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What Is Geopath? How Billboard Impressions, Reach and CPM Are Measured (2026)

Updated 2026-10-29

Short answer: Geopath is the not-for-profit that measures out-of-home audiences in the US. It estimates how many people are likely to see each billboard, bus or screen, using traffic data, anonymized phone location data and eye-tracking research. Those estimates are the impressions on most rate cards, and the basis for the CPM, reach and frequency numbers sellers quote. They're modeled estimates, not counts, but they're the industry's common currency, which makes them the fairest way to compare one board with another.

Who Geopath is

Geopath was founded in 1933 as the Traffic Audit Bureau, which audited the circulation of billboards much like publishers' circulation was audited. It renamed itself Geopath in 2016 as it shifted from counting inventory to measuring the people who see it (Billboard Insider). It's governed by a board of advertisers, agencies and media companies, and media owners pay to have their inventory measured. See the Geopath profile.

How Geopath measures a billboard

Geopath's published process works in stages (Geopath measurement process):

  1. Circulation. Estimate how many vehicles and pedestrians pass on every road and walkway, by hour, day and direction. The data comes from anonymized phone location data (from Teralytics), adjusted for season.
  2. Assign each ad to its roads. Every board, shelter or screen is linked to the roads or places from which it can be seen.
  3. Opportunity to see. Use the ad's facing direction and its share of time (for digital screens sharing a loop) to estimate how many passing people could see it each hour.
  4. Likelihood to see. Apply a visibility adjustment based on the ad's size, distance from the road, angle and other factors, built from Geopath's eye-tracking studies. What's left are impressions: people likely to have seen the ad.
  5. Reach and frequency. Use phone data on how often the same people pass to split impressions into how many different people saw it (reach) and how many times (frequency).
  6. Demographics. Infer each device's home area from where it spends the most time, and apply that area's Census characteristics. That's how a rate card can quote "adults 18+" or "Hispanic adults" impressions.

The eye-tracking research behind the visibility adjustment included a study of about 300 people viewing about 50 billboards in Atlanta and Dallas (Digital Signage Pulse). Geopath released an updated 2025 planning dataset with new road network definitions and post-pandemic travel patterns (Billboard Insider), and it now measures place-based screens such as gyms, bars and airports too (Research Live).

The key idea: Geopath impressions are much lower than raw traffic counts. A road with 100,000 cars a day doesn't give a board 100,000 impressions, because not every driver is facing it, close enough or likely to look.

The rate card numbers, decoded

Term What it means How it's calculated
Impressions Estimated times people were likely to see the ad Geopath's model, usually per week or per 4 weeks
Reach Share of the market's population that saw it at least once Different people reached ÷ market population
Frequency Average times each reached person saw it Impressions ÷ people reached
GRPs (gross rating points) Total exposure as a share of the population Reach % × frequency, or impressions ÷ population × 100
CPM Cost per thousand impressions Price ÷ impressions × 1,000

Out-of-home has also traditionally been sold in daily GRPs, or "showings." A "50 showing" means a package whose daily impressions equal 50% of the market's population.

A worked example: a 2026 DC bus package

A 2026 Washington DC transit rate card we reviewed lists this market-wide package of side ads on 200 buses, for 4 weeks, using Geopath 2025 data for adults 18+ (see our DC rates guide):

Metric Value
Price About $165,000
Impressions About 17.4 million
Reach 31.7% of DC-area adults
Frequency 9.6

Here's how the numbers fit together:

  • GRPs: 31.7 × 9.6 ≈ 305 GRPs over 4 weeks.
  • People reached: 17.4 million ÷ 9.6 ≈ 1.8 million adults, each seeing an ad about 10 times.
  • Market size it implies: 1.8 million ÷ 31.7% ≈ 5.7 million adults in the DC market.
  • CPM: $165,000 ÷ 17,400 ≈ $9.50 per thousand impressions.

Doubling the package to 400 buses on the same card raises impressions to about 34.8 million but reach only to 47.6%. That's typical: more units mostly add frequency, because many of the extra impressions land on people you'd already reached.

How to use Geopath numbers when buying

  • Compare CPMs, not prices. A $3,000 board with 600,000 impressions ($5 CPM) is cheaper per view than a $2,000 board with 150,000 ($13 CPM). Run your quotes through the CPM calculator. Our city rate guides list CPMs where sellers publish impressions.
  • Ask whether impressions are Geopath-audited. Small operators often aren't measured. Their figures may come from raw traffic counts, which run much higher than Geopath's likely-to-see impressions and make CPMs look artificially low.
  • Ask for the audience you care about. Geopath can split impressions by age, income and other groups, so a seller can quote adults 25 to 54 instead of everyone.
  • Plan reach and frequency, not just impressions. For a local business, high frequency in a small area usually beats thin reach across a whole metro.
  • Check the period. Some sellers quote weekly impressions and others 4-week. Make sure you're dividing the right price by the right number.

What Geopath can't tell you

  • Whether people acted on the ad. Impressions measure exposure, not results. For that, use promo codes, search lifts and store visit data. See out-of-home advertising ROI.
  • Creative quality. A clear seven-word ad and an unreadable one get the same impressions. See billboard design tips.
  • Exact counts. These are model estimates built from samples and phone data, updated periodically. Treat them as the best common yardstick, not a census.

Geopath and programmatic DOOH

For digital screens sold programmatically, the estimated audience of each play (the impression multiplier) is often derived from Geopath data or a similar model. That's how a programmatic buyer paying a CPM is charged per play. See programmatic DOOH explained.

Frequently asked questions

What is Geopath?

The not-for-profit organization that measures out-of-home advertising audiences in the US. Founded in 1933 as the Traffic Audit Bureau, it estimates impressions, reach and frequency for billboards, transit and place-based media, and its data is the standard currency on rate cards.

How does Geopath calculate impressions?

It estimates traffic past each ad using anonymized phone location data, works out how many people have the opportunity to see the ad, then adjusts for visibility (size, distance, angle) using eye-tracking research. The result is people likely to have seen the ad.

What's the difference between impressions and traffic counts?

A traffic count is every vehicle or person that passes. Impressions count only the people likely to have seen the ad, so they're much lower. Comparing a board's traffic count with another board's Geopath impressions makes the first look much cheaper than it is.

What is a good CPM for a billboard?

Static bulletins in many markets run about $2 to $10 per thousand impressions; digital and premium big-city locations run higher. Compare CPMs within the same market and format. See how much a billboard costs.

What are GRPs in out-of-home advertising?

Gross rating points: reach percentage times frequency. A campaign reaching 30% of a market an average of 10 times delivers 300 GRPs. Out-of-home has also traditionally used daily GRPs, called showings.

Do all billboards have Geopath data?

No. Large operators measure most of their inventory, but many small operators aren't Geopath members. Ask how a seller's impressions were estimated.

Sources

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