If you're buying outdoor in general, it's one message and that's it.
You have a few seconds, no more.
If folks take out one message and know who it's from, you've done your job.
With 'Street Talk' stuff like phone box's you have less. These are mainly for getting for people on foot in their day to day.
The ad I saw (below) really doesn't do that. Intentionally or not, it's three messages.
"Always fresh and tasty"
"Prepared in shop everyday"
And then a promotion to activate in that order.
The first has no place on street talk really, especially if they're trying to activate footfall.
The second is a very valid 'quality message' that gives me a reason to think "Might try Greggs today or soon".
The third MIGHT make me go in today to take advantage of the promotion, but I won't have noticed it.
Either the client has insisted to put in multiple messages.
The agency can't help trying 'brand stuff' in what is response stuff.
Or one or both hasn't a clue.
Now here's Oasis, with a 6 sheet, who are activating 'thirst' at point of need with a personality wrapper -rather than confusing brand with activation.
Scratch that, they've realised that single-mindedly tapping into a needs and doing it with the right tone and wit can deliver crisply - doing a longer term brand job as well as 'activation'