TRB’s National Cooperative Highway Research Program (NCHRP) Web-Only Document 156: Optical Sizing and Roundness Determination of Glass Beads Used in Traffic Markings is the final report on a project that examined the precision and bias of both optical and traditional mechanical methods for different glass bead types. The project also compared the precision and bias of various measurement methods, and explored development of a practice for the use of computerized optical methods and equipment to measure the size and roundness of glass beads used in traffic markings.

A summary of the project was published as NCHRP Research Results Digest 346.