Switching From Cars to Buses: Public Transport Could Cut Pollution per Traveller

The researchers found that buses produced the lowest estimated pollution per passenger across the vehicle categories studied, with the largest potential gains coming from people switching from cars to public transport.

A crowded bus can carry dozens of people through a city using far fewer vehicles than the same journeys made by car or motorcycle, giving transport planners a practical way to reduce pollution from everyday travel. The study "Public Transit as a Catalyst for Emission Mitigation and Air Pollution Reduction: A Case Study in India," published in Gases, examines this opportunity in Bhubaneswar, Odisha, and estimates how different travel choices affect emissions per passenger.

The researchers found that buses produced the lowest estimated pollution per passenger across the vehicle categories studied, with the largest potential gains coming from people switching from cars to public transport. Passenger numbers made a substantial difference because the emissions from one bus were shared across many more travellers.

Understanding Bhubaneswar's Everyday Traffic

The research team collected data between October and December 2025 through traffic videos, roadside questionnaires and bus passenger counts, covering morning and evening peaks and quieter weekday periods. They completed 385 driver surveys and checked passenger occupancy across 85 bus trips, using conductor records to help validate the bus counts.

Drivers provided information about vehicle age, fuel mileage and Bharat Stage emission standards, giving the researchers a picture of the vehicles actually using the surveyed roads. Their calculations combined these observations with regulatory emission limits and an activity-based approach informed by IPCC guidelines.

Two-wheelers accounted for about 61% of observed vehicles, making them the dominant form of traffic. Cars represented roughly 26%, three-wheelers about 12%, and buses less than 1%. Buses carried approximately 11.8% of the travellers despite their small share of vehicles, illustrating how road traffic counts can hide the contribution of public transport. Average occupancy was 1.67 people for two-wheelers, 2.83 for three-wheelers, 3.08 for cars and 43 for buses. Commercial freight trucks fell outside the passenger transport assessment.

Why Passenger Numbers Change the Pollution Picture

The study focused on carbon monoxide, or CO, and combined hydrocarbons and nitrogen oxides, written as HC+NOₓ. Its central comparison measured the estimated grams of these pollutants associated with transporting one person over one kilometre.

Cars had the highest estimated CO emissions per passenger, at 1.44 grams per kilometre, compared with 0.35 grams for three-wheelers, 0.25 grams for two-wheelers and 0.13 grams for buses. For combined HC+NOₓ emissions, three-wheelers had the highest value at 0.27 grams per passenger-kilometre, followed by cars at 0.24 grams, two-wheelers at 0.15 grams and buses at 0.07 grams.

A bus emitted more pollution per vehicle-kilometre than the smaller vehicles in the model, but its greater passenger load spread those emissions across enough people to give it the lowest figures per traveller. At the observed occupancy, one bus could carry approximately as many people as 14 cars or 26 two-wheelers. Two-wheelers averaged 48 kilometres per litre, three-wheelers 33, cars 16 and buses five, showing why a vehicle that consumes more fuel can still offer a lower pollution burden for each person transported.

India's stricter Bharat Stage standards remain valuable: the paper reports substantial reductions in permissible emissions between BS-III and BS-VI, including around 70% for diesel-car HC+NOₓ and more than 90% for bus NOₓ. Growing private vehicle use can offset these improvements, making occupancy and transport choices important parts of pollution control.

What Switching to Buses Could Achieve

The researchers modelled four hypothetical scenarios, each replacing a particular group's journeys with bus travel or moving everyone to buses. The reported reductions describe average emissions per passenger-kilometre across the modelled transport mix:

• Two-wheeler users switching to buses: CO fell by 8%, and HC+NOₓ by 17.37%.
• Three-wheeler users switching to buses: CO fell by 4.84%, and HC+NOₓ by 14.52%.
• Car users switching to buses: CO fell by 67.26%, and HC+NOₓ by 29.71%.
• Everyone travelling by bus: CO fell by 80.10%, and HC+NOₓ by 61.60%.

Car travel offered the largest opportunity among the individual categories because it combined a substantial passenger share with high estimated CO emissions per traveller. The all-bus scenario provides a theoretical benchmark rather than a prediction of what a city could immediately achieve.

The paper identifies a more gradual shift of 25–50% of car users to buses as a practical planning option, estimating average CO reductions of approximately 16.8–33.6% under its assumptions. Bus occupancy remained important when the researchers tested passenger loads 20% above and below the average. Estimated bus CO emissions ranged from roughly 0.11 to 0.16 grams per passenger-kilometre, retaining a clear advantage over cars within that tested range.

Making Public Transport Work for More People

Reducing pollution through bus travel requires more buses, frequent services and dedicated bus lanes that make everyday journeys easier. Expansion depends on funding, available drivers, depot space and charging facilities for electric buses. Bhubaneswar's hot, humid climate makes passenger comfort a practical concern.

The paper reports that engine-driven air conditioning can increase fuel use by 15–25% and suggests exploring low-energy personal cooling, which was not tested in this study. Electric vehicles remove direct exhaust emissions, but electricity production, tyre and brake wear, and road dust still affect the environment.

The results are estimates based on Bharat Stage emission limits, rather than actual exhaust measurements. Traffic jams, idling, acceleration, cold starts, ageing vehicles and changing passenger loads can affect real emissions. Carbon dioxide, particulate matter and methane were not assessed, so the reported reductions do not represent equal cuts in overall city pollution or greenhouse gases.

Future research needs on-road measurements, portable emission monitors and models that reflect changing traffic conditions. The study suggests that better bus services can reduce exhaust pollution per traveller when enough people use them.