We ride on them, our tyres stick to them, and our bikes lean on them – literally. But have you ever stopped to wonder where roads came from, where they’re going, and what are the materials we use to make them? Follow along as we break down all of these questions to learn how the modern road came to be.
Anyone who rides a bike knows a fresh pair of correctly inflated tyres is vital. Tyre grip is what stops a moving motorcycle - and you - from falling over. But tyres will only stick if a surface sticks back to them. (If you want to test this, take the stickiest road tyres you can find and go for a ride on wet grass). Whether you did this experiment or not, you’ll soon find out tyres prefer to stick to the road.
Roads were invented during the agricultural revolution 12,000 years ago when early humans stopped hunting and gathering, and settled down to live in caves. Clearing a route between you and your neighbour’s cave made popping round for dinner or going shopping much more accessible. These early trackways were fine on foot and gradually widened to make room for livestock when animal farming began in 8000 B.C. But it was when the wheel started to revolutionise transport, around 4000 B.C., that roads steadily began to evolve too.
The earliest dirt roads were simply tracks cleared down to the topsoil. But soon, people began to use different materials, such as wood, gravel and stone slabs, to build roads. Apart from some notable exceptions, like the 50 km paved road built by the Minoans of Crete in 2000 B.C., it wasn't until the Roman army started marching and pulling chariots all over Europe that the need for durable, reliable, weatherproof roads arose.
With a network covering tens of thousands of miles, Roman roads needed several layers to become stronger. Typically, the base layer contained sand or light gravel, followed by larger rocks (sometimes set in mortar) then a light layer of smaller or crushed stone and finally a top layer of paved stone blocks. Workers dug ditches on either side of the road and used a cambered top layer to aid drainage. Adding a pedestrian area and milestones to track distance were innovative at the time, and are still in use on our modern roads today.When the Romans withdrew, the roads they created fell in disrepair, (although many of the routes and much of the construction remain today). It wasn't until the 18th century that systematic road construction began again in Europe. While Europe took a brief hiatus, the rest of the world continued to pioneer road construction. For example, in the Middle East, the Arab Empire started using a sticky binding substance derived from oil to bind and seal the top layer of their most prestigious roads. This substance would come to be known as tar.
Flash-forward to the early 19th century when Scotsman John McAdam developed a cheaper and quicker road building technique that no longer required base layers of stone, but relied on compacting the existing subsoil and using smaller stones.
The method was popular until the invention of motorised vehicles in the early 20th century, when higher speeds meant dust and wear became a problem. The solution was to use molten tar mixed with sand on the road surface to seal the top layer — and tarmac was officially born.
Today, bitumen (asphalt) has replaced tar. Also a byproduct of refining oil, asphalt is preferable because it remains stable across a broad range of temperatures and is easier to manage.
A modern asphalt road, (i.e. blacktop or pavement), contains an aggregate (stone particles, such as gravel or granite) mixed with a bitumen binder, and fillers. There are also numerous mixes, variants and grades, modified by adding chemicals such as polymers, fibres, waxes, zeolites and emulsions, to tailor the asphalt to the specific weather and traffic conditions of the country or location.
In practice, this means there are thousands of possible "recipes" to create the layers of an asphalt road. But, generally, the layers are made up, top to bottom, with something like this:
The top layer is made of hot rolled asphalt and is noise and skid resistant, and durable against wear in all weather.
The second layer is made of hot rolled asphalt and gives structural stability to rural roads.
This layer gives structural stability on main, trunk and industrial roads and motorways.
This is the primary foundation of a road, and its thickness depends on the underlying soil and road conditions.
Similar to the sub-base, this often contains a lower-spec material. The capping layer is 150 - 350 mm thick.
Geotextile is a permeable mat from synthetic fabric that stops the layers above it from merging with the subsoil.
Sub-grade is created for load-bearing and stability, then consolidated and compacted into a natural bottom layer.
The construction of a road depends on the soil conditions in the area, how frequently it’s used (measured in millions of standard axles), the range of vehicles that will use the road, design life (a road typically lasts 40 years in Europe), and finally, the cost of construction. Roads fall into distinct categories, such as:
A trunk road or motorway is the most expensive and must adhere to the highest standards possible. Principal route building also follows similar specifications, and has a national body maintaining it rather than local authorities.
