I'd say a big part of it is that drivers already filter out billboards, but those info signs often have important information on them that drivers are expected to read
Rent a billboard just to put the most distracting image possible on it and then watch the traffic reports for that area and see if there is an increase in accidents. 😈
Thinking of way back in the day in CS 1.5/1.6 when aimbots worked by searching for a specific color you'd mod the player skins to, someone had a spray to counter them that was a patch of those colors in the middle of the butthole on goatse. Lol
I'd say a big part of it is that drivers already filter out billboards, but those info signs often have important information on them that drivers are expected to read