Is public transport less fuel-efficient than cars?

Question

Someone who is opposed towards energy saving and climate change (I did not realise they existed until I met him) has told me that public transport (buses, trains and so on) are less efficient than individual cars.

I found it difficult to believe. A bus is very big, but it can hold many people. A car could carry maybe 5 people maximum and usually, when commuting only one or two people will come. A bus could hold 30 or more. That means that an average car doing 35 mpg would only be better if a bus did about 1.2 mpg, which I find difficult to believe. I don't know about trains, though. This is also assuming the bus is full which it might not be, but probably is most of the time, especially during morning commutes, in my experience. Maybe not all day, though.

Is this guy lying to make a point, or does he actually have a point?

Answer 1

Yes, public buses appear to be worse than cars, at least based on the data I found for the US. (At present and on average. See the "Caveats" section for a discussion about this.)

Edit: the answer from DJClayworth appears to be based on the same information, I just noticed. bbc.co.uk just links to the overview for the Dept. of Energy report, but doesn't break things down.

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Update: I used data from National Transit Database (LINK) to calculate BTUs/passenger-mile for every US city. I was initially unsure of how to do this, but I just wasn't looking at the right documents. The data I used is available in the bundle of Excel files titled, "RY 2009 Data Tables - Complete Set (Self-extracting xls)" (LINK). The necessary files are "T17_Energy_Consumption.xls" and "T19_Op_Stats_Service.xls." From these, I was able to compare fuel used to passenger-miles traveled for all buses in the country. The results are as follows:

The Excel files above list all public transportation by type, so I sorted and pulled out all bus listings, then summed the listed passenger-miles for each state. I also used the conversion factors mentioned below to convert all of the fuel listing into BTUs and then simply divided BTUs by passenger-mile to find the rates. I used the value of 3,400 BTUs/passenger-mile as listed below and inserted "Car" into the chart. It appears 6th in the list; in other words, only five US states have achieved an efficiency higher than a car for transporting individuals.

See below for discussion about BTUs

I left the rest of the answer mostly as it was -- it jives fairly well with this data, other than the bus average coming out to about 5,000 BTUs/passenger-mile with the National Transport Database data vs. the Department of Energy value below of 4,300.

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Data/Analysis

Data to attempt to answer this question may reside in The Transportation Data Energy Book, published by the US Department of Energy (DoE). We will be pulling from Edition 29, published in June 2010 (available HERE).

Table 2.12 is shown here (LINK):

To highlight the pertinent values for 2008:

• Cars: 137 million cars traveled 1.6 quadrillion miles for a total of 2.6 quadrillion passenger-miles, and consumed 8.8 quadrillion BTUs of energy to do so.
• Buses: 67,000 buses traveled 2.4 billion miles for a total of 21.9 trillion passenger-miles and consumed 95 trillion BTUs of energy to do so.

Passenger-miles are a summation of (passenger_n * miles_n) for all passengers (1 passenger mile = 1 passenger travelling 1 mile, 2 passengers travelling 0.5 miles, etc.).

The key value is BTU/passenger-mile:

• Cars: 3,437 BTUs/passenger-mile
• Buses: 4,348 BTUs/passenger-mile

This means that, on average, it is taking buses more energy than it takes cars to transport a given number of individuals a given amount of distance.

By using BTUs, fuel types are normalized by converting to energy per volume. See Table 2.5 in Chapter 2 (LINK) for a breakdown of fuel type usage by vehicle type. Also, Appendix A (LINK) provides the conversion factors for different fuels to BTUs.

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BTU Discussion

The British Thermal Unit is a unit of energy. Analogs would be the calorie and the joule. Thus, the Department of Energy has taken each fuel type and translated it to an energy per volume output. Thus, if we know that a gallon of fuel X outputs Y BTUs, and we know the average number of BTUs required to move vehicle type Z a given distance, we have an efficiency value for each type of vehicle that has been normalized from fuel type to energy. Then we can analyze the energy required to transport a vehicle with it's typical passenger load (9.2 passengers per vehicle for buses, and 1.57 passengers per vehicle for cars) and determine energy per "passenger-mile."

We want a lower value here, since lower BTUs/passenger-mile means that it takes less energy to transport a given number of passengers a given distance.

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See Table 11.11 in Chapter 11 (LINK) for a breakdown of emissions by fuel type. Buses primarily use diesel, while cars primarily use gasoline; the emissions for these two aren't all that different, with diesel at 10,000 grams/gallon emitted and gasoline at 8,800 grams/gallon.

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Caveats

First off, this is only US data. I have no idea how the rest of the world compares.

