Geographic overlap between roadside and in-store formats
A shopper who passes a roadside panel on the way to a store and then sees a screen inside it has been reached twice by the same plan. That is an opportunity — sequential exposure in a single journey — and a measurement problem, because the two audiences are not additive.
Both halves deserve attention when the two formats appear on one plan.
Where does the overlap come from?
Shared geography. A panel on a route and a store on that route serve overlapping populations, and the closer they are the greater the overlap.
In dense urban areas with walkable catchments, the overlap can be substantial. In dispersed areas where a panel serves a driving route and a store serves a walking catchment, it can be small.
Why is that useful?
Because it allows a journey-based sequence: a broader message outside and a specific, proximate one inside. The formats have different strengths and the combination uses both.
Planning it deliberately, with creative designed as a pair, is more effective than buying them separately and hoping they compound.
What is the measurement problem?
Adding the two formats' reach figures counts the overlapping audience twice. The combined figure then overstates the plan's reach, sometimes materially, and the overstatement grows with proximity.
Deduplicating requires an estimate of the joint audience, which in turn needs data about movement between the panel's exposure area and the store's catchment.
How is it usually handled?
Badly, by addition, or honestly by reporting the formats separately with a stated caution. The second is less satisfying and considerably more defensible.
Where a deduplicated figure is produced, the method should be named, because the assumptions carry the number.
Does the overlap change the value?
It changes what the plan is doing. High overlap means frequency and sequence; low overlap means reach. Neither is wrong and a plan should know which it bought.
A brand aiming for reach that unknowingly bought heavy overlap has paid for repetition.
How should the pair be planned?
By deciding which effect is wanted, then choosing proximity accordingly. Panels near the stores for sequence; panels away from them for reach.
That is a straightforward planning decision that requires the locations of both, which is the practical constraint rather than the concept.
What about creative?
Designed as a pair where sequence is intended: a broader idea outside, a specific prompt inside. Running the same execution in both wastes the difference between the formats.
Frequently asked questions
How do I estimate the overlap?
From the geography of the panels and the store catchments, with an assumption about movement between them. It is a modelled figure and the assumption should be stated, because the estimate is sensitive to it.
Should the formats be bought together?
Where a sequence is intended, planning them together is what makes the sequence real. Buying them from different suppliers without coordination produces two campaigns in the same place rather than one.
Can reach be combined at all?
Only with a deduplication method and a stated assumption. Adding them is an upper bound rather than an estimate.
Which format comes first in a journey?
Roadside typically precedes the store, which suits a broad-to-specific sequence. Whether an individual experiences it in that order is not controllable.
Does this apply to transit formats?
The same logic applies with a different geography, and transit audiences and store catchments can overlap heavily near stops.
How should the plan report reach?
By format, clearly not summed, with the overlap question named. Any combined figure should state its method.
Is the sequence effect measurable?
Only with a design that varies the combination across markets, which is possible and requires planning. Without it, the sequence is an intention.
Does proximity always increase overlap?
Generally, and it depends on movement patterns rather than distance alone. A panel on a route people do not take to the store may be close and barely overlapping.
What is the practical rule?
Decide whether you want sequence or reach, place accordingly, and never add the reach figures. Out-of-home measurement practice is discussed by bodies such as Geopath.
### How should a paired plan be evaluated?
With a design that varies the combination: some markets with both formats, some with one, some with neither. That is the only way to establish whether the pairing does more than the sum of the parts.
It needs enough markets for power and a flight long enough to detect the difference, which makes it a deliberate exercise rather than an analysis run afterwards.
Without it, the combined campaign's result cannot be attributed between the two formats, and the sequence claim remains an intention.
Should the two formats share a budget line?
Planning them together helps and reporting them separately is usually clearer, because their delivery is measured differently. A shared line that cannot be decomposed makes evaluation harder.
Does the sequence direction matter?
Broad-to-specific is the natural order given the journey, and the reverse occurs for anybody leaving the store past the panel. Designing both executions to work independently handles it.
Should the two formats share creative?
A shared design system with different executions is the usual answer, because the formats have different viewing distances and attention patterns. Running an identical file in both wastes the difference.
How should the pair be budgeted?
Separately, with a stated rationale for the split. A combined line that cannot be decomposed makes the evaluation harder than it needs to be.
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