BILLBOARD ADVERTISING GUIDE
Weekly Impressions, Traffic Counts & DEC
Understand the numbers behind a billboard location so you can compare your options with confidence.
A busy road can be a good starting point, but roadway traffic and billboard audiences measure different things. Here is what the numbers mean when you are reviewing a billboard proposal.
The differences at a glance
| Measure | What it counts | Time period | What it helps explain |
|---|---|---|---|
| Weekly impressions | Estimated ad exposures, including repeat exposures | Seven days | The audience delivery estimated for a billboard or ad slot under the stated method |
| Traffic count | Vehicles passing a roadway point | As reported, often a daily average | How busy the road is |
| DEC | People in vehicles potentially exposed to the display | One day, within applicable viewing hours | Daily potential exposure under a legacy measurement method |
Weekly impressions: estimated ad exposures
For Geopath-rated billboards, weekly impressions estimate the number of times people are likely to notice an ad during a week. The estimate considers more than traffic volume, including the display’s orientation and visibility.
The same person can contribute several impressions by passing the billboard repeatedly. Impressions are estimates, rather than a record of every individual looking at the ad. Other providers may report opportunity-to-see estimates, so ask which method is being used.
Traffic counts: vehicles passing a point
For roadside billboards, a traffic count measures vehicles passing a specified roadway point during a stated period. AADT stands for Annual Average Daily Traffic: the average daily vehicle volume across a year.
A count may include traffic traveling in both directions, even when only one direction faces a particular billboard. It also counts vehicles, rather than the people inside them.
DEC: Daily Effective Circulation
DEC is a legacy estimate of the people in vehicles passing and potentially exposed to a billboard in one day. The applicable viewing period is commonly 12, 18 or 24 hours, depending on the display’s illumination.
DEC describes potential exposure, rather than the likelihood of noticing the ad. It differs from a raw vehicle count because it measures people. Geopath retired DEC as its trading standard in 2010 in favor of likelihood-to-see impressions.
Why you cannot simply multiply by seven
Daily traffic × 7 gives weekly vehicle passes using that daily average. It does not give weekly billboard impressions.
DEC × 7 gives weekly potential exposures under the same assumptions. It does not convert DEC into Geopath likelihood-to-see impressions.
There is no single conversion factor that makes these three measures interchangeable. Compare figures using the same source, audience definition, reporting period and measurement method.
What to ask when comparing billboards
- Source and date: Who supplied the number, and which data year or release does it use?
- Period and audience: Is the figure daily or weekly? Does it cover all ages or a specific age group?
- Direction and face: Does it apply to the traffic approaching the specific billboard face?
- Measurement method: Is it a traffic count, DEC, opportunity-to-see estimate or likelihood-to-see estimate?
- Digital advertising: Does the estimate apply to your ad slot or the whole display? Confirm operating hours and the ad rotation.
Not every listing has Geopath-audited measurements. We can help clarify the source and meaning of the figures provided for the locations you are considering.
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Measurement sources
- Geopath Glossary of Terms — AADT, DEC, impressions, reach and frequency.
- Geopath Circulation — vehicles, people and the transition from DEC.
- Geopath High-Level Media Measurement Process — visibility, ad units and audience estimation.
References reviewed October 16, 2026. Audience figures are estimates and do not guarantee sales results.
Billboard CPM calculator: compare quoted options
CPM is the cost per 1,000 estimated ad impressions. Enter a quoted cost and the estimated impressions for the same campaign period. Use the same cost basis when comparing quotes: media rental alone or the full campaign cost including production.
Enter your quote and estimated impressions to calculate CPM.
Formula: quoted cost ÷ estimated impressions × 1,000. For example, $1,000 ÷ 200,000 impressions × 1,000 = $5.00 CPM.
For digital billboards, use the estimate for your ad slot and agreed rotation. A lower CPM alone does not establish a better location: also compare audience fit, viewing direction, visibility and proximity to your business. CPM does not measure sales or unique people. Road traffic counts and DEC are not interchangeable with ad impressions.
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