|Retroreflection Subcommittee Supports Highway Safety with New Tests for Pavement Markings
ASTM Subcommittee E12.10
on Retroreflection in ASTM Committee E12 on Color and Appearance has completed a new test method, and is developing several others, that offer transportation safety engineers a solid way to measure the reflectivity and color of road stripes and other pavement markings materials.
E 2302, Standard Test Method for Measurement of the Luminance Coefficient under Diffuse Illumination of Pavement Marking Materials with CEN-Prescribed Geometry Using a Portable Reflectometer
Now available, the subcommittee has developed E 2302 to assist technicians who test the daytime lightness of pavement markings in the field, or the material performance of samples in the lab before installation. E 2302 employs a portable reflectometer to measure the luminance coefficient Qd under diffuse illumination of horizontal pavement markings such as stripes, symbols, and pavement surfaces.
The luminance coefficient Qd gives the lightness of horizontal pavement markings and pavement surfaces in a particular viewing direction in daylight or road lighting. Diffuse illumination approximates daylight illumination from the overcast sky or road lighting as an average of locations on the pavement surface. The viewing direction approximates an automobile drivers slightly downward view at 30mm distance from the marking, says subcommittee chairman Dennis Couzin, a senior optics engineer with Avery Dennison Corporation, Niles, Ill. The co-viewing angle of the reflectometer will affect the reading, and the standard provides a 2.29º co-viewing angle, as specified by CEN (European Committee for Standardization).
To further assist transportation safety engineers and researchers, the subcommittee is developing test methods in relation to two new classes of instruments. The major instrument makers in the U.S. and Europe participate in ASTM Subcommittee E12.10, notes Couzin.
Work Item WK2310, Proposed Test Method for Measurement of Daytime Chromaticity of Pavement Marking Materials using a Portable Reflectocolorimeter
Pavement markings (e.g., road stripes) are generally made from retroreflective materials, making them appear about 50 times brighter to drivers at night than simple paint, says Couzin. Frequently, they are comprised of small glass spheres partly embedded in white or colored binder. They are optically tricky materials.
Another ASTM Test Method E 1710 [Standard Test Method for Measurement of Retroreflective Pavement Marking Materials with CEN-Prescribed Geometry Using a Portable Retroreflectometer] covers the measurement of the markings nighttime brightness (for drivers) using a portable instrument, he says. For that measurement to have relevance, E 1710 specifies how the instrument approximately simulates the road geometry. The instrument illuminates the marking at a nearly grazing angle, as a vehicles headlights would, and the instrument detects reflection at a higher, but still nearly grazing, angle, as a vehicles driver would.
Besides nighttime brightness, he continues, three other appearance qualities of pavement markings are also important and have been specified: nighttime color, daytime lightness, and daytime color. In the past 10 years there has been increasing awareness that these qualities also require approximately realistic geometry for relevant measurement.
For example, the daytime color and lightness used to be measured in the same way that the color and lightness of a painted surface or a piece of paper is commonly measured, Couzin adds. Readily available, handy instruments illuminate the surface at 45° off normal and detect the reflection at normal (or vice versa). Such 45/0 (or 0/45) measurement can be misleading for a pavement marking which will be illuminated by day by the whole vault of the sky while viewed from a near-grazing angle. For example, some markings have wells of irrelevant color or lightness (since drivers dont see them) that figure prominently in the 45/0 measurement.
The new test methods [under development] require approximately realistic measurement geometry: diffuse illumination, near grazing detection, he says. Even more important than the absolute daytime lightness of pavement markings is the lightness relative to that of the road surface. We expect the new test method [for Measurement of Daytime Chromaticity of Pavement Marking Materials using a Portable Reflectocolorimeter] to also be used with road surfaces, which have their own complex textures and sheens and require approximately realistic geometry for lightness measurement.
Work Item WK358, Proposed Test Method for Measurement of Nighttime Chromaticity of Pavement Marking Materials Using a Portable Retrocolorimeter
The main stimulus for the development of a new test method was the banning of lead chromate yellow pigments from pavement marking materials, says Couzin. Substitute pigments produced the same yellow appearance in the binder, but the retroreflections from the glass spheres werent the same. Some new yellow stripes looked white at night! This dangerous situation ended a long complacency about nighttime color.
Nighttime color of markings hadnt previously been specified, he continues. It was presumed that if the daytime color (itself mismeasured) was within spec, then the nighttime color was acceptable too. There is an ASTM laboratory method D 4061 [Standard Test Method for Retroreflectance of Horizontal Coatings] for measuring the nighttime color of pavement markings, but since many markings are formed in situ, and since they all undergo extreme wear in situ, there was need for a field method. The simplest method was to interchange colored filters over the detection system of the ASTM E 1710 instruments. [The proposed] new test method keeps the geometry of the E 1710 instruments while allowing various color detection methods.
For further technical information, contact Dennis Couzin (phone: 847/588-7261). Committee E12 meets Jan. 12-14, 2004. For meeting or membership details, contact Tim Brooke, director, Technical Committee Operations, ASTM International (phone: 610/832-9729). //
Copyright 2003, ASTM