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WHEN first phoned a local asphalt company, she didn鈥檛 mince her words. 鈥淚 have something to tell you,鈥� she said. 鈥淵our material is really hard 鈥� too hard. People are getting hurt.鈥� Her comments didn鈥檛 go down well. 鈥淭hey were like, 鈥榃ho is this crazy scientist?,'鈥� she recalls. Asphalt is supposed to be hard, they said. But a few days later, the company rang back. It was the beginning of a journey that could reinvent the ground we walk on.

Wallqvist鈥檚 passion is rare. It is more than two millennia since the Romans laid their first pavimentum, from where we get the word 鈥減avement鈥�. Since then, very few people have questioned the fact that the pavements we walk on are, in effect, extensions of the road surface, made of stuff with properties that almost exclusively reflected the needs of horse-drawn and then motorised vehicles rather than pedestrians. Wallqvist, a materials chemist at the Research Institutes of Sweden in Stockholm, is determined to change that.

Meanwhile, in London, plans are afoot to build a giant research facility to test new, spongier walking surfaces. It is the brainchild of at University College London, who is also convinced that pavement pounding is harming us. The average person takes around 200 million steps in a lifetime, he notes, and we aren鈥檛 evolved to deal with such hard surfaces.

So, after waiting more than 2300 years for a pavement evangelist, two have come along at once. You might not read anything into that. On the other hand, perhaps, it鈥檚 a sign that sidewalks are about to get a makeover.

The Romans were meticulous engineers, famous for their road building. Excavating down almost a metre, they placed flat stones at the bottom, then small stone fragments in mortar. Next came a compacted layer of broken pottery and brick, crushed stone and gravel, mixed with lime mortar. Atop they placed irregular stones about 15 centimetres thick 鈥� the pavimentum. This remained the pinnacle of pavement technology until the 18th century, when the first modern roads were built. And it wasn鈥檛 until the 19th century that engineers really began to innovate.

In the 1820s, British engineer Thomas Telford recommended that London鈥檚 main streets be surfaced with granite slabs. However, hooves and wheels on granite created an ear-splitting din and, by the early 1840s, businesses in busy Oxford Street were so they could hear their customers. They weren鈥檛 alone. In the US, cedar blocks were laid on roads in Minneapolis and cypress in Omaha. 鈥淟ower Manhattan had a network of wood streets in the early part of the 20th century,鈥� says , an architectural and urban historian at Savannah College of Art and Design in Georgia. 鈥淚t鈥檚 shocking, but it made sense. It was quiet for the banks and trading floors.鈥�

Yet wood surfacing had drawbacks. Horses could fall when it became wet or icy, and it decayed in less than five years. The main alternative was macadam, named after Scottish inventor John Loudon McAdam. First laid in the 1820s around , it consisted of layers of rocks and gravel for drainage, with crushed stones on top. 鈥淭he stones had to fit comfortably into your mouth, a very human measure,鈥� says Williams. Poorly made macadam streets still turned into a quagmire of glutinous mud and equine deposits, however. The solution came with the addition of asphalt (a semi-solid form of petroleum also known as bitumen), to create a smooth, super-absorbent surface called tarmacadam. Even so, around half the streets in progressive, 19th-century cities remained unpaved. 鈥淧avement was expensive,鈥� says Williams. Savannah even tried feet-slicing oyster shells, which were a fraction the cost of the alternatives. It also had roads of vitrified brick, granite blocks, cobblestones, macadam and asphalt, a typical late 19th-century medley.

In the 20th century, asphalt gradually became the material of choice. It first gained ground in the US in the 1890s after bicycles were mass-produced and the League of American Wheelmen, a cycling lobby, launched a . In 1901, the tarmacadam recipe was perfected with the addition of angular, interlocking stone chips, or aggregate. From the 1920s, with the rise of automobiles, asphalt came to dominate because it allowed for smooth acceleration, whereas car wheels spun on macadam surfaces and chewed them up.

On the margins

Amid all this innovation, pedestrians were barely considered. For most of history, they had to share the highway with other road users. Pavements began appearing in the mid-19th century on the busiest London streets. Initially, all that delineated walkway from street were upright posts, but, by 1881, something like the pavements of today appeared, with granite and limestone slabs used to elevate walkers above the road surface. Nevertheless, engineers mostly neglected the needs of pedestrians. Things haven鈥檛 improved much since then, according to Tyler. 鈥淔ootways are not thought about,鈥� he says. 鈥淭hey are the gap between the traffic bit and the buildings, both of which get a lot of thought.鈥�

Although modern urban roads usually have pavements, Tyler believes they are made of inappropriate materials. He particularly rails against the common use of concrete, a superhard composite of cement, water and sand, gravel or stone. 鈥淭he human species was not evolved to walk on concrete,鈥� he says. 鈥淲e evolved to walk on savannah.鈥� As a result, he argues, unforgiving pavements are responsible for increases in knee and hip replacements, as well as cumulative damage to cartilage, tendons and bones. Intuitively, that makes sense, but evidence is sparse (see 鈥淏orn to run 鈥� on grass鈥�). 鈥淚t is really hard to study,鈥� says anthropologist at Harvard University. Tyler is giving it a go though.

