What is out-of-home advertising, and how is it measured?
Advertising in public space — billboards, transport, street furniture, retail and increasingly digital screens — and its measurement problem is fundamental: you cannot ask a poster who saw it.
The formats:
Classic (static) posters, sold by fortnight or month.
Digital out-of-home (DOOH), screens showing rotating content, which is now the majority of revenue in many markets and which changed the medium substantially.
Transport — rail, underground, bus, airport.
Street furniture — bus shelters, phone boxes, kiosks.
Place-based — retail, gyms, petrol stations, medical waiting rooms.
Special builds, physically constructed to attract coverage.
How it is actually measured. Industry audience currencies model likelihood to see rather than counting viewers, combining traffic and pedestrian counts, travel surveys, visibility modelling — angle, distance, obstruction, illumination — and eye-tracking research. The output is opportunities to see and estimated reach and frequency, at a level that is a modelled estimate, not an observation.
What digital added:
Programmatic buying, so inventory can be bought impression by impression.
Dynamic creative triggered by conditions — weather, time, traffic, sports scores, pollen count — which is genuinely effective when the trigger is relevant and gimmicky when it is not.
Mobile location data, allowing exposure to be linked to subsequent store visits or online behaviour, which is the main advance in attribution.
Flexible scheduling, replacing the two-week minimum.
What the medium is genuinely good at:
Broad reach and fame, which is what brand building requires.
Unskippable, unblockable presence, in an environment where most advertising can be avoided.
Contextual proximity — near the store, at the station, on the commute.
High attention scores relative to cost in some research.
Its limits: no targeting precision comparable to digital, limited message complexity — six to eight words is the usual guidance — attribution that remains modelled, and weather and vandalism affecting physical sites.
Production and installation lead times are longer than digital media buyers expect.