Fuel Efficiency in Transportation - Caveats

Caveats

Comparing fuel efficiency in transportation has several challenges:

• All transportation methods require a different infrastructure. The building, repairing and renovation of this must also be considered, and heavy usage of infrastructure will require earlier maintenance, which will also be energy consuming. Road transport requires an extensive network of well-maintained paved roads and trains require railroads and stations along the whole length of the track. Meanwhile, ships and aircraft only require a more limited amount of harbours or airports, without any en route infrastructure except for the occasional navigation beacon.

• Some types of transportation can take shorter routes, thereby (partly) offsetting an increase in energy consumption per unit of distance travelled. For example, a trip from the UK to Italy would take a boat an enormous detour via the Strait of Gibraltar, while cars need to travel much less distance and aircraft can (theoretically; in reality it's going from good to better) take by the shortest route or, in case of favourable winds, a more efficient route.

• There is a distinction between vehicle MPGe and passenger MPGe. Most of these entries cite passenger MPGe even if not explicitly stated. It is important not to compare energy figures that relate to unsimilar journeys. An airline jet cannot be used for an urban commute so when comparing aircraft with cars the car figures must take this into account.

• There is currently no agreed upon method of comparing electric vehicle efficiency to heat engine (fossil fuel) vehicle efficiency. However, current typical emissions and thermal energy consumption can be compared. Vehicle speed is also an important parameter, and a peer-reviewed evaluation which convolves these criteria may be found at http://www.bentham.org/open/toefj/articles/V001/11TOEFJ.pdf

• If the issue is rapid investment in new electric mass transit it is important to use emissions associated with the most polluting fuel because increased demand for electricity increases the use of polluting fuel used in generation for the immediate future, as well as low emissions fuels in the case of some countries.

• Systems that re-use vehicles like trains and buses can't be directly compared to vehicles that get parked at their destination. They use energy to return (less full) for more passengers and must sometimes run on schedules and routes with little patronage. These factors greatly affect overall system efficiencies. The energy costs of accumulating load need to be included. In the case of most mass transit distributing and accumulating load over many stops means that passenger kilometres are inherently a small proportion of vehicle kilometres see Transport Energy Metrics, Lessons from the west Coast Main line Modernisation and figures for London Underground in transport statistics for Great Britain 2003. Lessons from the west coast mainline modernisation suggest that long passenger rail should operate at less than 40% capacity utilisation and for London underground the figure is probably less than 15%.

• Most cars run at less than full capacity, with the usual average load being between 1 and 2. Cars are also subject to inefficiencies because of congestion and the need to negotiate road junctions. The impact of transport road building to reduce congestion should always be considered as should the improving efficiency of cars see http://www.hm-treasury.gov.uk/media/9/5/pbr_csr07_king840.pdf,

• Vehicles are not isolated systems. They usually form a part of larger systems whose design inherently determines energy consumption. Judging the value of transport systems by comparing the performance of their vehicles alone can be misleading. For instance, metro systems may have a poor energy efficiency per passenger kilometre, but their high throughput and low physical footprint makes the existence of high urban population densities viable. Total energy consumption per capita declines sharply as population density increases, since journeys become shorter.
• See also Logistics and Transport Focus (the Journal of the Charter Institute of Transport) vol 9 number 10 through volume 10 number 6 for a series of articles debating the general issues of fuel efficiency in transportation in the context of impact on climate change.