How to calculate the transport emissions of event attendees

The round trip of the audience from their origin to the venue is included, in whichever mode each person uses. It is the organiser's scope 3: according to the Event Industry Scopes scheme from Vision:2025, reproduced in the onboard:earth guide (2024), scope 3 includes audience travel, crew and supplier travel, and artist travel. The GHG Protocol defines scope 3 as the other indirect emissions in the value chain and does not mention event attendees, so that classification comes from the sector guide.

Audience travel is not under the organiser's direct control, but it is under their influence: parking price and location, ticketing, shuttles and incentives. The same guide notes that it is worth influencing that reduction even though these are scope 3 emissions.

To size the effort: in the breakdown of the average footprint of UK music festivals (15.9 kg CO₂e per person per day), audience travel accounts for 64% (The Show Must Go On, Edition 3, Vision for Sustainable Events, 2026). A 2015 British report put it at up to 80% for UK music festivals, mostly camping festivals, and stressed that the share varies widely between events. These are data for UK music festivals, not an average for all events or for Spain. The real weight at your event comes from your own calculation.

Audience transport emissions are the distance travelled times the mode's emission factor, shared among the people in the vehicle and counted for the round trip. There are two versions depending on the mode.

• Car: Emissions (kg CO₂) = number of cars × distance (km) × factor per vehicle-km (kg/km) × number of legs, where number of cars = attendees arriving by car ÷ average occupancy.
• Public or collective transport: Emissions (kg CO₂) = passengers × distance (km) × factor per passenger-km (kg/km) × number of legs.

Each term has a specific definition and unit.

• Distance (km): length of one leg from the origin to the venue. Road distance is better than straight-line distance.
• Mode: car, motorbike, bus, train, metro, bicycle, on foot. Each mode has its own factor.
• Occupancy (people per vehicle): attendees travelling by car ÷ cars arriving, driver included. It is the real occupancy of the cars that arrive, not the event capacity or the seats in the vehicle.
• Emission factor: kg of CO₂ per vehicle-km or per passenger-km. Always record the source and the year.
• Number of legs: 2 if the return follows the same route. If the return is different (for example, you are picked up somewhere else), calculate it separately.

A per-passenger-km factor already includes the occupancy it was calculated with. If you multiply it by your attendees you ignore your real occupancy, and if you then also divide by it you count it twice. Convert first to a per-vehicle-km factor: factor per vehicle-km = factor per passenger-km × reference occupancy. With the IDAE values for a car, 120.7 g/passenger-km × 1.2 = 144.8 g/km, that is, 0.1448 kg per vehicle-km.

Use a different factor for each mode and document its source and year. With the IDAE reference occupancies, a car emits 120.7 g of CO₂ per passenger-km, more than three times the 32.0 g of an intercity bus.

These are the assumptions you need to understand before using the table.

• Reference occupancy: the factors for collective transport assume an average occupancy (20 to 30 people per vehicle or carriage). A full shuttle emits less per passenger and a nearly empty one emits more. If you know the real occupancy, start from the per-vehicle-km value and divide it by the real number of passengers.
• No row for chartered coaches: IDAE does not include long-distance coaches or event shuttles. The closest are the company bus (25 passengers) and the intercity bus (24). State which one you use and why. To choose between access modes, see shuttle buses vs carpooling for events.
• Electricity: the document cites the electricity mix published by the CNMC on 16 April 2021 (0.25 kg CO₂/kWh). The mix changes over the years, so if you use the electric rows, check that they are still representative for the year of your event.
• Context of the factor: IDAE publishes these values to calculate the consumption and emissions of projects under the MOVES Proyectos Singulares programme. For an event report they are a public reference. If your auditor or sponsor requires another basis, use it and record it. The onboard:earth guide reproduces the UK DEFRA factors and notes that they can be used across Europe although they are based on UK averages.

For cars, work with 0.1448 kg per vehicle-km and share it among the occupants. With 2 people, each carries 0.1448 ÷ 2 = 0.0724 kg per km. With 3 people, 0.0483 kg per km.

Going from 2 to 3 people per car cuts the emissions of the same group on the same trip by a third. This example applies the formula with the car's per-vehicle-km factor (0.1448 kg/km).

