The opt-out screen is on the hardware installed on the billboard, you might need a pair of cutting pliers to access it.
Should be opt-in anyway, given all the data that is retrieved
This probably doesn't collect GDPR protected data. If the billboard aggregates the data to say that 30% of the people that passed by have an iPhone and the other 70% have an Android phone then GDPR won't come into play here. Or that 40% of the people that went past the billboard are between the ages of 40 and 60 and so on.
But what if you added facial recognition (for walkers) and license plate readers (for drivers). Now you know exactly (assuming you have a good DB and the recognition was perfect which is not the case today) who saw the billboard. Now that's creepy.
Well, yes, but then they would run afoul of the GDPR, which may prevent this from happening in the first place. But it can't prevent them collecting aggregate stats.