Local highway authorities are in charge of looking after non-trunk roads and side roads, which tend to abide by a less expensive maintenance plan due to lack of traffic.
As with all roads, the type and speed of the traffic will also determine the skid resistance. Trunk roads and motorways will get a better product and increased skid-resistant surfaces, but less travelled roads can demand it if the environment dictates it, e.g. it’s an accident black spot, near pedestrian crossings, an entrance to a roundabout, or contains sharp bends, etc.)
Cost also plays a significant role in determining the specifications of a road. It’s entirely possible to build roads that never wear out, never suffer from potholes, and will never need lifetime maintenance. However, the cost of materials and scale of the engineering make it impractical.
Shell Cariphalte Racetrack contains an asphalt with a modified bitumen binder used in high-performance locations, such as race tracks. Many MotoGP circuits (not Silverstone though) use this stuff. It’s very stiff and resistant to stress, fretting (which is general surface wear and tear) and deformation. It also provides high grip for tyres.
The reason the materials in this racetrack are not for roads is simple: its unique contents and specific construction comes at a high cost that it much higher than the construction cost to build a normal road, and frankly, that kind of performance isn't necessary for everyday driving.
The duration of construction varies depending on the type and size of road — it could take a few months to up to three years. Tests are regularly carried out during construction to ensure the materials conform to specification and performance needs. After construction wraps up, it’s normal for the client (the local authority or national body) to withhold a percentage of the costs for a year, a practice known as a "defects correction period," or a road-repair warranty.
The cost to build a road also depends on the category of road, which means it can cost between 1,300 euros to 11,000 euros or more, per metre. The workforce also rests on the type of road and contractor. Around 20 workers would be involved in building a minor road, but that could rise to a few hundred if they're creating a motorway.
Thousands. And there are just as many companies making them. There are standards in place that companies must conform to regarding the type, size and ratios of aggregate used, but manufacturers will also use different additives and other compounds to create their specific blends.
Shell Grip is a popular type of high grip surface. We usually think of Shell Grip as the sandy-coloured, textured surface when drivers approach roundabouts or that’s used on hairpin turns.
It’s a high-friction, anti-skid material using a calcined bauxite aggregate with an epoxy or polyurethane resin to give a skid resistance value (SRV) of 70+. Skid resistance is measured using a particular machine called a SCRIM (Sideways Force Coefficient Routine Investigation Machine), which contains a unique test wheel, angled at 20 degrees, towed behind a car at 50 kph.But Shell Grip, and many other surface coatings like it, are expensive. In the end, what differentiates road types is the balance between road use, road life and cost.
The bottom line is it’s cheap and effective in prolonging a road surface’s life (assuming drivers adhere to the recommended speed limit).Typically, problems arise when people drive too fast, and they either skid on loose gravel or their front tyre flicks a stone through their vehicle's radiator.
There's also the chance that another speeding vehicle could flick a stone at another driver, smashing a headlamp or chipping paintwork. But this is not always the case, even a bike at a low speed can lose control on gravel.
The ingress of water causes potholes, so good drainage is vital to a road's longevity. If not, a road is vulnerable to erosion, corrosion (if there's road salt) and a general breakdown of road layers.
For some countries, winter weather also causes the degradation of roads. Repeated cycles of freezing (expansion) and then thawing (contraction), will quickly break and collapse sections of the road. As long as traffic still runs over the damage, it only gets worse.
Bumps, on the other hand, are from poor soil conditions caused by the presence of tree roots or adverse road design and construction. For example, the entrance to sharp bends, where a lot of heavy braking by large vehicles occurs, can ripple the road surface or wear our grooves with repeated use. But, generally, when a road surface ages and wears, the asphalt binder deteriorates which leaves the exposed, abrasive aggregate to become over-polished and slippery.
Now you know the origins of the road, the different types and we make them, you can call yourself a road expert. But don’t forget another vital part of the road: the tyres.
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