Research
•One for the road
Highways infrastructure presents great opportunities to save carbon and embrace circularity. This article explores the sustainability of roads and footways, delves into what roads are made from, and highlights how we could make them better.
Introduction
When we talk about sustainability in the built environment, it’s natural to focus on the obvious and visible: the glassy facades, the soaring concrete cores, and the interior design of the spaces we occupy. Roads and pavement are often (literally) overlooked, a sort of filler in the gaps between the buildings. We don’t really appreciate how important they are until they let us down, perhaps via a pothole or a wobbly paving slab.
When roads are mentioned in the context of decarbonisation it is usually as an end destination for demolition byproducts, a way to divert waste from landfill by instead burying it in the ground and driving over it.
Embodied carbon can often hide in places that receive little design attention. Adding levels of basement is far more carbon intensive than adding storeys above ground, thanks to the thick concrete walls and slabs required. The Portland cement that makes up a small proportion of traditional concrete accounts for 90% of the embodied carbon. In our recent deep-dive on structural steel for a built project, we found that 10% of the steel was responsible for 40% of the embodied carbon – see the article here.
This research is a collaboration between HTS+ and the HTS Transport and Highways team, exploring and mitigating the carbon beneath our feet.
A closer look at roads and carbon
Roads are as ubiquitous as they are unnoticed – the UK has over 240,000 miles of road – almost ten times the circumference of the earth. In 2024 the UK built about 300 miles of new road and maintained another 4,800 miles. To achieve this, over 20 million tonnes of asphalt was used, emitting 1 million tonnes of CO2e1.
1 We used Highways England data for the total asphalt consumed, and the ICE database General Asphalt carbon footprint of 0.0556tCO2e/t.
Carbon emissions associated with road construction over a 40-year lifespan. Adapted from Lokesh, K., Densley-Tingley, D. and Marsden, G. (2022) Measuring Road Infrastructure Carbon: A ‘critical’ in transport’s journey to net-zero.
As expected, building a mile of multi lane motorway emits far more carbon than a mile of single lane country road, but for all road types it is the materials which account for 70% of the whole life carbon footprint.
It takes eight years for the emissions of the cars driving over a new dual carriageway to match the emissions from its construction2. This underlines an important point: decarbonising transport means looking at the roads themselves, as well as the cars we drive.
2 To work this out, we knew that vehicle emissions are 100mt CO2e/y (gov.uk) and the UK has 246,500 miles of road (DfT), so vehicle emissions are 406tCO2e/mile of road/year. The paper cited above indicates the average lifecycle emissions to build a mile of dual two lane road is 3248t CO2e/mile.
What are roads made from?
Roadways are built from layers of increasing durability and further tuned to consider things like drainage and lifespan. At the bottom there will be compacted aggregate, and above that, layers of different mixes of asphalt.
So what is asphalt? There are many types and variations but broadly speaking, asphalt is 95% aggregate, bound together using bitumen. This 5% bitumen is an embodied carbon hotspot – it’s between 20x and 50x more carbon intensive than aggregate, as it is produced from crude oil and imported, whereas aggregate is made domestically from crushed rock. That small amount of bitumen holds the road together and holds in smaller chippings on the top surface to increase grip.
Bitumen is very durable, but when heated it becomes much softer, allowing it to be mixed with aggregate and laid onto roads to form a bound surface layer. This is also why roads can soften on hot days. Heat is another reason for the high carbon intensity of asphalt – as well as heating the bitumen to soften it, the aggregate must be kiln dried prior to mixing, and both processes are typically powered by fossil fuels.
So what can we do?
We have identified two prime levers which when used in tandem can have a powerful impact.
Firstly, we propose swapping the base and binder asphalt courses for something called CRBM3 (Cold Recycled Bound Material). This approach uses foamed bitumen which can be mixed at lower temperatures and greatly reduces the heating requirements from traditional asphalt.
CRBM also uses a small amount of cement as a binder. Cement is carbon intensive, but by increasing recycled content and decreasing the heat energy required, we get a compounded benefit which far offsets this increased carbon.
Secondly, we propose greater use of recycled asphalt, removed during prior roadworks, for new roads. Asphalt is so durable that it can be scraped off a road as recycled asphalt planings (RAP), processed, and reused. Traditional asphalt can include up to 15% recycled content, but CRBM can use up to 95% reclaimed asphalt, saving carbon and increasing circularity.
3 CRBM is still heated, but only to 60°C rather than >150°C.
How much better is it?
We have conducted a layer-by-layer carbon analysis of a typical roadway, and a typical footway, plus a CRBM version of each buildup, which indicates carbon savings between 25% and 35%.
These levers can be pulled right now
Sometimes carbon reduction measures are cost prohibitive or come with additional risk as they are outside of normal standards or practices. The CRBM material described here is completely compliant with current standards, and although it is a deviation from business as usual, discussions with the industry suggest that for amounts over 1000 tonnes, there would be no cost increase. To put that in perspective, 1000 tonnes of asphalt makes about 300m of a two-lane road including footways on each side.
We are incorporating this approach into our designs, supporting our clients to deliver carbon savings and increased circularity across the whole project, all whilst mitigating uncertainty and staying on budget. Stay tuned for part two of this series, which will feature an in-depth case study using CRBM as part of a wider strategy to decarbonise the roads, paths, excavation, and water attenuation of a real project.
