Calculating the Impact of Outdoor Advertising: OTS, Impressions, Reach, and Frequency

### Calculating the Impact of Outdoor Advertising: OTS, Impressions, Reach, and Frequency

Introduction:

Outdoor advertising—both traditional OOH (Out-of-Home) and digital DOOH—plays a significant role in many marketing campaigns. But how does one truly measure the effectiveness of an outdoor campaign? For marketers and advertisers, it is crucial to understand the metrics used to calculate reach and exposure in outdoor channels. In this guide, we will clearly and inspiringly explain how to calculate the impact of outdoor advertising. We will focus on key concepts such as OTS (Opportunities To See), impressions, reach, and frequency—what they mean, how they are used in media planning, and how they differ from digital advertising metrics online. Additionally, we will discuss how these numbers should be interpreted in practice, including the opportunities and limitations of outdoor media measurements.

What is OTS (Opportunities To See)?

OTS stands for Opportunity To See, which literally means “the chance to see.” In outdoor advertising, OTS indicates how many potential contacts or exposure opportunities there are for a specific ad over a given period. It is a measure of potential exposure—essentially, how many times a target audience could see the message, provided they are in the right place at the right time. OTS is a traditional way of estimating a campaign's exposures: each time a person from the target group passes a billboard, it is counted as an “opportunity” for the ad to be seen.

It's important to emphasize that OTS does not guarantee that the ad has actually been observed by all these individuals—it’s about chances to see the ad, not confirmed viewers. Historically, advertisers have often paid for this “opportunity to see” rather than for the number of people who actually saw the advertisement. While one can estimate how many might pass by a billboard or advertising space, it has long been impossible to know exactly what proportion of these individuals actually look at the ad. Therefore, OTS provides us with an estimate of the extent of exposure, although in practice it is a kind of "gross contact" measure.

Impressions in Outdoor Advertising

Impressions refer to the actual number of times an advertisement is seen or comes into contact with the audience, providing a more concrete measure compared to OTS. In outdoor advertising, impressions account for the estimated number of views and are calculated based on the traffic or footfall around the ad location. This metric allows advertisers to gauge the effectiveness of their placements more accurately and understand the true reach of their campaigns.

As technology continues to evolve, the way we measure impressions is becoming more sophisticated, integrating data from various sources such as traffic counts, mobile data, and proximity marketing. This evolution enables advertisers to optimize their campaigns in real-time and refine their targeting strategies based on actual consumer behavior patterns.

In conclusion, understanding these metrics—OTS, impressions, reach, and frequency—is essential for any marketer looking to leverage outdoor advertising effectively. By grasping the nuances of how these measurements work, advertisers can develop more impactful campaigns that resonate with their target audience.

Impressions are closely related to OTS (Opportunities to See) and refer to the total number of times an ad has the potential to be seen. An impression can be viewed as a delivered exposure—each time the ad is displayed constitutes an opportunity to reach a person. In digital marketing, the term impressions is used to measure how many times an ad has been shown to a browser, regardless of whether the user clicks on it or not. In outdoor advertising, however, impressions are more representative of the number of potential contacts with people in the target audience. According to a common definition, impressions are "the total number of times an ad could potentially be seen."

How are impressions calculated in OOH/DOOH? Typically, traffic data, mobility statistics, or sensor technology are used to determine these figures. For example, if approximately 100,000 vehicles pass a digital billboard in a day, and each vehicle has an average of 1.5 passengers, the estimated number of impressions for that billboard per day would be around 150,000. In this way, real mobility data is leveraged to estimate how many "eyes" are exposed to the ad. In Sweden, for instance, there is an official measurement tool called Outdoor Impact 2.0, a planning system that helps calculate the number of contacts (impressions), reach, and frequency for outdoor advertising. This system collects data on traffic flows and audience composition for tens of thousands of advertising spaces, allowing advertisers to forecast how many contacts a campaign will generate.

It is worth comparing this with digital advertising: online, an impression corresponds to an ad being shown on a screen, typically to one user. Outdoor advertising, on the other hand, is a "one-to-many" medium—one single poster can be seen by many people simultaneously or within a short period. Consequently, the number of views is often multiplied by factors based on audience size. In DOOH contexts, media owners apply an impressions multiplier per ad display on a screen to account for the fact that multiple people can see the signage at any given time.

