Modern roads are engineered for safety and for efficiency. This article of mine will touch the fundamentals of how roads are put together, which can enrich us as drivers and help us appreciate strong and weak points of the roads, and drive according to the conditions.
It all starts with Paving
In fact, it starts by setting the base for paving. The ground over which the road is to be constructed is flattened by rollers to a point where it feels very rigid. It is than covered with a base layer (HMA layer) and than covered with tarmac. In old roads, the "tarmac" was simply black tar. The problems of this compound was that it became jelly-like in hot temperatures, and slippery when wet, and got rutted by cars' tires. The alternative of a gravel surface was also slippery and made cars project a lot of dust in the air.
The solution was to combine both into tarmac. Modern tarmac is a combination of tar with aggregates, which are simply pieces of crushed mould and rock, which make the tarmac's face rugged to provide tire grip, and rigid and resistant to influence of weather. Old roads, mainly in town, are made of grey tarmac which holds a greater percentage of sand, which makes it wear more and lose it's grip more quickly. Newer tarmac roads are made of darker tarmac, with more tar. New roads utilize one of three kinds of tarmac:
- Silent Tarmac: A tarmac mixture where the upper layer is mixed with limestone. The lime gives it a hallowed surface which is rugged for grip, but also "swallows" tire noise (reducing of noise by 80%!) and allows water to sink down and into the drainage, rather than create a deep film over the tarmac.
- S-Type Tarmac: Developed in the US by reinforcing the base layer with Basalt. Provides rigidity, high grip and silent ride.
- SAM-Type Tarmac: Much like the Silent Tarmac, this tarmac is also combined with special fybers that provide a rugged surface for high grip, with good water drainage and a silent ride.
Concrete roads are more rare and are used in areas of heavy traffic of trucks and in tropical regions. Concrete is less grippy, but cheaper (given the oil crisis), can live up to 9 times longer than tarmac and provides a smooth ride with less tire wear and less pollution. It's downsides come in the expensive repairs and lower grip levels.
Distresses and repairs can be seen on all types of roads. I will specify some of the most common distresses:
- "Alligator Cracks": A series of interconnected cracks all over a given section of road. It is very common and can be caused due to moist or frost in the base layer, bad construction or sudden load of too heavy a rig. Under pressure these cracks develop into potholes.
- Bleeding: All roads wear at the point where most cars place their wheels (typically the center of the lane). However, when there is a superfluous amount of binder in the tarmac, or when the road is not properly constructed to meet the heat of the region, the wheel ruts can form black, shiny (and slippery) films of black binder.
- Block Cracking: Rectangular, interconnected cracks at a much bigger scale than alligator cracks. This is the result of the aging of the tarmac and it's subsequent fatigue under the loads of traffic.
- Shoving: When the base layer (HMA) is full of moist (due to bad compaction of the ground beneath the road, infiltration of tree roots, etc...) or badly put together, the force applied by cars, especially heavier cars, under braking, especially around the left and right lanes of intersections, will make the upper layer of the tarmac shove and "fold". This can also take the shape of crescent-shaped cracks against the direction of travel.
- Depression: Frost or moist will reach into the base because of bad drainage at the edge of the road, or poor compaction of the earth beneath it, creates a sudden drop in the elevation of the road or possibly a "wave"-like road.
- Perpendicular cracks: The result of putting the joints (where the different surfaces of the base layer connect) in the way of the cars' wheels, causing excessive effort to fall on them and create cracks or "step" over the length of the road.
- Raveling: Seems like loose debris and pieces of ultra-rugged and/or crumbling patches of tarmac over the road surface or over it's length. This is caused by infiltration of dust into the upper layers of the tarmac when compacted and later during it's use.
Other than replacing bad layers of tarmac, repaving or replacing the base layer, there are superficial repairs that can be made by cheap authorities, and can pose a hazard themselves:
* Patches: Be placed over parts that suffered from brake slip or alligator cracks, but will reduce traction, create a bumpy ride, and can wear themselves and reveal potholes.
