Side friction in highway design

Introduction

Friction between tyre and road surface is an important element in highway design. For example, friction values are used in the calculation of stopping sight distance and horizontal radius. Friction is considered in two ways – longitudinal friction along the line of the road (or tangential to the curve of the road); and lateral friction, which is taken at right-angles to the line of movement of the vehicle.

Lateral (or side-) friction is used in the calculation of horizontal radius, as in the standard formula:

Rmin = V2 / 127 (e+f)

This relates radius to values for speed (V), road superelevation (e) and side friction (f)

If we suppose that the assumptions behind this formula are correct, then it is clear that highway engineers need to have a good idea of what the value of friction is if they are to produce a safe, economic design. In a study of quoted values for side friction, details have so far been collected for more than 30  different sets of values, taken from highway design guidelines and standards from different countries published over the last 40 years or so.  The following are some preliminary comments on the collected data.

Factors affecting side friction

Friction between a tyre and the road surface can be affected by a largenumber of factors. Some of these are shown in table 1, which is taken from  a 2012 paper published in the USA titled “Development of surface friction guidelines for LADOTD” (ref. 183). The table lists 36 different factors.

Limiting values for friction

Maximum  values for side friction can reach 0.90, although on a wet road values fall to 0.35 or less, depending on speed (it is generally agreed that friction varies with the speed of the vehicle).

Maximum and minimum values from design guidelines

Table 2 gives some values taken from the design guidelines studied so far. They cover quite a range – for example, from 0.12 to 0.31 for a speed of 40 km/hr. The values in the table for a speed of 100 km/hr are as low as 0.11 or less (less than a third of the limiting value of 0.35 found in table 1).

Early and recent values for side friction

Table 3 gives some values for side friction quoted in publications issued between 1973 and 2013. There does not appear to be any large change in the values  – for example compare the values shown for 1973 and 2013.

Multi-country values for side friction

There are a number of publications which propose guidelines for highway geometrics applicable to several countries. One example is given by the documents related to the Asian Highway Network. The values they quote for side friction seem to be similar; and they also seem to be lower than the available values shown in table 1 (compare the  available value of 0.35 at 100 km/hr with values around 0.12 in the table below).

Factors affecting quoted values

Since friction values with speed, then for the 36 factors listed in the first table above, taken together with 12 different speed values (20 to 130 km/hr at 10 km/hr intervals) then in theory, any design guideline would need to cover something over 400 scenarios. In practice, many guidelines just quote one value for side friction against a particular speed value. Other guidelines are a little more specific, and quote values for a given scenario, such as vehicle type (cars, trucks), road type (urban/rural), speed type (low speed/high speed) etc.

Side friction and comfort

One source (ref. 777) says that “the side friction factors that are employed in the design of horizontal curves should accomodate the safety and comfort of the intended users”. The source quotes 1994 AASHTO values for comfort:

Different values of side friction are needed for different sections along a road. For example (ref. 180) says that “in places where it is known that drivers frequently need to brake or turn at speed …., higher friction levels are likely to be needed than would be adequate elsewhere”. The following table is just one example of this (taken from ref. 174):

Comment

The basic formula which relates curvature and friction quoted above may be out of date. For example (Ref. 709 / 2009) says that “a group of researchers in Germany iinvestigated  several  European  country  cases  and  proposed  a  new  relationship  for finding relevant side friction factors in highway curve design.”

The existence of multi-country standards on highway geometrics suggest that road surfaces and the vehicle s which are driven on them  are similar, whichever country we happen to be looking at.

A glance at table 3 suggests that guideline values for side friction have not changed over time. However, the composition of tyres and road surfaces has changed over time, as has also the understanding of what affects side friction. It would be more reasonable to expect different guideline values to be recommended today.

It is not always clear whether quoted values for side friction are based directly on observed values, or whether factors of safety have been applied first. The principle should be that guidelines should always specify the conditions for the values of which they refer to  (for example do the values refer to tyres in good condition on wet road surfaces, are they based on observed values , what is the source and date of these observed values, do they quoted figures include for a factor of safety – and if they do include a FOS, what is it?)

Different methods  are available for measuring friction and side friction. The problem is that they give results which are at best difficult to compare – and even questionable for use in the basic formula quoted at the beginning of this post.

Work should be done to develop a common understanding of what scenarios highway design guidelines should be prepared for. Perhaps there could be a manageable number of (say) only 5 to 10 scenarios. These scenarios could include for vehicle types such as trucks and bicycles, and for different road surfaces.

The impression gained is that at present, guidelines on side friction in different countries are imprecise and dubious, and untrustworthy for use in modern highway design .

Footnotes

There is a hidden problem with the information on sources. In some cases there is a primary source (here taken as the publication in hand) and a secondary source (where the information actually comes from ). The year and country of the secondary source can be different from the year and country of the primary source.

The data in this post is being re-checked and updated. A new post will follow when this has been completed.

This post is intended to encourage discussion on design guidelines in general, and on suitable values for side fricton in particular. If anyone has comments or corrections please contact the author.

References

61  – “Roads in urban areas (3rd edition)”, London, HMSO 1973

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

174 – NCHRP “Guide for pavement friction”, USA, 2009

183 – FHWA, “Development of surface friction guidelines for LADOTD”, USA 2011

635 – “Overseas road note 6, A guide to geometric design” England, TRL 1988

709 – Choi and Kim, “Review of side friction factors in highway curve design of higher speed freeways”, REAAA 2009

714 – Ministro delle Infrastruture e dei Trasporti, “Norme funzionali e geometriche per la costruzione delle strade”, Decreto Ministeriale 5, November 2001

726 – UNECE “Trans European Motorway standards and recommended practice third edition”, Poland 2002

756 – Asian highway handbook, full version 2003

763 – Malaysia, “A guide on geometric design of roads”, 1986

771 – SATCC “Draft code of practice for the geometric design of trunk roads”, CSIR, South Africa 2001 (SATCC = Southern African Transport and Communications Commission)

777 – University of Idaho, webpage with notes on “superelevation and side friction”, downloaded July 2013

803 – Chowdhury et al, “Are the criteria for setting advisory speeds on curves stil relevant?”, ITE Journal Feb. 1998

831 – AASHTO, A policy on the geometric design of highways and streets 2011

837 – Yemen Highway Authority, Development of National Highway Master Plan, “Design Standards”, prepared by Dar al Handasah, 1986

892 – Manual Centroamericano de Normas para el Diseño Geométrico de Carreteras, 3ª. Edición, 2011

922 – Hb265: Linjeføringsteori, from the Statens vegvesen NPRA Norwegian Public Roads Administration;

943 – Changes in horizontal alignment standards in Australia and Canada, presented at the International Symposium on Highway Geometric Design Practices, TRB 1995