Roads are a major source of greenhouse-gas emissions worldwide. In addition to the gas burned by cars and trucks, roads require construction and maintenance, consuming millions of tonnes of aggregate, cement and tar every year. In the US, road transportation accounts for nearly one third of energy consumption and 27% of greenhouse-gas emissions. As a result, reducing congestion, improving public transport and encouraging cycling and walking can all bring significant reductions in emissions.

However, many well-meaning road-improvement schemes could actually result in greater congestion and more greenhouse-gas emissions. Why? Because local governments often fail to take into account the wider impact of redesigned roads. For example, it might be assumed that encouraging cycling through bike share and hire programmes like those in Paris, London and New York City will help to reduce carbon emissions, but this is only true if the extra cyclists do not slow the traffic down.

To better understand the impact of increasing the number of cyclists on the road, Conrad Gosse and Andres Clarens from the University of Virginia, US, developed a model that incorporates transportation engineering, economics and life-cycle analysis. They assessed the impact of different lane configurations on two-way urban roads, taking into account the physical parameters of the site, such as road gradient, distance between junctions and road width. The costs they considered included annual road maintenance, vehicle fuel and travel time. They used the model to understand when interaction between cyclists and vehicles would increase costs for all users and when it would make costs go down.

Encouraging people onto their bikes can reduce costs for all users by reducing congestion. But perversely, increasing bike use can actually add to pollution and greenhouse emissions if roads are not updated to accommodate these bikes. "Cities cannot hope to significantly increase cycle ridership without negatively impacting traffic unless they build bike facilities," said Clarens. In particular, the researchers show that bikes can significantly slow traffic down on steeper sections of road and where roads narrow. The findings are published in Environmental Research Letters (ERL).

Gosse and Clarens also discovered that in many cases it may make economic and environmental sense to convert areas of curb parking into dedicated cycle lanes. "It's important to recognize that curb parking does not cause congestion per se, it's just that in places that lack the width for parking and a bike lane, the bike lane may be a better choice," explained Gosse.

The researchers believe that this type of integrated approach is also a better way to think about improving other types of infrastructure. For example, water treatment, freight transport and power supply all have direct costs and impacts, but they also have wider and longer-term implications that need to be considered at the planning stage. But for now, Gosse and Clarens have set their sights on improving our roads. "We're trying to do our part to make the roads a better place for all of us," said Clarens.

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