* Tar snakes: Fresh, black tar poured into block cracks. The black tire itself is quite slippery, especially to bikers, and becomes more slippery both when wet or when it expands under great heat in warm days, where tarmac temperature can reach 70-80 degrees celsius. This reduces the grip levels on hot roads where otherwise they should have been increased due to tire heat.
The goal is good grip
The grip levels are measured by the variant of coefficient of friction, marked by the greek letter µ. Race cars and especially Drag-race cars, achieve up to 2.4! On a normal road, the maximum level is 0.88 or so. Lower tarmac quality, and poor tires/dampers reduce it to a typical 0.7, where worn tarmac or bad tires can yield 0.6.
Wet roads achieve similar grip levels of 0.6 to 0.5, given tires with sufficient tread depth, reasonable driving speeds and films of water of 2 to 3.5mm of water. This, however, is an idealized model. In real life, cars and bikes constantly leak grease from the bottom of the engine bay, bands and gaskets around the engine, gearbox, spools, and faulty oil pans, valves and engine gaskets. There are also fallen leaves, sap and fruit from trees, dirt from gardens, wind and car tin, and all sorts of contaminations.
These contaminations reduce grip levels at all times, but when soaked, they are set afloat over the water layer. This effect is more pronounced in areas of heavy traffic and especially in the routes of heavier vehicles (which more oil and lubricated parts), and at slow speeds (in towns) where constant accelerations, decelerations and cornering makes the oil drip about at stoplights, bus stations and roundabouts. Under these conditions, enough grease and dirt can be built up so that a small downpour after merely five dry days will create a slippery "mush" with a coefficient of friction of 0.4. In smoother, faster and better drained roads the same process might take two weeks. The normal standard is to refer to any downpour after one dry week as similar to "first rain."
Dirt and gravel layers on the road also simulate similar grip levels of 0.5 to 0.4, and a reasonably-sized spill of diesel or petrol will yield a coefficient of 0.3. The same grip level be reached by presence of frost or hail on the road.
Snow, sleet and some kinds of mud will be even more slippery, 0.2, much like motor-oil spilt on the road. Whereas a deep puddle of water, a patch of ice, an epoxy surface or very viscous transmission oil for trucks, will simulate grip levels of 0.1 and below that. Ice will be most slippery when it's around the temperature of 32F, where it's just covered by a thin film of water. Oils will be most slippery when a substantial amount of it has been mixed with water or moist in a sunny day. The oil can be seen from afar as a spectrum of colors, due to a physical effect of thin-film interference. As a rule of thumb, the more diverse are the colors, the thicker the oil film is.
Another important subject in grip is the painting of road markings. If simple, conventional paint would be applied directly onto the tarmac, it would be quite slippery. Sadly, in some countries, like mine, this is the case. Particularly slippery are the crosswalks because, unlike other markings, they are planned to be driven on so the color film is made intentionally thicker. Over time it wears and is repainted which can reduce the coefficient of friction further.
The simple solution is to make the color more rugged by using sand, crushed glass or special polymers. The problem is that these wear out, so once the crosswalk has been a tad worn by cars, it becomes extra slippery. A better solution is to use a different choice of paint. The different types of paint include:
- Acrylics: Colors based on a synthetic solvent made of Acrylic Acid. Defined as "plastic paint" and is very slippery.
-Alkyds: Color based on oil as a solvent. Basically a refined oil paint. Very slippery.
- Thermoplastic paint: Much more expensive (about 75%) but less slippery and reflects light better for good visibility, while being more durable for longer usage before repainting.
- Dual-Component Paint: Even more expensive than the former, but more durable and less slippery.
This is only part of the solution. The other part of it is to mix the said paint with the tar before it's paved, so that the tarmac itself be colored, and not have color over it. This technqiue is applied widely in the USA, and is also universally used in the paint marks in parking lots, and in racetracks.
Streaming Traffic
Traffic is streamed through use of long, open-view bends, straight roads where possible, replacement of controlled intersections with roundabouts (in towns) and interchanges (in highways). The road is divided into lanes, including special acceleration and deceleration lanes. A deceleration lane is a lane made exclusively for cars turning right (or left) at the near corner. An acceleration lane allows cars coming from the right or left to merge into the traffic which is moving forward.