Second, this is a snapshot. If buses were to increase their average "load factor" (persons/vehicle) such that their passenger miles were 30,000, they would be more efficient than cars on average. If buses reduced their BTU consumption, this would also help a great deal (I was blown away by the fact that buses require almost 8x the BTUs to travel 1 vehicle-mile compared to cars and 7x for trucks). Now, this is the data for all buses in all cities in operation. Thus, there are probably some cities doing quite well, while others are doing horribly. This paper abstract seems to confirm the same (emphasis mine):

The simulation results show that substitution of bus for car travel generally decreases the overall costs, particularly the costs of congestion, but increases exhaust emission costs if bus load factors are insufficiently high. In order to reduce exhaust emission costs from car to bus transfer at given load factors, the most effective policy option is to encourage the reduction of particulate emissions from bus engines. In terms of the overall costs, increasing bus load factors by relatively modest amounts can lead to substantial reductions in these overall costs. (SOURCE)

So, it seems to depend on the usage efficiency of the vehicle. I also note that the rail figures in Table 2.12 shows that "rail" travel does better than cars, so whatever circumstances are surrounding that mode may be of interest.

Lastly, suggested by @Ian, is that this comparison doesn't take into account the travel by individuals to the public transport departure location. This could be neutral in the case of walking, but it could be optimistic if folks are driving cars to a pickup location.

Answer 2

David JC MacKay's excellent book Sustainable Energy - without the hot air (free ebook), has a nice chapter on public transport.

At its best, shared public transport is far more energy-efficient than individual car-driving.

• A diesel-powered coach, carrying 49 passengers and doing 10 miles per gallon at 65 miles per hour, uses 6 kWh per 100 p-km (passenger-km) – 13 times better than the single-person car.
• Vancouver’s trolleybuses consume 270 kWh per vehicle-km, and have an average speed of 15 km/h. If the trolleybus has 40 passengers on board, then its passenger transport cost is 7 kWh per 100 p-km.
• London underground trains, at peak times, use 4.4 kWh per 100 p-km – 18 times better than individual cars.
• Even high-speed trains, which violate two of our energy-saving principles by going twice as fast as the car and weighing a lot, are much more energy efficient: if the electric high-speed train is full, its energy cost is 3 kWh per 100 p-km – that’s 27 times smaller than the car’s!

However, we must be realistic in our planning. Some trains, coaches, and buses are not full (figure 20.6). So the average energy cost of public transport is bigger than the best-case figures just mentioned. What’s the average energy-consumption of public transport systems, and what’s a realistic appraisal of how good they could be?

In 2006–7, the total energy cost of all London’s underground trains, including lighting, lifts, depots, and workshops, was 15 kWh per 100 p-km – five times better than our baseline car. In 2006–7 the energy cost of all London buses was 32 kWh per 100 p-km. Energy cost is not the only thing that matters, of course. Passengers care about speed: and the underground trains delivered higher speeds (an average of 33 km/h) than buses (18 km/h). Managers care about financial costs: the staff costs, per passenger-km, of underground trains are less than those of buses.

Answer 3

First of all, public transport is not just buses. In fact buses are the least efficient. Metro (aka subway) and electric trains and trams (streetcars) are more efficient and can use green power. For example all of Amsterdam's trams run on green power.

Even if you're talking buses, they can run on all kinds of green fuels, as they don't usually depend on random gas stations. There are buses running on ethanol, on hydrogen etc.

Also while comparing MPG you have to take in account huge difference in MPG in urban cycle, mixed cycle, and highway cycle. Buses ride mostly in the cities, while 20-something MPG for cars quoted above "assumes 55% city and 45% highway-miles". Difference between urban and combined with car is about 20-40% more fuel burned in the city. Thus 20MPG in combined cycle translates to something like 14-16MPG in urban cycle.

Talking about massive transport we should also include air transport. Airbus A380 has fuel efficiency of 2.9L/100km per passenger (that's 82 passenger-miles per gallon).

Answer 4

Here is an interesting article that seems to back up your friend's claims. The article says that bus occupancy is about 9, and car occupancy about 1.57, and cites a study that buses are more polluting than cars at these figures.

Answer 5

Other answers have shown evidence that public transport, particularly buses, is less efficient than car transport.

However, from the data, it looks like this is due to the buses being under capacity. They are inefficient compared to cars because they are much bigger and use more fuel while only providing transport for a few more people. From the table:

Transit buses provide an average load factor of 7.61 people/vehicle more than cars. Their energy use is much higher such that for these load factors, it is less efficient to use bus transport.

