The sustainability of transport by sea, air and land

Overview

The basis on which modes can fairly be compared, and how sea, air, road, rail, freight, walking and cycling each stand up to it.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   On what basis should the sustainability of different transport modes be compared?

   Model answer

   On a set of criteria applied consistently, and per unit of transport work rather than per journey: energy use and greenhouse gas emissions per passenger-kilometre or per tonne-kilometre; emissions of local pollutants such as nitrogen oxides and particulates; noise; land take for infrastructure and parking; safety and casualty rates; the capital and embodied carbon cost of the infrastructure; accessibility and equity, that is who can use it; and the speed and usefulness of the service. The essential point is that no mode is sustainable or unsustainable in the abstract. A near-empty train and a full coach reverse the usual ranking, so load factor, distance and, above all, what the journey would otherwise have been made by, determine the answer.

2. 2

   How sustainable is transport by sea?

   Model answer

   Shipping is the most energy-efficient way of moving freight over long distances, with emissions per tonne-kilometre far lower than road or air, and it carries the overwhelming majority of world trade by volume with very little land take. Against that, marine fuels have historically been heavy residual oils producing sulphur dioxide and particulates, now limited by international rules on sulphur content; shipping is nonetheless a substantial and growing source of carbon dioxide; there are risks of oil spills, of chronic pollution, and of transferring invasive species in ballast water; port development involves dredging, land reclamation and habitat loss; and emissions and lorry traffic concentrate in port cities. Measures to improve it include slow steaming, larger and more efficient hulls, shore-side electricity so ships do not run engines in port, and alternative fuels such as liquefied natural gas, methanol and ammonia. Cruise shipping is a separate case, with high emissions per passenger and concentrated pressure on the small ports it visits.

3. 3

   Evaluate the sustainability of air transport.

   Model answer

   Aviation is fast and makes long-distance travel and high-value, perishable freight possible, and it supports tourism economies and connects peripheral regions and islands. Its sustainability problems are serious. It has the highest greenhouse gas emissions per passenger-kilometre of the common modes, and emissions released at altitude have additional warming effects, notably from contrails and nitrogen oxides, so its total climate impact is greater than its carbon dioxide alone. It causes noise and reduced air quality around airports, which is why runway proposals are so strongly contested, and airports and their access roads take large areas of land. It is the hardest sector to decarbonise, since batteries are impractical for long-haul aircraft and sustainable aviation fuels are currently limited in quantity and cost. Improvements come from newer, fuller aircraft, better air traffic management and shifting short-haul journeys to rail. There is genuine and unresolved disagreement about how far technology can be relied on and how far demand itself must be managed through taxation or limits on capacity.

4. 4

   How sustainable is road transport?

   Model answer

   Roads are flexible, provide door-to-door service, need no interchange, serve dispersed origins and destinations that public transport cannot, and are the only practical option for many rural journeys and for most local freight distribution. The costs are large. A single-occupancy car has high emissions per passenger-kilometre; roads and parking consume very large areas of urban land; congestion wastes time and fuel; exhaust emissions and particulates from tyres and brakes damage health, particularly beside busy roads where deprived populations are often concentrated; noise and severance divide communities; and road casualties remain significant. Electric vehicles remove tailpipe pollution and, where the electricity is low-carbon, cut greenhouse gas emissions substantially, but they do not reduce congestion, land take, road casualties or particulates from tyres and brakes, and they carry their own demands for battery materials, so electrification is a necessary part of a strategy but not a substitute for reducing car use.

5. 5

   Evaluate the sustainability of rail transport.

   Model answer

   Rail moves large numbers of people and large tonnages of freight with low emissions per passenger-kilometre or tonne-kilometre, especially where the line is electrified and the electricity is low-carbon. It uses very little land for the number of people it moves, a double-track railway carrying far more people per hour than a motorway lane, and it is among the safest modes. Its limitations are the very high capital cost and the long lead time of new lines, and the substantial embodied carbon in tunnels, viaducts and earthworks, which takes years of operation to repay. It runs on fixed routes, so it needs concentrated demand and good onward connections to be useful, and it is difficult to justify in low-density rural areas, where a bus is usually the appropriate mode. Empty off-peak trains also weaken its per-passenger advantage.

6. 6

   Compare the sustainability of the modes for moving freight.

   Model answer

   For emissions per tonne-kilometre the usual ranking, from lowest to highest, is sea, then rail, then road, then air, with air very much the highest and sea very much the lowest. That ranking does not settle the choice of mode, because value, perishability, urgency, distance and the availability of terminals all matter: high-value or perishable goods travel by air despite the cost, and almost everything needs a road leg at each end. The sustainable approach is therefore intermodal: containerisation and rail freight terminals allow the long leg of the journey to be made by ship or train while the road leg is kept short, and urban consolidation centres combine deliveries so that fewer, fuller vehicles enter town centres.

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