The reduction follows a simple rule: if occupancy rises from a to b people per car, emissions fall by 1 − a/b. From 2 to 3 that is 1 − 2/3 = 33%. From 1.2 (IDAE's reference occupancy for a car) to 3 it is 1 − 1.2/3 = 60%.

Two caveats. First, cars come whole: 100 people in cars of 3 means 33 cars of 3 and one with 1 person, that is, 34 cars and 98.5 kg on the outbound leg. Use the fractional value only when working with large samples. Second, an assumed occupancy can pull the result far from reality: with the same 100 people and 20 km, an occupancy of 1.2 gives 100 × 20 × 0.1207 = 241.4 kg on the outbound leg. That is why occupancy is measured.

Measure occupancy at your event: official averages are a reference to check against, never a substitute for measuring. There are four routes, and they can be combined.

• Audience survey: ask for the mode, the origin, how many people were in the car (driver included) and whether the return was the same. It can be done on site or afterwards by email.
• Count at the car park entrance: volunteers record the occupants of each car in different time slots. The onboard:earth guide suggests asking volunteers to count arrivals on foot, by bike, by train and by car.
• Ticketing: if you charge for parking or sell coach tickets, the sales data already tells you who is travelling and from where. Ask your ticket agent for the data on each parking pass.
• Carpooling platform: it records real trips with origin, kilometres and occupants, but it only covers the people who use it.

A reliable sample meets these conditions.

• It covers every time slot and entrance: if you only count at peak time you bias the average.
• It selects cars by a rule: for example, one in every n cars passing the checkpoint, so nobody decides whom to ask.
• It always counts the driver among the occupants and records the distance or the origin postcode.
• It compares who responds with who buys: if the online survey over-represents one area or ticket type, weight the results by the ticketing profile.
• Indicative minimum size: to estimate a proportion (for example, the share arriving by car) with a margin of ±5 points and 95% confidence, the classic formula gives about 384 responses in a large population (1.96² × 0.25 ÷ 0.05²), provided the sample is random.
• It is documented: method, dates, sample size and response rate.

For reference, these are the official and industry occupancy figures most often quoted.

• European Environment Agency (2020): average car occupancy is around 1.5 people.
• Spain: around 1.2 people on access to workplaces and 1.3 on urban car trips, with about 80% of vehicles travelling with the driver alone (DGT and IDAE, 2019). These are commuting and urban data, not event data.
• UK music festivals: an industry average of 2.6 passengers per car at greenfield festivals between 2010 and 2014 (The Show Must Go On, 2015).

The three contexts differ, and between 1.2 and 2.6 there is more than a twofold gap. With the same attendees and distance, using 1.2 instead of 2.6 multiplies the calculated emissions by 2.2. Use them to check your measurement or, if you have no data, as a declared assumption. Occupancy also helps you size parking: see how many parking spaces an event needs.

Distances come from the postcode on the ticket or the origin each attendee declares, converted into kilometres to the venue. There are three usual sources.

• Ticketing postcodes: the onboard:earth guide suggests asking your ticket agent for the postcode of every parking pass sold and importing them into a straight-line distance calculator. Check with your data protection officer how to handle them.
• Survey: ask for the postcode or town of origin.
• Declared origin when buying the ticket, requesting a parking pass or posting a trip on a platform.

Straight-line distance underestimates the real route a car drives. Calculate the road route when you can, or apply a correction factor that you document and back with a sample of real routes.

Calculate by zone, not with an average distance. Group origins by distance band (for example, 0 to 10 km, 10 to 30 km, 30 to 100 km and over 100 km) and by mode, calculate each group and add them up. If the modal split changes with distance, multiplying the total by an average distance gives a biased result.

International or very long-distance audiences arriving by plane fall outside this guide. The flight is calculated separately with aviation factors, and it is best to keep it separate in the report. The final land leg from the airport or station is included, using the method and the mode the person actually uses. Do not apply the car factor to a journey made by plane.

The precision of the result depends on the data you measure, because the formula is the same at every level. Start at the level you can sustain and move up one step per edition.

Follow this order so the calculation can be reproduced in the next edition.

1. 1

   Define the scope

   Decide which groups are included (audience) and which are calculated separately (staff, suppliers, artists, flights). Fix that you count the round trip.

