There is nothing more inefficient than a car on the road with the
engine on going nowhere.
—Anthony Albanese, Federal Minister for Infrastructure, May 2008
Cars use less petrol when travelling at a moderate constant speed than in ‘stop-start’ traffic, and the road lobby frequently uses this fact to argue that freeways, by allowing traffic to flow freely, will cut petrol consumption and reduce pollution.
The argument is false, firstly because freeways don’t actually reduce traffic congestion and secondly because the additional travel freeways generate swamps any energy savings which might occur.
So while each car might be obtaining more ‘miles per gallon’, there are many more miles being travelled.
A comprehensive international review, by Western Australian transport researchers Peter
Newman and Jeff Kenworthy, found that people living in cities with the fastest traffic used the most petrol and spent the most time in their cars. The same is observed even within cities: more fuel is used in the relatively
uncongested suburbs than in congested inner-city areas. And the pollution levels follow the fuel use trends; emissions per person are lower in the cities with slower traffic (and in the slower-moving parts of cities) than in those with faster traffic!
It is clear [from the Perth traffic study] that while congestion diminishes significantly from central to outer areas….and vehicle fuel consumption improves, actual per capita fuel use by residents in these areas increases significantly…. [Compared to the Perth average] central area residents use 22% less actual fuel, and conversely, congestion-free outer suburban driving is 12% more fuel efficient than average but residents use 29% more actual fuel. It is important to stress that the per capita fuel data incorporates the factor of vehicle fuel efficiency due to traffic conditions.
These results [suggest] that there is indeed a trade-off in energy terms between attempting to improve the fuel-efficiency of the traffic system through raising average speeds and trying to foster an urban system which is more fuel-efficient overall.
—Newman and Kenworthy, Cities and Automobile Dependence: An
International Sourcebook, 1989, pp.151-2
New York is a very different type of city to Perth as it has extreme levels of congestion in its high density central and inner areas…. The same qualitative pattern is evident linking lower overall fuel use with slower traffic speeds, however in New York it is even more extreme…. [T]he evidence suggests that other cities, most of which have greater congestion than Perth, are likely to have an even more positive relationship between congestion and the fuel-efficiency of their city.
—Newman and Kenworthy, Cities and Automobile Dependence: An
International Sourcebook, 1989, pp.159-60
At the same time, while the more congested inner parts of cities use less actual fuel than outer areas, our congestion page explains that efforts to reduce this further by building more roads to reduce congestion virtually always fail. To paraphrase Mr Albanese, the only thing more inefficient than 1000 cars on the road with the engine on going nowhere, is 2000 cars on the road with the engine on going nowhere – yet the evidence shows that is the consequence of building roads in congested areas.
The road lobby is not deterred by evidence, however. In an intriguing submission to the Garnaut Climate Change Review in April 2008, the Australian Automobile Association claimed that the Eastlink tollway was reducing fuel use by $23 million a year, even though Eastlink had not yet opened! It turned out the AAA was simply repeating a claim made in a speech by then Opposition Transport Spokesman Martin Ferguson in 2007, which gave $23 million as the projected benefit of Eastlink based on the output of a computer model. Computer models are, of course, only as good as the assumptions put into them – and the models used in cost-benefit analysis of road projects have consistently underestimated the volume of traffic using a new road. Needless to say, no evidence exists of any reduction in fuel use following the construction of a new road (and if there were, fuel retailers would likely complain!).
In any case, efforts to reduce fuel consumption by increasing average traffic speeds can only go so far. Fuel wastage due to stop-start driving is important at low average speeds, but at higher speeds other effects such as wind drag become more important. Once a certain speed threshold is passed, increased average speeds will lead to higher fuel consumption, along with increased emissions. This is why the International Energy Agency recommends that governments implement speed restrictions in the event of an oil shock; the IEA’s 2005 study found that cars are 11% more fuel efficient at 90kph than at 100kph, and more efficient still at 80kph.
And of course there is more to pollution than just tailpipe emissions. There is also tyre dust, sent into the air continously by rolling friction between rubber tyres and the road. As long as road vehicles use rubber tyres this is a fixed source of pollution and will not be reduced with higher speeds, smoother flow or cleaner engines. Only moderating the level of traffic will solve this particular pollution problem.
Further evidence comes from Sydney, which between 1995 and 2005 was building freeways faster than almost every other city in the world, in a frenetic attempt to catch up with Melbourne’s level of car dependency. According to the road lobbyists, pollution should have reduced in Sydney as a result. Yet in April 2005 the NSW Auditor-General found that Sydney’s air quality was deteriorating and likely to get worse, because
government attempts to tackle rising car use had failed:
The Government is not succeeding in encouraging people to reduce their reliance on cars and promoting greater use of public transport. Private car use is growing faster than population.
We found a substantial gap between the Government’s stated objectives and trends in transport and travel.
—Bob Sendt, NSW Auditor-General, quoted in The Sydney Morning Herald, 7 April 2005.
In California, the home of congested mega-freeways, it is now illegal to build a school or childcare centre within 500 metres of a freeway due to the health risks to children from air pollution. Far from reducing pollution, freeways and car culture in California are now a recognised danger to the next generation. Worryingly, research from Los Angeles in 2013 reveals that dangerous concentrations of air pollutants can occur up to 1.5 kilometres downwind of freeways, not just a few hundred metres as previously thought.
Even the well-known fact that stop-start driving wastes fuel can’t be taken for granted if ‘hybrid’ petrol-electric cars increase in popularity. Much of the fuel efficiency advantage of hybrid cars comes from two factors: the ability to shut down the petrol engine while idling, and the use of ‘regenerative’ braking (which means kinetic energy is returned to the battery, instead of being burnt up in the brake pads). These are the very same factors that account for the vast bulk of fuel wastage and attendant pollution in stop-start conditions. Of course, this doesn’t make hybrid cars a panacea, as we explain on our fuel efficiency and alternative fuels pages: the fundamental cause of air pollution is growing volumes of car and truck traffic arising from increasing dependence on road transport, and tinkering around the edges with engine technology won’t change this.
As the Sydney and California experiences show, freeway building creates a vicious spiral of more traffic and more air pollution, completely negating the effect of cleaner engines. Our only real hope for getting permanent reductions in air pollution is to shift more car trips to public transport, which creates less pollution, and to walking and cycling, which create none at all.
Last modified: 23 April 2013