Traffic jams and air pollution due to CO₂ emissions of cars are two challenges caused by rapid urbanization and increasing mobility needs. Eco-friendly public transportation systems like metro trains and suburban railways contribute to sustainable development of cities.

Cities are now home to over half of the global population, and have a large role to play in reducing carbon emissions and improving air quality. This will need new technologies and new ways of organising cities, alongside efficiency gains.

Transport accounts for around one third of the global green house gas (GHG) emissions today. Car traffic in particular not only contributes to global climate change but causes local air pollution, noise and reduced freedom of mobility for pedestrians and cyclists due to the risk for accidents. Furthermore, transport is related to extensive area development for infrastructure like roads and parking spaces which often replace green spaces.

Transport is growing. To avoid further growth of negative environmental impacts, transport needs to be planned to be more sustainable. Alternatives to car use need to be found and promoted. Cars can become cleaner and more efficient with the latest technological developments. However, a more comprehensive approach to the transport problem needs to be taken. A combination of measures will be most effective.

In the chart below, you can see the comparison between carbon intensity of different types of passengers transport on per passenger kilometer basis.

Sustainable Transport is sometimes known as Green Transport and it is any form of transport that does not use or rely on dwindling natural resources. Instead it relies on renewable or regenerated energy rather than fossil fuels that have a finite life expectancy. For this reason it is said to have a low or a negative effect on the environment since it makes use of energy sources that are sustainable.

Walking, cycling and sailing are excellent examples of sustainable transport.

What are the alternatives?

In many areas there have been moves to encourage people to use public transport networks rather than travelling by car. Cycling has also gained in popularity, both as a means of maintaining fitness, but also because it is a cheap and often swift form of transport.

There have been moves to introduce cleaner and more fuel-efficient means of urban transport. There have been experiments using different types of fuel and there has been much effort and progress made in the development of so-called ''green vehicles''.

Hybrid vehicles have been developed as an alternative to conventional cars. These are gaining in popularity. Generally they are of a hybrid design, combining an internal combustion engine with an electric engine. Other vehicles run on natural gas and some are electrically powered.

Biofuel: Biofuels have also become very popular in some areas. Bioethanol is made by fermenting plant materials and biodiesel is made from vegetable oils, animal fats or recycled grease. In 2008 biofules provided 1.8% of the world's transport fuel. One of the problems is that bioethanol production relies on the cultivation of large amounts of plant material often on land that was formally tropical rain forest, or on land that would otherwise have been used to produce food.

Solar power is another possible future option, but this will depend on a technical leap in the conversion rates of the photovoltaic (PV) cells that convert sunlight into electricity. Battery power would be required at nighttime or when there was no sun during daylight periods.

Hydrogen can be used to power future transportation. This might in fact be the power of the future because hydrogen is the most common element in the Universe. Power can either be through the use of electric motors powered by fuel cell technology or by improved internal combustion engines. In both cases emissions would be zero. Hydrogen power is currently prohibitively expensive, but progress is being made in the technology to achieve this. A big challenge is to source the hydrogen from renewable resources. Honda has produced the first 'commercial' hydrogen powered vehicle in the form of the Honda FCX Clarity, although this has limited availability.

Nuclear power was once advocated as the clean alternative to coal, oil and gas for generating electricity and for producing hydrogen through electrolysis. A number of accidents at nuclear  plants, together with the inherent problem of dealing with large amounts of radioactive waste, has led many countries to reconsider this method of power generation.

Looking to the future

Transport is a vital part of our everyday life. The prevailing 20thcentury view was to rely on fossil fuels for most of our transport needs and to ignore any consequences. In the 21stcentury we have woken up to the fact that there is a need for us to care for the world around us.

References-

https://www.carbontrust.com/news/2015/01/sustainable-transport-cities/

http://www.earthtimes.org/encyclopaedia/environmental-issues/sustainable-transport/

http://www.scp-knowledge.eu/knowledge/sustainable-transport-examples-europe

http://shrinkthatfootprint.com/5-elements-of-sustainable-transport