Public or private transport - we have fixed ideas about which is best forthe environment. But, says Dr David Timoney, we could be quite wrong.
We are constantly told of the merits of public transport, quality bus corridors and suchlike. Yet the discussion is often focussed on a small fraction of the complex needs of the travelling public.
The standard claim seems to be that public transport is "good" and private transport "bad" from a "sustainability" perspective. However, look deeper and a more complex picture emerges.
First, consider the energy expended in putting the car on the road. Vehicles involve substantial use of energy in various stages, some more obvious than others. Primarily energy must be supplied as fuel to the tank. However, preparation and delivery of the fuel to your local filling station involves many hidden energy burdens, such as the extraction of crude oil, processing and transporting it to your tank.
The vehicle's production also involves substantial use of energy, as does the manufacture of raw materials used to make individual components. Energy is also used in ongoing maintenance. Even washing and cleaning requires energy.
Further analysis can include the energy used in creating and maintaining the road infrastructure, a significant task and expenditure of constant energy.
Total energy use in passenger transport is usually expressed in terms of the energy needed to move a passenger through a distance on one kilometre. The units used are therefore in kilojoules (kJ) per passenger kilometre. A number of issues have to be considered in calculation of a representative figure for different modes of transport.
Excluding the primary energy used in vehicle operation, the energy use incurred over its useful lifetime falls mainly under the headings of raw material sourcing, manufacture, and maintenance. This total can reasonably be shared out equally over each kilometre travelled, even though a range of occupancy rates will apply over this lifetime.
The energy then used in vehicle operation depends mainly on distance travelled but will be affected by vehicle weight, engine size and efficiency, traffic congestion, average speed and the weight of passengers on board.
Taking all the factors into account, a petrol car typically uses a total of about 4,000 kJ per kilometre travelled. This compares with about 22,000 kJ per kilometre for a city bus, 50,000 kJ per kilometre for a tram, about 178,000 kJ per kilometre for an urban train, and about 330,000 kJ per kilometre for a heavy rail train.
The total energy used per passenger per kilometre travelled in a vehicle depends very strongly on the number of people travelling together. For example, we see that a car with two people requires about 2,000 kJ per passenger kilometre. To do better than this, a bus needs more than 11 passengers, a tram more than 25, an urban train more than 89 and a heavy rail system more than 165 people on board.
This raises questions about quality or level of service offered by the public a transport system. If a relatively small number of people wish to complete the same journey at the same time, the private car has very significant energy advantages.
If, on the other hand, a relatively large number of people can be persuaded to climb on board a public service vehicle at the same time for the same journey, then public transport yields energy advantages.
The average occupancy levels required to lower energy use levels for the bus and rail modes are often incompatible with the expectations of modern life.
Under-utilised transport capacity leads to greater energy consumption, pollution and road damage per unit of transport service delivered. Movement of population towards suburbs and less concentrated employment patterns lead to lower transit system occupancy levels.
What about walking and cycling? Non energy using forms of transportation? Maybe not. Cycling at 16 km/h (or about 10 mph) gives rise to an increase in human food energy requirement (above that for 'sedentary' activities) of about 72 kJ per kilometre (km) travelled. This is equivalent to about 104 miles per gallon of extra milk. Adding in allowances for bicycle manufacture and tyres, repairs and maintenance, we get a total of about 360 kJ per kilometre.
Also, if we include the primary energy used in growing, processing, transporting, selling, and preparing of food (based on a normal diet, rather than on 100 per cent milk), the total primary energy use associated with bicycle travel rises to about 900 kJ per kilometre. This figure excludes some overheads associated with showering and clothes washing but is still relatively low compared to other transportation modes.
Walking at normal speed gives rise to an increase in food consumption requirement of about 330 kJ per kilometre, equivalent to 23 miles per gallon of milk. Including the primary energy used in food production etc., the total for walking rises to almost 3000 kJ per kilometre.
Note that walking is therefore ultimately responsible for energy consumption at a level equivalent to that of a private car at an average occupancy of 1.5!
You may enjoy walking. I enjoy driving. Is there really such a difference between the environmental impacts?
Dr David J Timoney is director of the energy conversion research centre at the department of mechanical engineering at UCD