Introduction

Texture depth or macrotexture are terms used to describe the profile of a road surface at a millimetre scale. Texture depth contributes to removing water at the interface between the tyre and surface, and supports the generation of skid resistance at high speeds. Texture depth can also influence material durability, rolling resistance and road noise. Ideally, texture depth needs to be optimised to produce a road surface that is quiet, smooth, safe, and durable.

Traditionally, for acceptance purposes, the texture depth of a new surface coarse has been measured using the Volumetric Patch Test (VPT) described in BS EN 13036-1.

(BS EN 13036-1. Test methods - Measurement of pavement surface macrotexture depth using a volumetric patch technique (BSI, 2010))

The test involves spreading a measured volume of glass beads into a circle on a road surface and calculating the average depth. In contrast, the in-service texture depth is measured using a laser-based system that currently calculates Sensor Measured Texture Depth (SMTD), which is used to monitor performance and assess any maintenance requirements. As these two measurement systems differ, it is difficult to optimise specifications as it is unclear how changing the requirement on new surfaces will affect in-service performance. It is also recognised that the VPT method provides a limited assessment of the length and width of a newly paved surface, and requires technicians to work near live traffic. There is therefore a desire to replace the VPT method with a laser-based contactless method.

Following recent research on transitioning from VPT to a laser-based alternative (Initial Investigation into the Effect of Transitioning to MPD, Unpublished report, TRL, 2020.), National Highways (NH) have proposed the measurement of Mean Profile Depth (MPD). Both MPD and SMTD are measurements calculated from raw texture (laser-based) profile. It was decided to use the MPD as it has a broad relationship with VPT and is an international ISO standard measurement (ISO 13473-1. Characterization of pavement texture by use of surface profiles. Part 1: Determination of mean profile depth (ISO, 2019)).

As a result, levels of MPD have been suggested for new roads, and the long‑term aim is to also measure MPD on in-service roads. Transport Scotland is interested in adopting the same approach. However, it is important that the proposed MPD values are also appropriate for surface coarse materials used in Scotland. In certain instances, Transport Scotland control high and low speed surface friction in a different way and in general use different, lower textured, materials.

Brief

The aim of the study is to analyse profile and texture data collected from the Scottish Trunk Road Network to understand the implications of transitioning to MPD. The work includes examining comparisons of SMTD and MPD at both a network and material type level, including Cl942 Thin Surfacing, TS2010 and HRA. An examination of data is also required to assess the impact of transitioning to MPD, rather than SMTD, with respect to how the new measure might affect the categorisation of the network in terms of maintenance thresholds.