Measured from ground level to the top of its highest tower, the Millau Viaduct in France is the tallest bridge in the world. At 343 metres, it’s taller than the Eiffel Tower or indeed any skyscraper in western Europe.
The two kilometre long bridge, which recently celebrated its 20th birthday, spans an entire valley and is an astonishing feat of architecture and engineering. But it also has a climate impact.
A megaproject like this, with several skyscraper-sized concrete and steel towers, involves a lot of carbon emissions. Yet the gains it has offered in operational efficiency – a shorter, straighter route, with fewer traffic jams – will probably offset these emissions within ten years. Given that the viaduct is now two decades old, it has far surpassed its carbon break-even point.
How to calculate the emissions impact of a bridge
The viaduct’s architect, Lord Norman Foster, estimates its annual CO₂ emissions savings from heavy goods vehicles alone at 40,000 tonnes. The methodology behind this figure is not fully transparent and it doesn’t appear to be based on peer-reviewed research. (Foster and Partners did not respond to a request for further detail). But, as an academic expert in sustainable energy and transport, I can make some rough calculations that show the Foster figure is at least plausible.
The viaduct forms part of the A75 motorway, a critical north-south route connecting Paris to the city of Montpellier and on to Barcelona. Before it was built, vehicles travelling on the A75 had to navigate a winding, congested route through the Tarn Valley and the town of Millau itself, adding a few stop-start miles to their journey.
The viaduct instead means vehicles can traverse the valley directly, cutting six kilometres from the journey. With about 4.7 million cars and 400,000 trucks using the A75 and the viaduct each year, all those savings add up.
We can estimate the emissions saved using standardised emissions factors of around 150 grams of CO₂ per kilometre for cars and 800 grams per kilometre for trucks. In all, it means the total savings from distance reduction alone amount to several thousand tonnes of CO₂ each year.
But there is more. Larger trucks who previously wanted a simpler and straighter drive generally took a different route through Lyon, a large city to the east, adding more than 60km to a journey from Paris to the south coast. The viaduct means these trucks can take the more direct route, saving perhaps 20,000 tonnes of CO₂. Of course it is hard to quantify exactly which trucks using the A75 would have taken which alternative route, but this is probably where the bulk of Lord Foster’s number comes from.
Before the viaduct was built, Millau was the main bottleneck on the French north-south motorway axis and experienced severe traffic congestion. The viaduct alleviated this congestion.
Research indicates that alleviating traffic congestion can reduce emissions by up to 25%. This is because vehicles consume less fuel when operating at steady speeds compared to frequent acceleration and deceleration in congested conditions. By applying this 25% reduction factor to the emissions saved from the 26km distance of the worst affected area, we can estimate the additional emissions savings attributable to improved traffic flow: a few thousand tonnes of CO₂ per year.
All considered, we can estimate the general emissions savings to be in the order of 25,000 tonnes of CO₂ per year, not too far from Lord Foster figure.
Construction costs vs efficiency savings
While the calculations provide a robust estimate of the viaduct’s emissions savings, it is only part of the story. For example, improved conditions on the A75 could mean more cars and trucks make the journey, partially offsetting the per vehicle fuel savings. This is an example of what’s known as a rebound effect.
That said, the rebound effect appears to be stronger for cars and individual people. For goods vehicles, who were mostly going to make these journeys anyway, research tends to show that new infrastructure like bridges mostly redirect and optimise traffic and do not generate a significant overall increase.
In a celebrated engineering feat at the time, the viaduct used structural components prefabricated off-site. This reduced on-site construction activities and limited the movement of heavy machinery and materials, minimising the impact on local biodiversity and the emissions associated with transportation and on-site operations.
The viaduct required 205,000 tonnes of concrete and 65,000 tonnes of steel. Concrete production emits approximately 75kg of CO₂ per tonne, while steel emits around 1,400kg. Based on these figures, the viaduct’s construction generated roughly 105,000 tonnes of CO₂.
To get a more complete picture of the viaduct’s environmental impact, we’ll need a comprehensive “lifecycle assessment” which would also look at maintenance, repairs, and its eventual decommissioning in 80 years time. For now, we can point to preliminary studies which estimate that around 40% of the carbon footprint of a bridge like this lie in the maintenance and decommissioning. So the bridge will still create a lot more atmospheric carbon in the rest of its lifetime.
Yet, even if the figures in this article are rough estimates, it seems clear that the emissions savings from the straighter and easier journey have already easily offset the carbon used to build and maintain the bridge. This shows how transport infrastructure policy can have a direct impact on decarbonisation. The Millau Viaduct has already prevented more atmospheric carbon than it has generated. From here on, those savings will only grow.
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Florimond Gueniat does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.
This article was originally published on The Conversation. Read the original article.
