I have talked in previous articles and posts about observation -- looking FAR ahead, searching for hazards and PLANNING your moves before actually doing them. One aspect of observation is reading the road surface itself. Admittingly, this is something that is more fun to write about/instruct over than it is in fact siginficant. It encorprates knowledge regarding the construction of the road and the tolls the tarmac undergoes, as well as knowledge of weather phenomena and other aspects that come into play. However, it is still important and I think you will find it interseting.
First, one "academic" distinction: The ROAD SURFACE is the actual surface or bed of the road: It can be tarmac, bitumen, tiles, dirt or ice. The ROAD CONDITIONS refer to things that might cover the road, like fallen leaves, snow, mud, grease, etc...We need to adjust our speed, concentration and position to both the surface and the conditions, and sometimes changes in the conditions are acute enough to force us to extreme measures (like performing full emergency braking to avoid a pothole or patch of ice).
Tarmac
Grip levels: Coefficient of friction between 2.1 (in drag racing) to 0.7 -- on bad roads.
Tarmac roads are constructed by pouring molten tar on a rocky bed with a mixture of materials like gravel that make the tar "rugged." There are different tar mixtures that differ in the grip levels they offer and in durability. The tarmac needs to be a bit rugged. Not actually "bumpy" to make the car rock, but just have very tiny undulations that allow the rubber fybers of the tire to "latch" into them. When tires go over the surface, especially those of heavy trucks and especially under mild braking and/or turning -- they polish the surface which flattens it and makes it harder for tires to "key" into it.
For us as drivers, it's important to look for parts of tarmac that still maintain this rugged appearence and slow down on smooth and "shiny" parts of tarmac, which might appear as "grooves". Darker tarmac tends to maintain it's grip levels better, but it is possible for a grey-shaded tarmac to still grip nicely. The main indication is the rugged nature of the surface.
Of course, serious bumps, presence of dirt, or "patches" of different layers of tarmac, also indicate lower grip levels and the latter can also indicate a risk of potholes ahead. There is one phenomena involving "black snakes", which are in fact parts of pure tar that was poured into a crack that has opened on the road. Roads riddled with such snakes grip poorly.
If we identify bad tarmac, we are ought to slow down a bit, even on the dry. We also need to attribute low grip levels to concrete roads (constructed due to economical and environmental reasons), and residential streets made out of tiles.
Wet roads
Coefficient of friction: 0.6-0.3
Tires hate liquids. Theoretically, there should be NO grip between tire rubber and liquids. Not less grip, NO grip. The only way that tires have to develop grip on such surfaces is for the tire tread to clear out the liquid and grip the tarmac below. The ability depends on the tire tread, velocity of the car and depth and viscosity of the liquid. It's a lot of liquid! In 60mph, it's about one gallon of water per second!
If you take a reasonable road, soak it with a bit of water and drive over it with a well-maintained car, you might be surprised to find a nice amount of grip over it. However, in other conditions, you might experience not half the amount of grip!
The first rule is that whatever you find slippery when dry, is twice as slippery when wet. So, everything we noted above as "low-grip" surfaces of tarmac and concrete, become quite slippery when wet. A road of seemingly glimmering tarmac becomes very slippery when wet. Of course, the amount of water also counts. We need to slow down further when we see deeper water, and slow down dramatically towards puddles (which we might choose to go around where possible). Another indication is the spary from other car's tires. A heavy spary indicates a deeper water build-up.
However, there is also another condition. This condition is in times called "first rain." It describes a phenomena of drainage where the rain sets existing slippery agents (dust, grease) afloat. Cars are naturally dirty, and they drop grease and dirt. Dirt can also come from gardens or fields near the road. These agents pile up on the road when it's dry, and float when first soaked in water. This reduces the grip levels to a half of those of a normal wet road, a mere 0.3 coefficient of friction. It than takes a day or two for the dirt to be washed away. It might even remain after the water has dissolved almost completly!
This does not only apply to the first rain of the winter. It can apply to any sign of moist (due to nightly dew or a misaimed fountain) in the summer or spring, and also to rains that fall in the wet seasons after a dry week or so. Mind you, it is possible that during a long drive, the road can in part be simply wet ("washed") and in part still greasy due to weather changes.
We therefore need to develop additional means to identify these situations. First, besides acknowledging when this condition occurs, we need to figure out where it occurs. The more traffic there is, and the slower it is, the more grease will pile up. This renders urban areas, and especially intersections, roundabouts and busy bus stops as particularly dangerous. Likewise, areas prone to heavy, jammed traffic will also make large amounts of grease pile up. Another special place is construction sites (on or alongside the road), where there is a large presence of sand and grease from machinary and from the joints of cranes.
It's also understandable that dirt will stream down inclines. When going downhill, there will be a pile-up of slippery agents on the edge of the slope. You can drive on a straight road that is accessed and crossed by roads coming downhill, making it slippery. Side-slops also count, and we need to assume that the normal drainage of the road makes the slippery pieces stream to the far edges of the road. So, it's best not to drive all too close to the edge of the road on the wet.
Besides, fixed presence of slippery agents like little gardens between the opposite lanes, fields alongside a rural road, trees over a residential street and in some countries even the pavement/cross-walk paint, will produce low grip levels. Fallen leaves and tree sap are slippery!