This isn't really a fair comparison because both vehicles could take more passengers. It is known that vehicles are most efficient per passenger when they hold the maximum number of passengers they can. This is because the extra energy cost of adding a person to a vehicle is much less than having another partially filled vehicle making the same journey.

To make a proper comparison, we would need data for the Btu per passenger mile for each vehicle used at its maximum capacity.

So this person isn't lying as such but the way he is using the data is misleading. The data used is based upon inefficient use of transport. If more people took the bus, the buses would become more efficient (a lower Btu per passenger-mile) and it would eliminate extra cars which themselves are currently expending energy.

Answer 6

I'm just adding one more source to the mix: http://www.delijn.be/over/milieu/co2_uitstoot_verkeer.htm

It is in dutch, but some highlights:

• avg traffic: 118 g/km
  • bus (14 persons): 85 g/km
  • tram (23 persons): 23 g/km (nuclear power)
  • train :28g/km

But do these numbers mean that adding new trams and trains will lower CO2 usage? Not necessarily:

• new lines may be less occupied
• people do change their behaviour when more transportation possibilities are present. Eg people start taking jobs further away if good train lines are there. See eg people moving to Lille which work in London: http://www.standard.co.uk/news/la-grande-commute-6842603.html

Answer 7

This page with 2010 statistics for Toronto's public transport includes ...

• 124 million km by bus
• 477 million passengers (total: by bus and subway)

If you assume that each passenger rides the bus for several ('x') km on average, that implies an average of approximately 4'x' passengers per bus (or actually a bit less because some number of those passengers only take the subway and not a bus ride).

I have to guess: and my guess is that this 'x' is greater than 2 (km per average passenger trip). The "Table 1.2" shown in other answers suggests you need an average of 11 passengers/bus to break even with cars (measuring fuel/passenger/distance), and therefore it seems to me that the TTC might break even on that metric.

Fuel consumption for a (unloaded) bus is relatively high (buses are neither light-weight nor aerodynamic, and outside the stop-and-start of the down-town core, expensive hybrid technologies like regenerative braking result in fuel economies of only about 10%).

It seems that the TTC doesn't publish and perhaps doesn't even collect statistics on passenger-miles: they'e only/mostly/more interested in vehicle miles, number of fares (passenger trips, not passenger distances, because fares are fixed-rate and distance-independent), and vehicle occupancy when and only when the occupancy get high enough to require scheduling more vehicles (in rush hour).

Outside of rush hour they are scheduled to provide convenient service (e.g. bus routes having service 4 or 5 times an hour throughout the day and into the night as well, and not only during rush hour), even when occupancy is low.

Two points to note:

• For each new passenger, it is 'greener' if they take the bus: because, the bus is running anyway.
• This communication published by the City of Toronto (which doesn't reference the source of every statistic, and which is written by an city counsillor who is an "environmental activist") says of public transit that we "we couldn't afford to do without it", according to a dozen or more other metrics (economic, social, and environmental).

On its page 11, Marilyn Churley's report cited above says, "average bus occupancy is about 20 people": but without explaining that figure. For all I know, that figure might be rush-hour-only: but if so that wouldn't be the first time that some expensive system is engineered to cope primarily with peak demand (it's ditto electricity generation, for example).

Answer 8

Yes, individual buses are ridiculously less efficient than cars. A Santa Barbara study (link blatantly stolen from Wiki) showed them to have a MPG rating between 4.8 and 6.0. This is obviously far worse than even the evil hummer.

Cars average something in the mid-20s MPG (according to bts.gov). In order to be more fuel efficient than individual cars, buses need to be carrying on-average 4-6 cars worth of people. I'm not finding any good statistics on the average number of people per car in US cities, and that's obviously needed to determine the "break-even point" for buses.

You also need to factor in the movement of the bus from parking spot to its route (not an issue for individual's cars), and include that time in the "average" vs the number of passengers.

Answer 9

Is this guy lying to make a point, or does he actually have a point?

It's difficult to know what his motive is (and if he does have a point then it's probably better if he expresses it than me).

The question is misleading, in various ways.

If you limit the discussion to only fuel efficiency, more relevant than average fuel efficiency is the incremental or marginal cost: for various (other, sufficient) reasons the buses are running anyway: so if you make a trip via public transport that uses no extra fuel; whereas if you take a private car that does use extra fuel.

Your using public transport increases total fuel consumption only at peak/rush-hour times (i.e. if you contribute to the bus being so full that they need to supply an extra bus). Note that this (peak/rush-hour utilization with full-buses) is exactly the time at which buses are actually more fuel-efficient per average-passenger-mile than cars.