Factors affecting horizontal radius – Friction

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There are three parameters which affect horizontal radius: speed, superelevation and friction. Friction is the only parameter which – in theory – has abstract, measurable values. Further, the materials used in road surfacing are generally similar from country to country, so we could expect that the values for friction  would be the same from country to country. However it seems that we don’t really know what friction is; for example:

  • The coefficient of friction is not  a  material  property.  In  other  words,  one  cannot  state  that  a  certain  surface  has  a specific  coefficient  of  friction.  Instead,  it  is  a  system  property  with  its  magnitude depending on both two surfaces that are in contact (185), (own emphasis), and
  •  the mechanisms of tire-pavement friction interaction are not fully understood (185)

The problem of estimating friction is made worse since there are different ways of measuring it, and that the results of these different methods are not necessarily comparable. Then, even where observed values for friction have been obtained, engineers have to answer the question, “what is the relationship between measured values for friction and values selected for use in highway design?”.  If there are more than twenty factors which affect friction (see below) how can someone decide on a value which will apply to all weather conditions, tyre types and patterns, road surface temperatures, road surface materials and so on – and do highway engineers in different countries make the same decisions and conclusions?

It is planned to post examples of friction values from different countries in later posts.

International friction index

Work has been done on the development of an international friction index (IFI) – see for example

“To harmonize the friction measurements by different devices, the World Road Association – Permanent International Association of Road Congresses (PIARC) performed an experiment in Belgium and Spain in 1992 and came up with a new friction index, IFI (Wambold et al., 1995). The IFI consists of two numbers that describe the friction resistance of pavement: speed constant (S P ) and friction number F(60).The general notation for IFI is IFI [F(60), S P ]. The number 60 in friction number F(60) denotes the test vehicle speed of 60 km/hr, though IFI can represent friction at different test speeds. The speed constant (S P ) is correlated with the result of a macro-texture measurement (Wambold et al., 1995)” (183)

  • but the results so far are not necessarily robust, see for example the conclusions of (185).

Other points about friction

  • Road surface improvements alone do not necessarily improve safety and may make it worse (172)
  • A driver making a journey should not expect to meet the same conditions of friction at all points along the journey.
  • Most skid resistance problems, skid resistance testing, and nearly all of the literature relate to wet surface skid resistance (172)
  • Engineers can design the pavements of sections of road so that they have higher than ‘normal’ values for friction. Higher values are useful at approaches to traffic signals and tight curves for example.

Values for friction

In Western European countries, the minimum permissible value of the friction coefficient ranges from 0.6 in Belgium to 0.4 in France (175 quoting Ren Fu-tian et al., 1993).

Wallmann and Astrom (169) say ” Roadway  slipperiness  was  measured  and  divided into four categories of friction coefficients: good grip (f > 0.45), fairly good grip (0.35 < f  ≤  0.45), fairly slippery (0.25 < f  ≤  0.35), and slippery (f  ≤  0,25)”. A German source quotes values for friction for a PIARC tyre of 0,36 at 60 km/hr to 0,19 at 120 km/hr (177).  New Zealand values quoted for “maximum design speed values of side friction for passenger cars on sealed roads” range from 0,33 at 60 km/hr to 0,11 at 120 km/hr (80).

One problem when comparing values for friction from different sources is that you cannot be sure that you are comparing “like with like”.

Factors affecting friction

One source suggests that “In a friction measurement there are often three bodies involved, the measuring tyre, the road surface and some kind of contaminant interacting with both tyre and road like for example water (wet friction), dust or wear particles etc. (169). If we add two more headings (‘pragmatic’, and ‘other’) then the features affecting friction are

the tyre

  • Material
  • Tread pattern
  • Inflation pressure
  • Load
  • Temperature / Research has indicated that tire pavement friction decreases if the tire temperature increases (180, quoting Hall et al 2009)

the road surface

  • Microtexture / at low travel speeds, surface aggregate microtexture is the primary contributor to skid resistance (see e.g. 172)
  • Macrotexture / … above 90 km/h, the macrotexture has been found to account for over 90 percent of the friction (184)
  • Road surface material
  • Age of road surface
  • Road temperature / “Temperature effect on friction is the main parameter responsible for  seasonal  variations  of  friction  measurements.  Therefore,  it  is  necessary  to  apply  a temperature  correction  to  friction  measurements  in  order  to  perform  comparisons between those at different temperatures or different seasons of the year” (185)
  • Drainage capacity


  • Temperature
  • Film thickness
  • Chemical composition

pragmatic features, such as

  • Factor of safety
  • Approximation
  • Simplification


This is a catch-all term for a miscellaneous group of factors which include

  • Season of year / “A significant variation has been observed in coefficient of friction values measured on the same pavement surfaces at different times of the year” (185)
  • The method and equipment used to measure friction
  • Type of road
  • Vehicle speed / The skid resistance provided by a wet road surface decreases with increasing travel speed (172)
  • Slip ratio
  • Direction of measurement (transverse friction values may be different to longitudinal friction values)
  • Type of vehicle (New Zealand quotes different values for friction for cars and for trucks (80))


 80 – “State highway geometric design manual section 2: basic design criteria”, New Zealand, Transit New Zealand 2003

169 – Wallman & Åström, “Friction measurement methods and the correlation between road friction and traffic safety. A literature review”, Sweden, VTI 2001

172 – McLean & Foley, “Road surface characteristics and condition: effects on road users”, Australia, ARR 314 1998

177 – Weiss, Durth et al, “Strassenbau, Planung und Entwurf”, Verlag für Bauwesen, Germany 1997

180 – Flintsch et al, “The little book of tire pavement friction (version 1)”, USA, Pavement surface properties consortium 2012

183 – Wu & King, “Development of surface friction guidelines for LADOTD”, USA Louisiana State University, 2012

184 – Andresen & Warmbold, “Friction fundamentals, concepts and methodology”, Canada, Transport Canada 1999

185 – Fuentes, “Investigation of the factors influencing skid resistance and the international friction index”, USA, University of South Florida, 2009


About roadnotes

Robert Bartlett is an international consultant with over 30 years of professional experience as a highway and traffic engineer with leading companies and organisations in several countries, including Germany, China (Hong Kong), Qatar and the UK. Specialised in urban studies, transport and the use of GIS, research has included new ideas on subjects such as the study of social justice using GIS, the dimensions of vehicles, and comparative geometrics (highways and transport).
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