It's possible to sense the grip levels through the steering, and also to look out for noticeable traces of grease, leaves, dirt and mud, especially on the sides of the road. The road surface can vary from "shiny" to "murcy."
Dirt roads
Coefficient of friction: 0.5-0.2.
Roads can be made of dirt, or riddled with it. I have always considered these road conditions as silent killers: Most people know to beware of wet surfaces, and clearly grease, snow and ice -- but know little to avoid mold or mud. These conditions offer a larger range of grip levels.
If the dirt is squeezed tight, it will behave much like tarmac and provide considerable grip levels, much like a wet road. Like a human is bound to slip over a loose stone, a car would not favor loose surfaces. However, loose ground has the advantage of making the slides more controlled. There is a so-called "wedge effect" which makes the sliding wheels dig into the surface, wedge it in front of them, which slows them down. In some cases, this can also help the tire find a grippier surface beneath.
Grass and verge would mean that a "network" of roots has been deployed under the surface which contributes to it's rigidity. Different types of earth will again tend to be more or less dense and provide different levels of grip. Rocks, as well as mold, will provide lesser amounts of grip. Piles of mold are to be particularly avoided.
Stangly enough, the dirt surface under deep puddles also tends to be firm. In flooded areas, loose rocks can be detected as little turbulances in the water. Of course, mud, as well as concealed rocks and pieces of metal are to be avoided. Wheeltracks form steady earth, but can create several troubles:
- A buildup of mud (and sometimes slush) inside them
- the high ground between the wheeltracks might rub against the car's ramp
- If you turn the wheel in the wheeltracks, the car will keep on moving forward as if "on rails" and when it suddenly finds grip, it will veer aside at once.
Other attributes of off-road surfaces include roads with two levels, where the lower one might host water and mud. The other pheonmena is a washboard surface, where soft earth is being pounded by wheels that bounce up and down over the little undulations of the road, leaving "waves" in the dirt.
However, dirty surfaces can also be referred to as cohesive. I.E. The friction occurs not only between the tire and road, but also between the different layers of the earth. This is especially true for dune driving, where you want smooth transmission of friction that will not "break" through the sand to dig down into it.
However, there are none-cohesive surfaces, like clay and mud, which are normally more slippery, but also change the manner of grip. Here, the grip is created by the tread, which spears lumps of clay/mud and pushes them back to push itself forward. These surfaces can be quite slippery. Mud is especially slippery when it's texture is similar to quicksand. This occurs with combination of large amounts of water with several types of earth, eithe rich earth from fields and with steed feces, but also loess (worn silt) which is prominent in the desert. Pools of mud can offer grip levels similar to those of snow, and lower than most types of grease. Ash and burnt sludge are also quite slippery.
Frozen Surfaces
Coefficient of friction: 0.3-0.1
In modern English, the distinction between snow and ice has become pedantic. However, there is a significant difference between frost, hail, snow and ice. Frost is created late at night, down in vallyes or up on mountains. Dew falls on the earth, and a sudden breeze of cold air freezes it into small portions of thing white snowy powder. Depending on how frozen it is, frost can offer grip levels quite similar to "first rain."
Hail is created up in the sky. It is the result of abnormal climate phenomena and unique cloud patterns, where there are large differences in the heat levels over the cloud itself from bottom to top. This creates turbulances inside the cloud. These air steams create rain drops and push them up towards the colder part where they freeze into little ice flaks. They than start falling off, only to be pushed up again and merge with other freezing water drops. The process repeats untill the hail is heavy enough to fall down as little pieces made of layers of ice. The grip levels are midrange between frost and snow.
Snow is simply rain drops that freeze. When fresh, a thin layer of snow provides relativelly reasonable grip, because the (snow) tires can clear it aside and find grippy, dry tarmac below. Deeper, colder snow becomes more dense and especially packed, which reduces grip levels siginficantly. There will also be frost or ice under it.
Ice is created where snow falls constantly and heavily enough to create pressure between the snow layers to create a solid, rigid layer of ice. Under normal winter conditions, small layers of ice can be bulit, and it is thin enough to transparent (seen as glimmering tarmac). This is known as "black ice" and it appears in tree shades, under and on bridges and withing puddles of water.
In colder conditions, there would be an actual layer of ice. This requires special studded tyres or chains to cope with and reduces grip levels more than anything else. The ice becomes more slippery when colder, up to a certain point where there is a thick layer of pure ice without a siginficant layer of either snow or water. This is known as "glare ice" and it is the most slippery thing one can find.
Oil
Coefficient of friction: 0.3-0.1
Oils form a difficulty because of their increased viscosity. Oily agents used as fuels (like petrol and especially diesel) can be found more easily, but are also least slippery, even relative to snow or mud. Coolant and motoroil are more slippery, and the lubricant used in the transmission, spindles and differentials, especially in heavy vehicles, are the most slippery and a bit of moist that these oils can abosrb makes the problem more acute. They can be recognised from a distance as a big "sheen" of colors, much like a rainbow. They can also form "foam" that can also be detected.
Another slippery agent to be mentioned is Epoxy. Wet epoxy is the material used in skid-pads (alongside Jennite and sometimes baslat tiles). This substance, much like plastic pigments or acrilics, offers ridiculously low grip levels, simulating glare ice. If you had a tuition on an Epoxy skidpad, I suggest not counting on it, because the simulation is very unrealistic.
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