When planning out-of-home advertising, actual reach is the central question: how many unique individuals truly see the campaign, rather than how many could potentially do so? To understand this, one must distinguish between pass-bys, traffic flows, and reach, and link these to VAC (Visibility Adjusted Contacts).

Pass-bys and traffic flows: Volume, not effect

• Pass-bys describe how many times people or vehicles move past a point during a given period. It is a volume metric, not a pure advertising metric. A single person can account for multiple pass-bys, such as on a daily commute.

• Traffic flows are a more structured way to describe these movements, mapping pedestrian, cyclist, car, and public transport flows to specific advertising sites and formats.

Together, these provide a picture of potential exposure—how often a site can be seen—but they do not indicate how many unique people are reached, nor how many actually perceive the message.

From potential to actual reach: Why VAC matters

Actual reach only occurs when traffic flows are adjusted for how well the site can actually be seen. International guidelines, such as ESOMAR’s global standards and JCDecaux’s “OOH Audience Measurement 101,” distinguish between:

• OTC (Opportunity To Contact): How many people pass by and could potentially see the ad.

• VAC (Visibility Adjusted Contacts): How many people have a high probability of having actually seen the ad.

To calculate VAC, factors such as the following are taken into account:

• Placement and height.

• Angle relative to the flow.

• Distance and speed (walking, driving, train, etc.).

• Obstructions and competing impressions.

• Dwell time in the environment.

Outdoor Impact 2.0 is an independent, industry-standard system for OOH measurement, developed by JCDecaux in collaboration with industry peers. The system uses VAC as its foundational metric. It calculates reach, frequency, and contacts by adjusting the contact volume based on how well each individual site can be seen according to defined visibility criteria. This means that two sites with an identical number of pass-bys may contribute very differently to actual reach; a screen or display in a slow-moving pedestrian walkway may yield more “real contacts” than a site along a fast-moving highway.

Networks, frequency, and unique individuals

In a network consisting of many sites, models are used to simulate how people move between different locations:

• How often the same individual passes multiple sites.

• How contacts are calculated without double-counting the same person.

Outdoor Impact 2.0 uses these models to derive reach (unique individuals), frequency (average number of contacts per individual), and the total number of VAC contacts for a campaign. This transforms raw volume figures into a realistic measure of how many people actually saw the campaign and how often.

DOOH: Combining movement with controllable digital exposure

In DOOH, additional dimensions are introduced:

• Share of loop and duration: Reach and contacts in a digital network are a function of both how many pass the screen (traffic flows) and how often your specific campaign appears in the loop (share of time, number of spot impressions).

• Time and location control: By steering playback toward specific times, days of the week, or segments (e.g., rush hour), you can increase the proportion of relevant contacts within a target audience rather than just maximizing total pass-bys.

The same logic applies to creative Innovate solutions. While they may have fewer total pass-bys, they contribute strongly to actual reach within a defined target audience thanks to high visibility, longer dwell time, and high memorability.

Why optimize toward VAC instead of just pass-bys?

International industry actors like ESOMAR and the Media Rating Council highlight the VAC mindset as essential for comparing (D)OOH with other media. It is about actual, visible contacts, not just potential ones. At JCDecaux, we summarize this by noting that the best (D)OOH measurement models “deliver figures on how many people actually saw your ad (VAC), not just how many could have seen it (OTC).”

Outdoor Impact 2.0 is developed on this same principle. By using VAC as our foundation, we can:

• Provide more accurate reach and frequency data.

• Optimize campaigns based on visible contacts rather than traffic volume.

• Make (D)OOH planning comparable to other media and channels that rely on viewability and real impressions.

Conclusion: Pass-bys are the starting point, but VAC is the destination. While traffic flows are important inputs that show where people move, Outdoor Impact and VAC translate those movements into actual, visible contacts and reach. In both OOH and DOOH, it is wise to treat pass-bys as baseline data, but to steer planning, optimization, and evaluation toward VAC-based reach and frequency in line with international measurement standards.

Read more about VAC in our article: [VAC and visibility – how do they connect?]