Outdoor advertising is bought the same way in most of this market: whatever site is available, at whatever rate is quoted, reported afterwards in impressions. All three of those defaults work against the person paying for the campaign.
A site is not just a location, it is an audience
The question is never simply "how many people pass this junction" — it is whether the people passing it are the audience the campaign is actually for. A highly trafficked site on the wrong commute is expensive reach wasted on the wrong people. Site selection has to start from the audience, not from what inventory happens to be free.
Rate card is a starting point, not a price
Outdoor media rates move with season, location and how badly the site owner needs to fill the slot. Paying rate card by default is the outdoor-advertising equivalent of paying a property's asking price without checking the guidance value first.
Impressions cannot be acted on
An impressions figure tells you how many eyes theoretically passed a site. It does not tell you whether any of them did anything afterwards. It is reported industry-wide because it is easy to produce and impossible to be wrong about — not because it is useful.
What to ask for instead
Qualified enquiries generated, and cost per qualified lead. Both are measurable — a dedicated phone line or landing page per site makes the attribution straightforward — and both tell you whether the campaign is working while there is still time to change something.
The structure and the media are not the same job
Building the hoarding, pylon or highmast is a construction problem: engineered for wind load and monsoon, installed safely, built to survive the campaign. Buying the site and running the media plan is a completely different problem. Treating them as one engagement, with one party accountable for both, is what keeps a good structure from sitting on a badly chosen site.