Tech Corner

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Q: How are pavement textures characterized?
Q: What methods can I use to measure traffic noise?

Q: How are pavement textures characterized?

A: In order to characterize different textures, one must first understand what pavement texture is and how to properly measure it. A pavement surface often consists of a combination of numerous "wavelengths" of texture - the length between physically repeating features. To the right is a table illustrating three basic texture categories, their wavelengths, and the surface characteristics they affect.

Understanding Texture Wavelengths
	Length	Affects
Micro-texture	< 0.5 mm	Skid resistance
Macro-texture	0.5 to 50 mm	Skid resistance Tire-pavement noise
Mega-texture	50 to 500 mm	Skid resistance Tire-pavement noise Smoothness

Several methods have been developed to measure mega and macro textures. Traditionally, a Sand Patch Method has been used to estimate the Mean Texture Depth, or MTD. This method involves applying a determined amount of sand or glass beads onto a textured surface, and spreading the sand outward into a circle. The more texture a surface contains, the smaller the circle that is created.
Another option available is a Circular Texture Meter, or CTM. Like the Sand Patch, this technique involves taking measurements at discrete locations along the pavement. The CTM uses a spot laser mounted on a rotating arm. Once in place, the unit triggers a computer that rotates the arm and measures texture height for one complete revolution. The resulting trace can be used to estimate the MTD, if desired. CTM
Laboratory measurements are also available, where a core can be extracted from the pavement under inspection and X-ray Computed Tomography (XCT) or Projected Moiré Interferometry (PMI) can be used. Some of the most recent advancements in texture measurement, however, are line-laser based devices such as RoboTex. The sensor mounted on RoboTex is a state-of-the-art Selcom RoLine line sensor from LMI Technologies. RoboTex is a six-wheeled, remote controlled robot that provides three-dimensional texture information for pavement surfaces. Developments like these help engineers, contractors and road owners understand and assess various pavement surface characteristics with greater precision.

Sand Patch Method	RoboTex	Circular Texture Meter

Q: What methods can I use to measure traffic noise?

A: Traffic noise measurements commonly fall into one of two categories: Wayside and Source. The Wayside methods measure noise at the "side of the road", and are the most common traffic noise measurement technique in use today. While this technique provides a good indicator of the exposure that an abutter may have, its application is limited due to strict test requirements, not to mention that it only represents the noise level for a very short segment of pavement.

Meanwhile, source methods measure noise close to the tire-pavement contact patch. The two most common Source measurements include On-Board Sound intensity (OBSI) and Close Proximity (CPX). Both are relatively advanced techniques that are becoming increasingly popular since they provide for a continuous measure of tire-pavement noise as one travels down a road. "Noisy" and "quiet" areas of pavement can be readily identified as a result. Currently, FHWA and AASHTO are reviewing a draft standard for OBSI, and similar guidelines for CPX testing is in the works, which will likely be modeled after an existing ISO draft specification.