying the phenomenon of "Highway Hypnosis."
In the early 1950s there was increasing criticism of the proliferation of billboards along the side of roads. People complained that they were ugly and possibly distracted drivers. So the OAAA sponsored Dr. Lauer to research the safety benefits of billboards, and specifically whether billboards distracted drivers.
Lauer came up with the result that the billboards did distract drivers, but that this was a good thing because it saved them from Highway Hypnosis —entering a trance-like state as they stared at endless, monotonous roads.
The OAAA then took out ads in newspapers promoting Lauer's research and the safety benefits of billboards.
The Des Moines Register - Mar 23, 1958