In both situations the concept is to allow free flow of the traffic in the forward direction by allowing cars that exit the road or enter it, to perform all changes of speed (acceleration or deceleration) besides the moving traffic rather than inside it, thus not interfering with it's flow. This condition would exist only if the lanes are long enough for the anticipated traffic flow, and if drivers know that they should only decelerate within the deceleration lane and not before it.
Interchanges are much more complex and fascinating. I will touch a few prototypes of it for knowledge. The most basic of interchanges is the cloverleaf interchange where two perpendicular highways, one above the other, connect by use of "loops." This distributes traffic smoothly and effectively, but forces a certain slow down to negotiate the loops safely.
This downside is cut down in the more sophisticated (which is also more expensive and room-consuming) Stack interchange pattern, where the loops are replaced with a crescent-shape ramp which can be safely negotiated with little to no reduction of speed. A Turbine or Whirlpool Interchange, which allows to built the interchange in a more compact and cheap way, by bending the access ramps into "whirlpools" that again force a certain slow down. There are other types of interchanges and hybrids of such prototypes, but the information regarding them can be easily found elsewhere.
Some highways have a control center. This control center involves a team of qualified people who work during rush hours, by monitoring cameras and the data from sensors in the road surface, to figure out the capacity of vehicles that the road holds. They can than decide on a need to keep traffic flowing by posting a reduced speed limit on a gleaming digital screen at the far end of the road. These signs are legally obligating for the drivers, and should help traffic flow.
There is an equation in Transportation Engineering that determinds the critical mass of moving traffic. It is defined as: F(Q)=KV. Where "Q" is a parameter for traffic flow (measured in vehicles per hour); K is density (cars per kilometer) and V is velocity or speed (Kilometers per hour). The function dictates that, up to a certain threshold, an increase of K does not reduce V or Q. However, beyond a maximal Q or a "critical" K, traffic becomes Congested into what is called (by one theory) Synchronised traffic, and further into an actual Jam. The control center achieves balance by altering the variant V accordingly.
Safety comes first
Modern roads have guardrails at their edges and between lanes of opposite traffic. The older forms of guardrails include the "New-Jersey Wall" which is a rigid concrete guardrail. It's advantage is that it makes it nearly impossible for a car to be projected over it and into oncoming traffic or off-road. Furthermore, it's lower part is carved in a slope which is intended to redirect a car that brushes against it towards the direction of travel. The downside is that at a sharper angle of impact it is too solid to hit, and the said slope can also contribute to rolling a car over.
A better guardrail is the steel-based "W" armco. This is a metal rail with a shape like the letter B. This rounded shape allows better absorption of energy, but it is still very rigid, might cause cars to slingshot over it, and also causes bikers to slide under it. To add to all of that, the sharp edge of of the guardrail can pierce a car that hits it. Therefore the edges are bent down and into the ground, but this can form a ramp that elevates and even rolls the vehicles. The best solution is to add to a finite element, called ABC, which is some sort of a polyurethan shock absorber that crumples when hit.
The same principal is applied in newer guardrails. The rail is designed to bend at impact (just like cars are planned to crumple) and this allows them to absorb the energy. These rails are tested in special tracks in Europe, and are rated for Containment Class, Acceleration Severity Index and Effective width of operation. The Containment Class offers a rating based on the test the rail has passed. The tests change in the angle of impact, the speed and the mass of the hitting vehicle. They range from N1 to N2 (the lowest two ratings) through H1 and H2 (the latter can be considered a reasonable standard), to H3, H4a and H4b, and an additional rating for heavy rigs.
The ASI rating (Acceleration Severity Index) portrays the damage to the vehicle's occupants under the loads of the crash in hitting the armco. It is rated between A to C, where the "deadline" is somewhere inside the range labeled under C. The Working Width describes how much the guardrail is planned to crumple and bend when hit. It varies from W1 (around 0.6m) to W8 (3.5m). Modern rails also have a rating for protecting for motorcycle riders, where a lower polyurethan "block" is added below the A and B pillars of the rail, to prevent fallen riders from going off road or hitting the rigid pillars of the rail.
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