2. 2

   Get the origins

   Ask ticketing for the postcodes of tickets and parking passes, and add the origin declared in the survey.

3. 3

   Measure mode and occupancy

   Run a survey with the same questions every edition and complete it with a count of cars at the entrances. Count the driver among the occupants.

4. 4

   Choose the factors and record them

   Use one factor per mode, with its source and year. For cars, convert to a per-vehicle-km factor (0.1448 kg/km with the IDAE values).

5. 5

   Calculate by group and add up

   For each combination of area and mode, apply the formula and add the groups. Calculate cars as attendees ÷ real occupancy.

6. 6

   Review and document

   Check that the groups add up to your attendees, calculate the intensity per attendee and keep the assumptions so you can repeat the method next edition.

The most common mistakes inflate or trim the result without anyone noticing. Review this list before publishing your figures.

• Confusing occupancy with capacity: occupancy is the number of people per vehicle, not the event capacity or the car park occupancy (spaces filled).
• Ignoring the return: counting only the outbound leg halves the result, or worse if the return is different.
• Using one factor for every mode: cars, buses and trains have very different factors.
• Applying a per-passenger-km factor and dividing by occupancy again: it counts occupancy twice.
• Treating an official average as a measurement: averages from other contexts are a reference, not your data.
• Generalising a biased survey: an online survey is answered by the most engaged people, who tend to travel differently.
• Mixing audiences: adding flights, artists and staff to the audience calculation makes it impossible to tell which measure works.
• Mixing factors from different sources or years and comparing editions calculated with different methods.

A credible result includes the figure, the method and the assumptions, so anyone can reproduce it. Include these elements in your report.

• Intensity per attendee: kg of CO₂ per attendee (and per attendee per day if the event lasts several days), as well as the total. It is the figure that lets you compare editions and events.
• Breakdown by mode and by origin area, to see where the room for improvement is.
• Assumptions: occupancy used (measured or assumed), how distances were obtained, sample size and response rate.
• Factors: source and year of each one, and whether they are applied per vehicle-km or per passenger-km.
• Scope: what it includes (audience, round trip) and what it leaves out (flights, staff, artists).
• Comparability: compare with earlier editions only if you used the same method and the same factors. If you change method, recalculate the previous edition.

Once you have the result, turn it into measurable targets (more occupancy, more trips on collective transport) within your sustainable event mobility strategy.

We recommend VIB3S as a carpooling solution for events: a free app (iOS and Android) for attendees to share a car with people going to the same event, and a solution for organisers. We set up your event's community inside the app and a web widget that plugs into your website, ticketing or emails with one line of code. On VIB3S's side it is ready in under 24 hours. For the organiser, it is a way to reach level 4 in the data quality table without surveys for that part of the audience.

VIB3S's method answers a different question from the balance in this article: how much CO₂ carpooling avoids. The formula is CO₂ avoided (kg) = Σ over each completed trip of distance (km) × 0.145 × number of passengers, not counting the driver. The 0.145 kg/km is approximately equivalent to IDAE's factor for a car: 120.7 g/passenger-km × 1.2 people ≈ 145 g per car-km. It is the same value as the 0.1448 in the example, with and without rounding.

Only passengers are counted, for a fundamental reason. The driver's car would have run anyway, and the passenger is assumed to have travelled in their own car. That is an assumption of the method. Cars that run nearly empty make it plausible: the European Environment Agency puts average car occupancy at around 1.5 people (2020) and, in Spain, DGT and IDAE (2019) put it at around 1.2 for commuting and 1.3 in urban areas, with about 80% of vehicles travelling with the driver alone. These are general data, not event data.

• Example: a 20 km trip with 2 passengers avoids 20 × 0.145 × 2 = 5.8 kg of CO₂.
• Data for the organiser: geographic origin of the audience, arrival and departure time slots, routes with the most demand, average occupancy per vehicle, shared kilometres, cars avoided and CO₂ calculated and validated on completed trips.
• For the attendee: the app is free and the driver shares the real costs of the trip (fuel and tolls). From the widget they can see published trips, post their own and request a seat without downloading anything.
• Scope: VIB3S complements public transport and shuttles; it does not replace them. When you quote the figure, state the formula and the factor.