Road slipperiness or skid resistance is the technical term for the cumulative effects of snow

Snow

Snow is a form of precipitation within the Earth's atmosphere in the form of crystalline water ice, consisting of a multitude of snowflakes that fall from clouds. Since snow is composed of small ice particles, it is a granular material. It has an open and therefore soft structure, unless packed by...

, ice

Ice

Ice is water frozen into the solid state. Usually ice is the phase known as ice Ih, which is the most abundant of the varying solid phases on the Earth's surface. It can appear transparent or opaque bluish-white color, depending on the presence of impurities or air inclusions...

, water

Water

Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

, loose material

Road debris

Road debris, a form of road hazard, is debris on or off a road. Road debris includes substances, materials, and objects that are foreign to the normal roadway environment...

 and the road surface

Pavement (material)

Road surface or pavement is the durable surface material laid down on an area intended to sustain vehicular or foot traffic, such as a road or walkway. In the past cobblestones and granite setts were extensively used, but these surfaces have mostly been replaced by asphalt or concrete. Such...

 on the traction

Traction (engineering)

Traction refers to the maximum frictional force that can be produced between surfaces without slipping.The units of traction are those of force, or if expressed as a coefficient of traction a ratio.-Traction:...

 produced by the wheels of a vehicle. Road slipperiness can be measured either in terms of the friction between a freely-spinning wheel and the ground, or the braking distance

Braking distance

Braking distance refers to the distance a vehicle will travel from the point where its brakes are fully applied to when it comes to a complete stop...

 of a braking vehicle, and is related to the coefficient of friction between the tyre and the road surface. Public works

Public works

Public works are a broad category of projects, financed and constructed by the government, for recreational, employment, and health and safety uses in the greater community...

 agencies spend a sizeable proportion of their budget measuring and reducing road slipperiness through drainage

Drainage

Drainage is the natural or artificial removal of surface and sub-surface water from an area. Many agricultural soils need drainage to improve production or to manage water supplies.-Early history:...

, snow removal

Snow removal

Snow removal is the job of removing snow after a snowfall to make travel easier and safer. This is done by both individual households and by governments and institutions.-De-icing and anti-icing:...

 and street sweeping

Street sweeper

A street sweeper or street cleaner can refer to a person's occupation or a machine that cleans streets, usually in an urban area.-History of street sweeping in the United States:...

 in order to maximise road safety; even a small increase in slipperiness of a section of road can increase the accident rate of the section of road tenfold.

A special road safety problem is Split friction

Split friction

A special road safety problem is Split friction or μ - split; when the friction significantly differs between the left and the right wheelpath. The road may then not be perceived as hazardous when accelerating, cruising or even braking softly. But in a case of hard braking, the car will start to...

 or μ (mu) - split; when the friction significantly differs between the left and the right wheelpath. The road may then not be perceived as hazardous when accelerating, cruising or even braking softly, but in a case of hard braking, the difference in friction will cause the vehicle to start to rotate towards the side offering higher grip. Split friction may cause jack-knifing of articulated trucks, while trucks with towed trailers may experience trailer swing phenomena. Split friction may be caused by an improper road spot repair that results in high variance of texture (roads)

Texture (roads)

Road surface texture are deviations from a planar surface, affecting the vehicle/tyre interaction. Pavement texture is divided into:* Microtexture with wavelengths from 0 mm up to 0.5 mm* Macrotexture with wavelengths from 0.5 mm up to 50 mm...

 and colour (thin ice on newly paved black spots thaws faster than ice on old greyish asphalt) across the road section.

Measurement

The two ways to measure road slipperiness are surface friction testing and stopping distance testing. Friction testing can use surface friction testers or portable friction testers, and involves allowing a freely moving object, usually a wheel, to move against the surface. By measuring the resistance experienced by the wheel, the friction between the ground and the wheel can be found.

Stopping distance testing involves performing an emergency stop

Emergency stop

-United Kingdom driving test:For the practical section of the driving test, an examiner will often request the candidate to perform an emergency stop. This is done in order to evaluate how well a candidate can react to a dangerous situation and bring their motor vehicle to a safe stop. This is to...

 in a test vehicle and measuring the distance required to come to a stop. This can be measured either from the length of the skid marks left by the vehicle, or by the "chalk-to-gun" method, where the brakes are connected to a small gun filled with chalk powder, which marks the point when the brakes were applied. This has the advantage of measuring the full stopping distance, while simply measuring the skid marks only measures the distance from the point where the wheels began to lock or slip.

