To be fair, I’ve *never* seen anyone type in a URL on their phone after seeing an advert, nor place a phonecall after transcribing the number printed on a poster. But I guess they do.
As of this month, it’s close to 200 scans per day. However, I don’t think that TfL pay for ad space on their own shelters (although you could count that as opportunity cost).
The question is, if you printed a unique phone number on your posters, you could measure how many calls they got. Has anyone done this?