A key difference is that impressions in OOH (Out-Of-Home) advertising are based on estimates and models derived from traffic and movement data—not actual pairs of eyes that have definitely seen the ad. In other words, OOH impressions are fundamentally estimated opportunities for an ad to be exposed to an audience rather than confirmed views. On the other hand, many digital impressions also do not guarantee actual exposure; a digital ad sent to a device does not ensure it was visibly displayed on the screen or that anyone actually looked at it. This has led to the emergence of concepts like viewable impressions online, to ensure that the ad was indeed visible. In classic outdoor advertising (such as a static billboard), visibility is essentially 100%—the sign is physically present—but it is still not certain that all passersby actually notice it. For example, a study by Lumen Research showed that a static billboard was seen by about 82% of the audience passing by (as the sign is large, well-placed, and hard to miss), while an average digital mobile ad was only registered by around 35% of those who had the opportunity to see it. This illustrates both the strength of outdoor advertising (great visual impact) and the limitation of the impressions concept—the numbers reflect potential pairs of eyes, not a guarantee of attention.

Reach – How Many Unique People Are Reached?

Reach is a central metric that indicates how many unique individuals are exposed to your ad during a campaign period. Unlike impressions (which can count the same person multiple times if they see the ad repeatedly), reach focuses on the unique number of individuals you reach at least once. Reach can be expressed as an absolute figure (e.g., 500,000 people in the target audience) or as a proportion/percentage of the target audience. Industry tools like Outdoor Impact define reach as the percentage of the target audience that has the opportunity to see the campaign at least once during the period. For instance, if a digital billboard at a traffic junction has 500,000 unique passersby in a week, the reach for a week-long campaign on that sign would be 500,000 (assuming everyone is exposed at least once).

High reach means that the campaign reaches a broad audience. Outdoor advertising is known for its ability to build wide reach relatively quickly, as advertising spaces are often placed where many people pass by daily. Net reach is a term sometimes used to emphasize that duplicates are filtered out – it focuses on unique individuals. In media planning, goals for reach are often set, such as aiming to reach 50% of a specific target group in a region. Since reach measures audience spread, it serves as an indicator of the campaign's breadth and potential to create awareness.

It's also worth noting that reach in outdoor advertising is based on population and movement data. Measurements like "VAC reach" (Visibility Adjusted Contact) also adjust for likely visibility – meaning they attempt to estimate what portion of passersby actually notice the billboard. These methods refine reach calculations by considering factors such as distance, angle, speed (e.g., cars driving by), and how long the ad is visible, providing a more realistic picture of how many people may have genuinely seen the message.

Frequency – how many times does each person see the ad?

Frequency refers to how many times, on average, the people included in the reach see the ad over the course of the campaign. While reach answers the question "How many unique individuals did we reach?" frequency answers the question "How often did we reach them?" Frequency is mathematically calculated as impressions divided by reach. For example, if a campaign generates 1,000,000 impressions and the reach is estimated to be 250,000 unique individuals, the average frequency would be 4 (meaning each person saw the ad 4 times on average).

In practice, discussions often revolve around OTS (Opportunities to See) per person or average contacts per individual, which essentially refers to frequency. A high frequency means that the target audience is exposed to the message repeatedly. This can be effective for increasing recognition and message retention—often it takes several exposures before a person truly registers and remembers an ad. On the flip side, excessive frequency can lead to "wear-out," meaning the audience becomes fatigued or the message loses its novelty. Therefore, media planning aims for a balanced frequency. You might want to ensure that each person in the target group sees the campaign at least a minimum number of times (e.g., 3+ times is often mentioned as a benchmark for ads to make an impression), while avoiding spending the budget on many repetitions to the same individuals when you could instead increase reach and connect with new audiences.

Reach and frequency go hand in hand. Together, they provide a comprehensive picture of a campaign’s spread and impact. In fact, the product of reach (%) and average frequency is what is known in advertising as GRP (Gross Rating Points)—a measure of total gross exposure. For example, 50% reach with a frequency of 4 yields 200 GRP. However, to assess the effectiveness of a campaign, it's often more meaningful to consider reach and frequency separately: How many did we reach, and how many times? As one expert put it: “To know if the advertising investments are doing their job, it's also essential to measure reach and frequency. [You need to know] how many have seen our ad and how many times.”

This is how the figures are utilized in practice (media planning).

In practical media planning for outdoor advertising, one starts from the aforementioned metrics to build an effective campaign. Planners can utilize tools and data (such as Outdoor Impact in Sweden) to simulate how a combination of advertising spaces – e.g., a certain number of digital screens in city Y for two weeks – will deliver a specific reach and frequency. By adjusting the number of units, location(s), and the duration of the campaign, they can see how reach increases (more locations mean more unique individuals reached) and how frequency is affected (a longer campaign or more viewing cycles boosts the number of exposures per person). The goal is often to optimize the balance: to reach as large a portion of the target audience as possible (reach) sufficiently many times (frequency) for the message to stick – without wasting impressions by showing the same person the ad too many times unnecessarily.

