Each mode has a real average emission factor EF, in grams of CO2-equivalent per passenger-kilometre (gCO2e/pkm). For a trip of distance d km, the surface-transport footprint is:
Footprint(d) = d × EF [gCO2e]
Flights get an extra factor: contrail cirrus, water vapour and NOx at altitude create additional warming that plain CO2 accounting misses. Aviation-climate studies apply a radiative forcing index (RFI), roughly 1.9×, to approximate it:
Footprint_flight(d) = d × EF_flight × RFI
Published per-passenger factors already assume an average occupancy (DEFRA/BEIS uses ~1.5 people per car). A car or EV's tailpipe/grid emissions are fixed per vehicle-km, so filling more seats divides that same footprint among more people:
EF_effective(N) = EF_avg(1.5) × 1.5 / N
- Trip distance — the distance every mode races, in kilometres.
- Occupancy — people sharing the car/EV; higher occupancy lowers each traveller's share of the same vehicle emissions.
- RFI — how strongly non-CO2 aviation effects are weighted; 1.9× is the commonly cited mid-estimate, shown adjustable here to explore the sensitivity.
- Base emission factors (gCO2e/pkm, average occupancy): domestic flight 246 (×RFI), petrol car 170 (occ 1.5), electric car 53 (occ 1.5), local bus 82, national rail 35 — in line with published UK DEFRA/BEIS greenhouse-gas conversion factors.
Top track: every mode moves along its own lane at a race-scaled speed (not literal cruising speed) — drag to pan, use the Follow button to auto-track the leader. Bottom chart: each mode's cumulative footprint traced against distance covered — the slope of each line is its emission factor, so the steepest line is the dirtiest mode per kilometre. Right-hand bars show the current standings. This is the arithmetic behind every travel carbon calculator and "flight shame" (flygskam) advocacy.