Measurement of skidding reistance is not yet universally harmononised despite a number of attempts such asFEHRL's HERMES project.A current EU initiative is the Tyrosafe project which aims to raise awareness, to coordinate and prepare for European harmonisation and to optimise the assessment and management of essential tyre/road interaction parameters in order to increase safety and support greening of road transport. This project will provide a synopsis of the current state of scientific understanding and its current application in different standards. It will identify the needs for future research and propose a way forward in the context of the future objectives of road administrations in order to optimise three key properties of roads: skid resistance, rolling resistance and tyre/road noise emission

Reduction

Road slipperiness is one of the largest contributors to car accident

Car accident

A traffic collision, also known as a traffic accident, motor vehicle collision, motor vehicle accident, car accident, automobile accident, Road Traffic Collision or car crash, occurs when a vehicle collides with another vehicle, pedestrian, animal, road debris, or other stationary obstruction,...

s — in 1997, over 53,000 accidents were caused by slippery roads in the United Kingdom

United Kingdom

The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...

 alone. A small change in road slipperiness can have a drastic effect on surface friction: decreasing the coefficient of friction from 0.45 to 0.35, equivalent to adding a dusting of wet snow, increased the accident rate by almost 1000%. As such, road agencies have a number of approaches to decreasing road slipperiness. Most roads are designed with a convex camber

Cant (road/rail)

The cant of a railway track or a road is the difference in elevation between the two edges...

 to provide sufficient drainage

Drainage gradient

Drainage gradient is a term in road technology, defining the resulting vector of a road surface cross slope and longitudinal gradient .If the DG is too low, rain and melt water drainage will be insufficient...

, thereby allowing surface water to drain out of the road. Trouble sections include entrances and exits of banked outercurves, where the cross slope

Cross slope

Cross slope is a geometric feature of pavement surfaces; the transversal slope [%] with respect to the horizon. It is a very important safety factor. One task is to make water run off the surface to a drainage system, as Cross Slope is the main contributor to Pavement Drainage gradient. Very low...

 is close to zero. Unless these sections have a longitudinal grade of at least 0.4 - 0.5 %, water will not run off the road surface. Storm drain

Storm drain

A storm drain, storm sewer , stormwater drain or drainage well system or simply a drain or drain system is designed to drain excess rain and ground water from paved streets, parking lots, sidewalks, and roofs. Storm drains vary in design from small residential dry wells to large municipal systems...

s may be installed at regular intervals and modern paving materials are designed to provide high friction in most conditions. Permeable paving

Permeable paving

Permeable paving is a range of materials and techniques for paving roads, cycle-paths, parking lots and sidewalks that allow the movement of water and air around the paving material. Although some porous paving materials appear nearly indistinguishable from nonporous materials, their environmental...

 allows water to soak through the paving material, reducing slipperiness in very adverse conditions.

Snow and ice removal also decreases road slipperiness; snowploughs and snow blower

Snow blower

A snow blower or snow thrower is a machine for removing snow from an area where it is not wanted, such as a driveway, sidewalk, roadway, railroad track, rink, runway, or houses...

s can remove the snow from the road surface while gritter

Gritter

A winter service vehicle , or snow removal vehicle, is used to clear thoroughfares of ice and snow. Winter service vehicles are usually based on dump truck chassis, with adaptations allowing them to carry specially designed snow removal equipment. Many authorities also use smaller vehicles on...

s drop road salt and sand

Sand

Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...

, which both melts the snow and ice from the road surface, and provide a rougher surface to grip onto. However, in dry conditions, sand and salt on the road surface can themselves increase road slipperiness and pose a danger to road traffic, and therefore, roads are cleared by street sweepers after roadwork

Roadwork

Roadwork is a novel by Stephen King, published in 1981 under the pseudonym Richard Bachman as a paperback original. It was collected in 1985 in the hardcover omnibus The Bachman Books, which is no longer in print...

s and gritting to make sure that all the loose material is cleared from the road surface.