His group is now working on a pilot study to compare the effects of walking on different surfaces, including concrete and the material used for the London Olympics running track, which has a subsurface made from two layers of vulcanised rubber to increase its give. More than 100 volunteers will be put through their paces, striding 700 times up and down different strips while kitted out with pressure sensors and accelerometers. The results will be fed into a model designed to simulate a lifetime of walking, developed by scientists at Aalborg University in Denmark. 鈥淲e will put data into that model to see what happens to knee cartilage after 200 million steps,鈥� says Tyler.

That鈥檚 just the start. The team is awaiting the construction of a 拢50 million research facility in London that will cover 4000 square metres, with 600 square metres of floor space that can be reconfigured with different materials. Tyler dubs it a scientific film set. When it opens later this year, he will be able to build streets 100 metres long and investigate how crowds up to 500-strong interact with the urban environment. 鈥淭here鈥檚 lots of work around designing materials for roads, but the only engineering around footways is how heavy a vehicle it can support. There is nothing about what friction or sponginess footways should be,鈥� says Tyler. He plans to put softer, spongy material down at the entrance to the facility so that politicians and other visitors can experience the difference. 鈥淚 don鈥檛 think there is a policy person in the world who has thought about this,鈥� he says.

Wallqvist has come to similar conclusions independently. 鈥淚t all went wrong from the beginning,鈥� she says. Asphalt and concrete are made for cars. 鈥淭hey are so hard. Why should we walk on them?鈥� As well as the cumulative damage they cause, she is concerned about falls. Research published in 2020 found that in Sweden, . In older adults, more than 60 per cent of . What鈥檚 more, around 30 per cent of the damage to someone hit by a car is due to the impact of being flung onto the asphalt. And surface impact is the main cause of non-collision cycling injuries.

Wallqvist decided to do something about this. For the past few years, she has been working with asphalt companies to develop softer asphalt by replacing the hard aggregate with rubber from shredded tyres. 鈥淚t has nice soft properties,鈥� she says. It is also plentiful, with enough waste tyre rubber produced each year to cover the entire surface of France. In 2017, Wallqvist and her colleagues published results from the . As well as being more impact-friendly, the addition of rubber also reduced ice formation, an important issue in Sweden. One blend even included phosphorescent silica so that it glowed in the evening.

Softer sidewalks

Since then, this bouncier asphalt has been laid on a small forest track outside Uppsala, Sweden, where walkers and cyclists can try it. An asphalt company also plans to use it to surface a stretch of busy road in Lund, also in Sweden. 鈥淚n our opinion, this softer material should be standard for every pedestrian and cycle track,鈥� says Wallqvist. She suggests areas outside hospitals could initially be paved with rubberised asphalt as a priority. These new surfaces would be particularly life-changing for older people who are deterred from walking for fear of an accident, she adds.

Rethinking pavements would also help tackle the issue of 鈥減avement poverty鈥� in places where vehicles take priority over pedestrians. Increased access to safe sidewalks could have big health benefits, not just in low-income countries, but also across the US. More than 30 per cent of adults in 122 countries 鈥� and nearly half of those in the US 鈥� , and studies show that . 鈥淲e evolved to be physically active and we increase our vulnerability to a wide range of disease when we are not,鈥� says Lieberman. These conditions include type 2 diabetes, osteoporosis and heart disease. Physical activity also

The potential benefits of a pavement revolution are clear. Nevertheless, Tyler accepts that cost is a barrier. His Olympic running track surface comes in at around 拢34 per square metre. Concrete costs just 拢5 per square metre and, although it might be ugly, it is extremely durable. Of course, there are the health costs of poor paving. 鈥淐heap concrete paving turns out to be pretty expensive when you count the injuries it causes,鈥� says Tyler. But there is another hidden cost: concrete production is a huge source of carbon dioxide. If the cement industry, which produces the chief ingredient of concrete, were a country, it would be the third largest emitter in the world, behind only China and the US. In 2015, it generated around 2.8 gigatonnes of CO₂. That鈥檚 8 per cent of the global total.

By contrast, Wallqvist鈥檚 approach comes with environmental benefits. Not only does it use recycled tyre rubber, but including this in the mix can also lower the temperature required for asphalt laying, reducing energy consumption and generating fewer fumes. The current formulation , but Wallqvist hasn鈥檛 finished innovating yet. 鈥淲e are trying to include even more rubber content. The more the better in terms of impact absorption properties and prevention of injuries,鈥� she says.

Tyler鈥檚 work is still very much in the experimental phase, and he anticipates that the key problem will be durability. 鈥淗ow do we get robustness without making the material hard? That鈥檚 the challenge,鈥� he says. He wonders about using natural materials such as grass or vegetation. The reason we left those behind is because they don鈥檛 cope well with rain and usage. 鈥淭he holy grail would be to use what we actually evolved to walk on,鈥� says Tyler. 鈥淭hat could be a massive success, but we would need an effective substructural system that could support good drainage.鈥�

Surely, creating better pavements isn鈥檛 beyond us, though. After all, it is more than half a century since the first moonwalk. That was the culmination of a massive technological effort 鈥� so perhaps what we need are more engineers who are passionate about pavements. 鈥淭he footway pavement is really the Cinderella of urban infrastructure,鈥� says Tyler. But, as we all know, Cinderella lived happily ever after.