For example, a local campaign might focus on high reach in a specific city over a short period to quickly build awareness (many different people see it at least once). Another campaign, say for a product launch, might prioritize frequency: wanting the primary audience to encounter the message repeatedly over a month to really drive the message home. OOH media is flexible in this way – by combining different formats (billboards, bus stops, digital screens in malls, etc.), one can either broaden the message or focus it, depending on campaign goals.

Because outdoor advertising generally has a low cost per contact, it is often possible to achieve high frequency within a reasonable budget. That is, one can reach the same person multiple times without costs spiraling compared to some other media. At the same time, it’s essential to utilize the strength of outdoor advertising in reach: with the right locations, one can reach a large portion of the population in, for instance, a city. Here, media planners should use OTS, impressions, reach, and frequency figures to calculate the optimal setup. A rule of thumb is to first ensure a good baseline reach – that a sufficiently large segment of the target audience is actually exposed to the campaign – and then ensure that these individuals see the message enough times for it to have an effect.

Comparison with digital advertising (online).

It's natural to wonder how OOH/DOOH metrics relate to the figures we're accustomed to in digital advertising. On the surface, they look quite similar—we're talking about impressions online as well, along with reach in terms of unique browsers or users, and frequency per user can also be calculated for digital campaigns. However, there are significant differences in measurement methods and implications:

Measurement Method and Accuracy:

In digital channels, each impression is technically logged when the ad is delivered to a device. This means that you can precisely state how many times the ad attempted to display. Conversely, as mentioned, not every digital impression is an actual view—the user might scroll past, the ad might be out of the screen area, etc. In OOH, on the other hand, flows of people/vehicles are measured and statistical assumptions are made. This is not an exact figure on an individual level, but the industry uses standardized models to ensure it's as reliable as possible. For instance, the Outdoor Impact model is continuously updated to better reflect reality. In practice, this means that OOH figures should always be interpreted as estimates. They provide a good overall picture of campaign reach and frequency, but they lack millimeter precision.

"Contacts" vs. "Impressions":

Another difference is that the outdoor advertising world traditionally talks about contacts (i.e., people reached), while the digital world refers to browsers/devices. As one expert pointed out: “Contacts are people, impressions are devices—you can't directly compare them.” This means that 1 impression online equals one ad sent to one device, but it’s unclear whether that person actually saw it. In contrast, 1 contact offline refers to an actual person who has been exposed (at least theoretically) to an ad. This distinction indicates that we shouldn't mix numbers from different media arbitrarily. An outdoor "impression" based on traffic data isn't quite the same as a digital "impression" in an ad server, even if the terms are identical—the contexts differ.

### Engagement and Clicks: Digital advertising is often measured by engagement in the form of clicks, video views, conversions, etc. Outdoor advertising rarely generates immediate clicks (after all, no one clicks on a poster), so its impact is measured more in terms of reach/frequency and subsequent behaviors. However, there are new opportunities for DOOH: if a screen displays a QR code, scans can be tracked as equivalents to “clicks.” Additionally, through mobile tracking, one can see if individuals who were near a sign later visit the advertiser's website or store (known as footfall attribution or website traffic lift). Despite these innovative methods, it's crucial to interpret OOH results with the right expectations—often the effect is about increased awareness, brand lift, and influenced attitudes that may not show up directly in click statistics but can be measured through brand surveys or sales data over time.

### The Time Aspect: Online, campaigns can be optimized in real-time based on performance data (increasing/decreasing budget if CTR or conversions fluctuate). DOOH is beginning to approach this through programmatic buying and dynamic content, but traditionally, outdoor advertising is booked weekly and monthly, with its effectiveness evaluated after the campaign ends. This makes OOH more suited for top-of-funnel goals (broad exposure, brand building) rather than direct conversions, while digital online advertising typically handles the lower funnel (driving clicks to purchases, etc.). The combination of the two can be powerful—OOH provides breadth and attention, while digital picks up to drive engagement and conversions.

