gta-4.003 factors affecting horizontal radius : friction

7/30/2019 gta-4.003 Factors affecting horizontal radius : Friction

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H or i zontal radius

GTA-4.003 January 2013 Page 1

Global Transport AtlasSeries 4- Comparative Geometrics

There are three parameters which affect horizontal

radius: speed, superelevation and friction. Friction isthe only parameter which - in theory - has abstract,measurable values. Further, the materials used inroad surfacing are generally similar from country tocountry, so we could expect that the values forfriction would be the same from country to country.However it seems that we don't really know whatfriction is; for example:

The coefficient of friction is not a materialproperty. In other words, one cannot statethat a certain surface has a specific coefficient

of friction. Instead, it is a system propertywith its magnitude depending on both twosurfaces that are in contact (185), (ownemphasis), and

The mechanisms of tire-pavement frictioninteraction are not fully understood (185)

The problem of estimating friction is made worsesince there are different ways of measuring it, andthat the results of these different methods are notnecessarily comparable. Then, even where observedvalues for friction have been obtained, engineershave to answer the question, "what is the

relationship between measured values for frictionand values selected for use in highway design?".

If there are more than twenty factors which affectfriction (see below) how can someone decide on avalue which will apply to all weather conditions, tyretypes and patterns, road surface temperatures, roadsurface materials and so on - and do highwayengineers in different countries make the samedecisions and conclusions?

In ter nat ional f r ic t ion index

Work has been done on the development of an

international friction index (IFI) - see for example

"To harmonize the friction measurements bydifferent devices, the World Road Association- Permanent International Association ofRoad Congresses (PIARC) performed anexperiment in Belgium and Spain in 1992 andcame up with a new friction index, IFI(Wambold et al., 1995). The IFI consists oftwo numbers that describe the frictionresistance of pavement: speed constant (S P )

and friction number F(60).The generalnotation for IFI is IFI [F(60), S P ]. The

number 60 in friction number F(60) denotesthe test vehicle speed of 60 km/hr, though IFIcan represent friction at different test speeds.The speed constant (S P ) is correlated withthe result of a macro-texture measurement(Wambold et al., 1995)" (183)

- but the results so far are not necessarily robust, seefor example the conclusions of (185).

Other points about f r ic t ion

Road surface improvements alone do notnecessarily improve safety and may make itworse (172)

A driver making a journey should not expect tomeet the same conditions of friction at all pointsalong the journey.

Most skid resistance problems, skid resistancetesting, and nearly all of the literature relate towet surface skid resistance (172)

Engineers can design the pavements of sectionsof road so that they have higher than 'normal'values for friction. Higher values are useful atapproaches to traffic signals and tight curves forexample.

Values for f r ic t ion

In Western European countries, the minimumpermissible value of the friction coefficient rangesfrom 0.6 in Belgium to 0.4 in France (175 quotingRen Fu-tian et al., 1993).

Wallmann and Astrom (169) say " Roadwayslipperiness was measured and divided into four

categories of friction coefficients: good grip (f >

4.003 Factor s affect i ng hor izontal r adius / Fr i ct ion


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A German source quotes values for friction for aPIARC tyre of 0,36 at 60 km/hr to 0,19 at 120 km/hr(177). New Zealand values quoted for "maximumdesign speed values of side friction for passengercars on sealed roads" range from 0,33 at 60 km/hrto 0,11 at 120 km/hr (80).

One problem when comparing values for frictionfrom different sources is that you cannot be sure thatyou are comparing "like with like".

Factors affecting fr ict i on

One source suggests that "In a friction measurement

there are often three bodies involved, the measuringtyre, the road surface and some kind of contaminantinteracting with both tyre and road like for examplewater (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

Inflati on pressureLoad

Temperature- research has indicated that tirepavement friction decreases if the tiretemperature increases (180, quoting Hall et al2009)

The road surface

Microtexture- at low travel speeds, surfaceaggregate microtexture is the primarycontributor to skid resistance (see e.g. 172)

Macrotextur e - above 90 km/h, themacrotexture has been found to account for over90 percent of the friction (184)

Road surface mater ial

Age of r oad sur face

Road temperatur e - "Temperature effect onfriction is the main parameter responsible forseasonal variations of friction measurements.Therefore, it is necessary to apply atemperature correction to frictionmeasurements in order to perform

comparisons between those at different

temperatures or different seasons of the year"(185)

Drainage capacity

Contamination

Temperature

Film thi ckness

Chemical composit ion

Pragmatic features, such as

Factor of safety

Approximation

Simplification

Other

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

Season of year - "A significant variation hasbeen observed in coefficient of friction valuesmeasured on the same pavement surfaces atdifferent times of the year" (185)

The method and equipment used to measure

friction

Type of r oad

Vehicle speed - The skid resistance provided by awet road surface decreases with increasing travelspeed (172)

Slip r atio

Dir ecti on of measurement - transverse frictionvalues may be different to longitudinal frictionvalues)

Type of vehicle - New Zealand quotes differentvalues for friction for cars and for trucks (80))

Refer ences

80 - "State highway geometric design manualsection 2: basic design criteria", New Zealand,Transit New Zealand 2003

169 - Wallman & strm, "Friction measurementmethods and the correlation between roadfriction and traffic safety. A literature review",Sweden, VTI 2001


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172 - McLean & Foley, "Road surface characteristicsand condition: effects on road users",Australia, ARR 314 1998

177 - Weiss, Durth et al, "Strassenbau, Planung undEntwurf", Verlag fr Bauwesen, Germany1997

180 - Flintsch et al, "The little book of tire pavementfriction (version 1)", USA, Pavement surfaceproperties consortium 2012

183 - Wu & King, "Development of surface frictionguidelines for LADOTD", USA LouisianaState University, 2012

184 - Andresen & Warmbold, "Friction

fundamentals, concepts and methodology",Canada, Transport Canada 1999

185 - Fuentes, "Investigation of the factorsinfluencing skid resistance and theinternational friction index", USA, Universityof South Florida, 2009


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GTA docum ents seri esGlobal Transport Atlas is a project with two aims. One isto encourage people around the world to share examplesof how they actually use transport in their daily lives. Theother is to compare and improve the design of transportinfrastructure. There are the following GTA documentseries:

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A collection of papers on transport infrastructure andvehicles from various countries, prepared by variousauthors and contributors. Typical size 2 to 3 pages.

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Papers with arguments and ideas on different aspects ofmodern transport and transport infrastructure.

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Working papers which look at different aspects of highwayinfrastructure design and compare values from differentcountries.

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Transport infrastructure has to be defined with an idea ofthe size and types of vehicle which will use it. This serieslooks into the dimensions of different types of vehicle atdifferent periods of time. Papers include technicaldiscussion notes and example dimensions. Typical size 20pages and more.

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H istor y and Change log

Version 403.01-a (December 2012) - first version

gta-4.003 (January 2013) - document number updated

Contact

We welcome comments on this paper, and also on newdevelopments in other countries in this field.

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About the contr ibutor

Rober t Bart let t , Ger many - is an experiencedtransportation and urban development studies engineerwith over 25 years of professional experience. Currentengineering work: includes technical research in highwaydesign standards and applications in areas such as urbanplanning and highway engineering. Interests include

applied GIS.

gta-4.003 factors affecting horizontal radius : friction

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