Roads are so much a part of our lives that it is easy to forget just how big a part they play – and what effect they might be having on our wellbeing.
For the record, there are more than 41 million kilometres of paved roads on the planet – if you travelled every single one of them you would cover the same distance as going to the moon and back more than 50 times.
When we think of the environmental impact of transport, that usually means the CO2 emissions and other impacts of vehicles that burn fossil fuels. But we probably overlook the fact that those millions of kilometres of roads are themselves consumers of fossil fuel and may well be significant contributors to atmospheric pollution.
The key here is the word ‘paved': in almost all cases a road is actually paved with asphalt, which is a mixture of crushed stone and bitumen. Bitumen is found in natural deposits and in ancient times was used as a binding adhesive in house building, but today it is mostly distilled from crude oil.
Rethinking bitumen
The construction industry uses nearly 90 million tonnes of bitumen a year, mainly in road building. But the important thing to remember is that bitumen is just another fossil fuel and has some of the same drawbacks as the others. In fact, recent research has shown that bitumen-paved roads may even be one of the most significant polluters in our environment.
The first and most obvious source of emissions from asphalt comes from transport: this material is heavy and fuel-intensive to deliver, and even more fuel-intensive to spread on road surfaces. This is why many countries are working to reduce emissions from road construction – the UK, for example, has set a target of net-zero carbon emissions from road construction and maintenance by 2040.
But it turns out that cutting construction emissions is the easy part. Research led by Yale University recently found that asphalt itself is the source of a great deal of particulate air pollution – possibly anything up to double the amount of polluting particles emitted by vehicles. When it gets hot asphalt also releases volatile carbon-based compounds, which means that the hotter the climate becomes, the worse the pollution.
The good news is that there are potential solutions, including alternative road-building materials. One of these is lignin, a natural plant-based product that is being trialled as a substitute for bitumen – early research shows that it is more stable than bitumen, as well as being cheaper. Other materials – including recycled tyres – are being used to create new sorts of composite road-building materials that last longer than bitumen-coated surfaces, and so require less emissions-producing maintenance.
Capturing energy
However, not all the proposed ways to make roads more sustainable have turned out to be quite so promising. One idea that has been around for a while is to embed solar panels into roads to turn them into carbon-free generators of power – an intriguing notion that led nowhere. Experimental versions of the solar road have been tried in the US, France and China. Unfortunately, everything that could go wrong did. The panels did not produce the expected power (because they could not be rotated to catch maximum sunlight). They also wore out quickly, got stolen or, in at least one case, caught fire.
A slightly more promising idea is to use roads to capture the compression energy created by heavy vehicles and use it as a power source. This involves energy-capturing devices called piezoelectric transducers, which turn pressure into electric current (similar transducers are commonly used in electric musical instruments such as guitars). But it is still early days for this idea: whether the transducers produce economic amounts of power or just break down like the solar panels remains to be seen.
It seems that the journey towards the truly sustainable highway is going to be a long one. But, as with other innovations, repeated failures are probably just part of the eventual solution – because, as any seasoned traveller knows, when you find one route blocked, you must find another.The road to the future
