Transport accounts for approximately 28% of total US greenhouse gas emissions — the largest single sector. Source: EPA Inventory of US Greenhouse Gas Emissions and Sinks, 2023. For most households, personal vehicle use is the dominant transport source. Switching to public transit, or carpooling, is one of the highest-impact single decisions available to urban and suburban commuters.
The comparison is not simple, because emissions per passenger mile vary significantly by mode, occupancy, and regional grid intensity. This article works through the numbers.
Emissions per passenger mile by mode
The EPA provides emission factors for passenger vehicles and major public transit modes. All figures below are per passenger mile — the standard unit for transport comparisons.
| Mode | Emissions (g CO₂e per passenger mile) | Key variable |
|---|---|---|
| Gasoline car — solo driver | 350–400 | Vehicle fuel economy |
| Gasoline car — 2 occupants | 175–200 | Occupancy halves per-person emissions |
| Gasoline car — 4 occupants | 88–100 | Carpooling approaches transit efficiency |
| Bus (average US occupancy) | 150–250 | Highly occupancy-dependent |
| Commuter rail (diesel) | 100–200 | Route and occupancy dependent |
| Electric rail (average US grid) | 80–150 | Grid intensity is the key variable |
| Electric rail (low-carbon grid) | 20–50 | Near-zero on hydro/nuclear grids |
Source: EPA Greenhouse Gas Equivalencies Calculator; EPA Office of Transportation and Air Quality; American Public Transportation Association ridership data.
The wide ranges reflect real variation. A near-empty bus during off-peak hours can have higher per-passenger emissions than a full carpool. A subway running on a coal-heavy grid produces more emissions per mile than the EPA average suggests.
Why occupancy is the most important variable
Vehicle emissions are fixed per mile driven — they do not change based on how many people are in the vehicle. Emissions per passenger, however, are divided across occupants. This is why carpooling has such a dramatic effect: two people in a car cuts per-person emissions in half. Four people in a car brings per-passenger emissions close to bus-level efficiency.
The same logic applies to transit. A bus at 40% capacity has higher per-passenger emissions than a full bus. US transit systems average around 20–25% capacity utilization, which explains why some transit modes perform less favorably than their theoretical emissions suggest.
Annual commute comparison
At 10,000 commuting miles per year — roughly the US average for a round-trip commuter — the emissions difference between modes is substantial.
| Mode | Estimated annual emissions (tCO₂e) |
|---|---|
| Solo gasoline car | 3.5–4.0 |
| Gasoline car — 2 occupants | 1.8–2.0 |
| Average US bus | 1.5–2.5 |
| Commuter rail (diesel) | 1.0–2.0 |
| Electric subway / light rail | 0.8–1.5 |
Source: EPA emission factors applied to 10,000 passenger miles annually.
Switching from solo driving to commuter rail saves approximately 1.5–3.0 tCO₂e per year — comparable in scale to switching from a gasoline car to an EV, and achievable without any capital expenditure.
Urban vs suburban context
Public transit performs best when ridership is high, routes are frequent, and the electricity grid is clean. Dense urban environments — New York, Chicago, San Francisco, Boston — have transit systems that consistently outperform solo driving by a wide margin.
Suburban and rural environments present a harder case. Lower ridership means higher per-passenger emissions on many routes. Park-and-ride systems, where drivers take transit for the urban portion of a commute, offer a practical middle ground.
The IEA confirms that electrified rail systems powered by lower-carbon grids produce substantially lower lifecycle emissions than internal combustion vehicles, and that the gap widens as grids decarbonize. Source: IEA Rail report, 2023.
The e-bike and walking gap
One mode not in the EPA’s standard comparison data: e-bikes and cycling. For commutes under 10 miles, e-bikes powered by average US grid electricity produce approximately 5–20 g CO₂e per mile — an order of magnitude lower than any motorized alternative. Walking produces near-zero transport emissions. Both are worth considering for short urban commutes before comparing transit options.
What this means for your footprint
For most US households, transport is the largest or second-largest emissions category. A commuter switching from solo driving to transit or carpooling can reduce their transport footprint by 40–75% on commuting miles alone.
The calculation depends on your specific route, transit availability, and grid intensity. That is exactly what a personal footprint calculator is designed to work out.
Want to see how your commuting choices affect your total annual footprint? Use our carbon footprint calculator to get a personalized breakdown in 3 minutes — free, no account required. →
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