Economic cost is certainly not the only reason to worry about environmental impacts, but it does offer a useful way to quantify and compare impacts.

When talking of the economic costs of transport, we can separate internal costs and external costs –

• Internal costs are those paid by the user, for example the costs of petrol used to run a car or the price of a train ticket.

• External costs are those borne by society generally and not by individual users. For example, if pollution from cars causes health problems among the general population, or damage to historic buildings, these costs are external.

There are obviously costs associated with the health impacts, climate change and other effects of these pollutants. If we look at health impacts in economic terms sickness costs money - through treatment, lost earning and of course the "value" of lives shortened or lost.

With climate change costs can be considered either in terms of the costs of prevention or the probable costs of the damage caused in the future.

The information below gives an indication of the CO² created by different forms of transport;

Typical emissions for different transport modes

For a 10 km (6.2 mile) journey:

• A car, typically carrying 1 to 2 passengers, consumes about 1.2 litres of petrol and emits about 3 kg of carbon dioxide, i.e. about 1.7 kg per passenger

• A bus, typically carrying 60 people during peak hours, emits about 15 kg of CO2, i.e. about 0.25 kg per passenger

• An electric train emits about 36 kg of CO2 (in electricity generation stations), and could easily carry 150 people per vehicle during peak hours, about 0.24 kg per passenger

These figures are approximate and issues such as occupancy rates, vehicle specification and the type of journeys limit comparability. But they do illustrate that cars cause more pollution than any other form of passenger transport.

It is estimated that emissions of CO² will have to be reduced by 60% from present global emissions if we are to stabilise the world's climate. If the current rate of CO² emitted around the world were to be averaged out, it would mean that each person would be responsible for emitting 4.21 tonnes (4210 Kg) of CO² a year.

This would give everyone an equal allowance, which would have to reduce over time by 60%. At present this is not the case:

• an average Indian emits 0.81 tonnes / CO² / year (810 Kg)

• an average American emits 19.53 tonnes / CO² / year (19530 Kg).

Reducing the average emissions by 60% would give each person an allowance of 1.682 tonnes / CO² / year (1684 Kg) for all life activities.

Other external costs are not related to pollution. One obvious factor is the cost of accidents. Insurance payments cover some elements of the immediate costs of accidents but do not nearly cover the full costs in terms of a person's own valuations of their injuries or their lives. Thus injury and death caused by road accidents represents a major external cost of transport and cars are the most significant contributor in this regard. Other costs worth considering include noise costs and congestion costs.

Traffic noise costs money: it causes stress, makes areas less pleasant and devalues property. It is difficult to put a price on traffic noise but estimates can be made.