“Skid resistance is a condition parameter characterising the contribution that a road makes to the friction between a road surface and a vehicle tyre during acceleration, deceleration and cornering. A distinction is sometimes made between longitudinal friction (relative to acceleration and deceleration) and transverse friction (relative to cornering and emergency manoeuvre) (QDMR 2006).”
The level of skid resistance available depends on, among other things, the micro-texture of the aggregate, the macro-texture of the road surface (i.e. surface texture) and the thickness of any water film on the road surface. Other factors that can influence the level of skid resistance available are discussed in Section 2.8.7.2.
2.8.7.1
Many crash studies have shown that a disproportionate number of crashes happen where the road surface has a low level of surface friction and/or surface texture, particularly when the road surface is wet (QDMR 2006).
It is noted that skid resistance is one of many factors that may influence the risk of crashes (e.g. other factors include driver behaviour, geometry and vehicle characteristics).
The following two sections are general in nature. For a detailed discussion about skid resistance and surface texture reference should be made to the following documents:
• For details about the management of skid resistance by TMR refer to the Skid Resistance Management Plan (QDMR 2006) and its addendum (TMR 2010c). This document includes guidance on investigatory levels.
• In the future, Polished Aggregate Friction Value (PAFV) and surface texture requirements will be published in the PDM (QDMR 2009) and/or the Pavement Surfacing Manual (future publication). In the interim TMR’s Pavements and Materials Branch must be contacted for advice about appropriate PAFV and surface texture values (and test methods).
The above documents also contain further references to which the designer can refer.
Test procedures and measures
Measurements of skid resistance are usually made on wet surfaces (the worst case) and in the longitudinal direction of travel (QDMR 2006).
Two types of friction measuring devices are currently used by TMR, namely:
• The ROad Analyser and Recorder (ROAR) (i.e. Norsemeter); and
• The Portable/British Pendulum Tester with testing undertaken in accordance with TMR Test Method Q704-1982 (QDMR 1982).
The latter is a portable device that is simple to use. It requires minimal establishment but only tests a very small area and interpreting its results requires consideration of a number of factors.
The ROAR is used more for network surveys. If they exist, ROAR results may be useful in an investigation.
However, for a project level investigation that includes skid resistance, portable pendulum testing should also be done. ROAR results can be compared to the results of the pendulum testing and an assessment made in the light of both sets of results. The Skid Resistance Management Plan (QDMR 2006) and its addendum (TMR 2010c) includes guidance on investigatory levels.
In the future the Sideways Co-efficient Routine Investigation machine (SCRIM) will be used for network surveys and the management of skid resistance (TMR 2010c). Its basic constituents are a heavy vehicle (truck) cab and chassis with an integrated water tanker body, test pod/s (which can be as required) and instrumentation. Each test pod contains a test wheel is held at an angle of 20º to the longitudinal axis of its normal travelling wheels (this is known as the ‘yaw angle’). The test wheel is free to rotate during testing, but is restrained by the yaw angle. A consistent vertical load of 200 kg, which is independent of the movement of the truck chassis, is also applied to the test wheel. A machine has at least one test pod to test one wheelpath. However, the Roads and Traffic Authority (RTA) of New South Wales and GeoPave SCRIMs have two test pods/wheels and collect data from the left and right wheelpaths concurrently. Currently TMR does not own and/or operate its own SCRIM and testing services are procured from an experienced operator
It is important to note that each piece of apparatus measures slightly different things and so the results obtained using one device or piece of apparatus can not be directly compared to results obtained using another device piece of apparatus.
Texture depth is an indicator of the space through which water may escape from the interface between a tyre and the road surface. It is an important factor affecting skid resistance at high traffic speeds as without sufficient texture depth vehicle aquaplaning can occur. Surface (water) spray and the noise generated from the tyre-road interaction are also affected by surface texture as described in the Austroads Guide to Pavement Technology Part 3: Pavement Surfacings (Austroads 2009a).
Generally for roads controlled by TMR surface texture is determined using one or both of the following methods:
• By converting the results of a laser profilometer survey (e.g. from a NSV survey) to an equivalent sand patch texture depth; and
• The “sand patch” test with testing undertaken in accordance with Test Method Q705-2010 (TMR 2010e).
Texture depth measured by the sand patch test is expressed in millimetres. Some laser profilometers (e.g.
those on the NSV) can measure texture depths expressing results as Mean Profile Depths (MPD) or, by use of correlation factors, as equivalent sand patch texture depths. Notwithstanding this, the use of equivalent sand patch texture depths derived from laser profilometers should be used with care. For a project level investigation that includes skid resistance, sand patch testing should also be done (as per TMR Test Method Q705-2010 [TMR 2010e]). Both sets of results can then be compared and an assessment made in the light of both sets of results.
2.8.7.2 Causes
It is noted that the available surface friction depends on a multitude of factors including vehicle speed, surface texture, water depth, tyre characteristics, tyre condition, vehicle suspension characteristics, distribution of mass, seasonal influences and road geometry (QDMR 2006). In the case of the latter how superelevation is applied, the degree of superelevation provided and horizontal curve radius are important influences.
Table B.4 of the AGTPT Part 5 outlines some factors contributing to reduced skid resistance. Additional factors include:
• the accumulation of surface water (e.g. from poor surface drainage or depressions);
• surface contamination (e.g. debris filling voids of Open Graded Asphalt [OGA]);
• pavement markings with low friction; and
• crack seals/sealants with low friction.
Other factors can exacerbate or otherwise contribute to reduced skid resistance such as:
• depressions (e.g. ruts or shoves creating depressions);
• inappropriate geometry (e.g. incorrect application of superelevation, the degree of superelevation provided is too low, a horizontal curve radius is too small);
• corrugations;
• poor maintenance;
2.8.9.1
• inadequate sight distances;
• changing the shape of the pavement through a treatment or intervention without checking surface drainage (e.g. checking for aquaplaning);
• inappropriate selection of a surfacing (e.g. inadequate surface texture) and/or inappropriate materials used in a surfacing (e.g. use aggregate with a polished aggregate friction value less than that recommended for a site); and
• changes to standards.