### Interpretation and Real Implications – Opportunities and Limitations

When evaluating OOH/DOOH campaigns based on OTS, impressions, reach, and frequency, we should keep in mind both the significant opportunities that the medium presents and its inherent limitations:

### Opportunities

Outdoor advertising (OOH) can deliver extensive reach and high frequency at a relatively low cost per contact. An effective OOH campaign can reach millions of people in a big city, exposing them to the message multiple times over the course of a week. The format itself—large, visual messages in public spaces—provides a type of guaranteed visibility that many digital ads struggle to match (a poster is “in view” whether you want it or not, as long as you are in the location). Studies have shown that when outdoor messages are actually seen, they can significantly impact brand awareness and attitudes, especially when viewers spend more time engaging with the message. Digital Out of Home (DOOH) adds flexibility and dynamism—ads can be swapped out, adjusted based on the time of day or weather, and through technological solutions, data can now be collected on how many people were nearby and even interacted (e.g., via QR codes or through anonymized mobile tracking). This makes outdoor advertising more measurable than ever and integrable into a data-driven ecosystem.

### Limitations

However, the uncertainty in measurements is something that must be acknowledged. Impressions and Opportunities to See (OTS) are based on traffic and movement data, not on actual pairs of eyes. This means that when we say a campaign generated 1 million impressions, we’re talking about 1 million potential contacts—not that 1 million people definitely saw the ad. In reality, some of those passersby might have been engrossed in their mobile phones, sitting on a bus looking the other way, or perhaps it was dark/rainy making the sign less visible. Fortunately, outdoor media somewhat mitigates this due to its nature—a well-placed sign will capture many gazes. But exactly how many is difficult to determine. This leads to evaluations of OOH often being supplemented with other methods: for example, interviews/surveys (asking a group if they recall the campaign), sales analysis during the campaign period, or newer solutions where exposure (via anonymized mobile locations) is matched against behaviors (such as store visits or web searches).

### Interpreting Reach and Frequency:

When you see figures like 50% reach and a frequency of 5 for an OOH campaign, interpret it as "half of the target audience had the opportunity to see my ad, on average, five times." This is a strong foundation for building impact—most people, as mentioned, need repeated exposures for the message to sink in. However, also remember that the quality of these contacts matters. An OTS (Opportunity To See) does not guarantee engagement. Two people may both count towards reach, but only one of them might actually notice the advertisement. This is why there's increasing discussion around attention as a complement to reach today. For example, longer exposure times and creatively designed ads that capture interest have been shown to significantly enhance effectiveness.

In summary: Calculating effectiveness in outdoor advertising involves understanding and balancing gross figures with reality. OTS, impressions, reach, and frequency provide us with critical metrics for how a campaign performs in theory and in planning. They assist us in purchasing the right number of billboards and ensuring the ad is shown enough times to reach our audience. At the same time, we must interpret these numbers with insight into what they represent. Impressions in OOH are not instantaneous clicks but rather estimates of potential exposure based on people's movement patterns. By being aware of this and combining OOH metrics with other analytical methods, marketers can obtain a credible and holistic view of the campaign's reach and effectiveness. This way, the value of outdoor advertising becomes clearer: when used correctly, it can provide broad visibility and repeated exposure cost-effectively, which in turn builds brand awareness and drives behaviors—even though the path from an OOH impression to an actual conversion can be both indirect and difficult to measure. With a solid understanding of OTS, impressions, reach, and frequency, you as an advertiser are better equipped to plan, evaluate, and justify your OOH/DOOH investments in an informed way, and to harness the full potential of the medium.

Sources:

[1] Folkbilda – Definition of OTS (Opportunity To See) [2] Sveriges Annonsörer – Advertisers have historically paid for "opportunity to see" rather than actual views [4] Broadsign – Impressions in DOOH: definition and calculation examples [6] Leeads/Outdoor Impact – Outdoor Impact 2.0: planning system for contacts, reach, and frequency [7] Broadsign – OOH as a one-to-many medium and the use of impressions multiplier [9] Sveriges Annonsörer – Study on actual visibility: comparison between mobile ads and billboards [3] Dagens Media – Reach vs impressions in digital advertising (explanation of concepts) [8] Sveriges Annonsörer – "Impressions" digitally do not necessarily mean shown or seen ad [11] Broadsign – Calculation of reach and frequency with examples [10] Folkbilda – VAC-reach: percentage of the target audience that can see the campaign at least once [18] Sveriges Annonsörer – Increased viewing time on ads leads to a significantly increased effect (attention time linked to effect) [16] Sveriges Annonsörer – Concept confusion: contacts (people) vs impressions (units) [13] Sveriges Annonsörer – The importance of measuring reach and frequency – knowing how many have seen and how many times [14] JCDecaux (Guidelines) – Outdoor advertising provides high frequency at low contact cost

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[3] The Expert Clarifies – What the Measurement Terms Mean - Dagens Media

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[15] New data on outdoor advertising – fewer contacts but increased reach

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