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This report presents the results of a 4-year research programme (from 2000 to 2003) involving a combination of field, laboratory based, and statistical modelling studies undertaken to identify critical aggregate properties from the perspective of in-service skid resistance performance of chipseal surfaces. Emphasis was placed on straight and level road sections to minimise confounding effects brought about by braking, cornering, and traction manoeuvres.

The principal finding of the research was that the critical determinants of in-service skid resistance performance of chipseal surfaces were:

• cumulative heavy commercial vehicle (HCV) passes
• the mean spacing between tips of aggregates.

As a result, a rational model was formulated that provides 95% certainty that the predicted value of skid resistance will be within ±0.08 MSSC of observed values. The model inputs are limited to PSV, HCV traffic, seal age and aggregate average least dimension (ALD). However, significant inter-relationships between aggregate microtexture and macrotexture were also identified, which require additional investigation given their implication to current seal design practice.

The preliminary indications are that:

• selection of 'rounded' alluvial aggregates for skid resistant surfaces should be predicated on PSV as is current practice
• selection of 'angular/sharp-edged' hard rock aggregates should be predicated on size (the smaller the better) and ability to withstand tip- and edge-wear caused by HCV traffic.

Keywords: aggregate polishing, bitumen film, chipseal, chipseal design, contamination, emergency braking, macrotexture, microtexture, predictive model, regression analysis, skid resistance, skid testers