|Are Your Roads
Safe at Night?
|Your safety when driving at night depends on the retroreflective
materials that dot roadways and warning signs. Justin Rennilson
talks about just how these important materials reflect light and
ASTMs role in standardizing them.
More accidents occur at night per capita than during daylight hours. Why? The visibility of objects depends largely on the illumination provided by vehicle headlights. If objects are dark (such as the very road surface you are driving on) and absorb the headlight illumination then no light bounces back to warn you. But optical elements added to these dark objects can return a portion of that light and render such objects visible. These optical elements redirect the headlight back toward the driver and thus are called retroreflectors. Today these retroreflectors are common in the road environment. Traffic signs, road markings, road markers, and delineators are all retroreflective and thus are visible when our headlight beams shine on them. The vests and clothing of maintenance workers have bands of retroreflective materials enhancing their conspicuity. Such retroreflective elements have also been introduced on all types of clothing for pedestrian safety.
The first such use of optical elements for retroreflection started shortly after the 1920s when road building was undertaken on a large scale. Transportation engineers realized that if some of the headlight from the vehicle could be directed back toward the driver, messages could be placed on signs that would be visible at night to direct the driver and warn against any traffic hazards. The first optical element was made of glass and used a silvered surface to reflect the light back to the driver. This is the same principle employed by the eyes of animals to increase the amount of light available at night and thus increase their sensitivity. When light is incident on this reflecting layer in the animals eyes, a portion of that light is returned to the vicinity of the light source. The eyes of these animals, especially cats, thus glow in the dark when light is directed toward them. Thus the first optical elements were (and still are) called cats eyes. Small glass buttons could be arranged on a flat surface to spell out words giving directions and warnings to the driver such as STOP. Other types of optical elements use a principle of three mutually perpendicular reflecting surfaces to redirect the headlight. Such optical elements are also called corner cubes or prismatic retroreflectors. Light that illuminates such elements reflect on each surface (three times) and travel back to the light source relatively independent of the orientation of the cube corner. In the early 1920s and the '30s these elements were made of red glass and later of red plastic and were used to mark the rear of vehicles. They were also combined with light bulbs and became the taillight assembly common today.
Since then, we have progressed a great deal in the production of retroreflectors. A wide variety of types are used in transportationat airports, on maintenance personnel, pedestrians, marine buoys, and railroad cars and crossings. The visibility of roadway work areas is of major importance because road constructions cause disruption of the usual driving patterns and unfamiliarity can result in an increase in accidents. Worker fatalities are on the increase since much road work occurs at night to minimize the impact on traffic congestion. By increasing the amount of retroreflective material these workers wear, their visibility is greatly enhanced and their safety improved.
In the early 1970s, ASTM members recognized that standards would be needed to measure the performance of these retroreflectors. The Society of Automotive Engineers had already directed a similar effort in the early 1940s. SAE directed their efforts toward reflectors called reflex-reflectors used on all types of vehicles. The use of reflex-reflectors was broadened in the 1940s for use as side delineation to aid in road navigation.
As an example of the accelerated progress of ASTMs committees in this field, note that at the time an article on retroreflection appeared in Standardization News in February 1987, seven standards in ASTM dealt with that subject. Today 19 standards exist that are concerned with retroreflection and ASTM has introduced a Technical and Professional Training Course on that subject.
There are three basic types of the reflection of light incident on materials. The first and most common is the diffuse reflections that occur when light falls on a material that has a rough surface. The reflected light is scattered in all directions and that surface can be observed in any direction by the reflected light. We see this light reflection everywhere we look. The next most-common reflection is the light falling on a mirror surface. If light strikes a mirror surface, its reflection makes an angle perpendicular to the mirror, equal and opposite to the angle the incident light makes. If we look toward the mirror at that angle of reflectance we will see the light source. It will be very intense. If only one light source illuminates the mirror and we move away from the angle of reflectance the mirror is dark. Most surfaces reflect light from the diffuse reflection to the mirror (or specular) reflection and all appearance modes in between.
The third type of reflection uses optical elements to reflect the light in the direction from which it came. We call that type of reflection retro from the Latin meaning backward. The light that is retroreflected returns to the light source in a cone, which may be symmetrical to the illuminating axis or distributed in preferential directions with respect to the viewing axis. The angle between the direction of illumination and the viewing axis is called the observation angle. For practically all materials this angle is the most important as it defines the spread of the retroreflected light. The amount of light retroreflected also depends on the angle of incidence that the light makes with respect to the normal to the surface. We call this angle the entrance angle. The ASTM Standard for describing retroreflection (1) illustrates the total angles that may be used for describing the optical properties of any retroreflecting material.
The entire geometry of retroreflection can be described with no more than four angles. Several different sets of four angles may be used to characterize a particular type of light return. Some are important in the design and testing of new materials, other sets are useful in laboratory measurement and field conditions. All these sets are mathematically related such that conversions from one set to another is easily performed.
The ASTM Standard Guide to Properties of High Visibility Materials Used to Improve Individual Safety(2) shows the most common types of retroreflecting materials. The addition of glass spheres or beads to paint, plastics, and other materials adds to the reflective properties of road markings and these are thus considered retroreflecting even though the returned light is diffusely reflected from the pigment that is in contact with the glass bead.
The use of all these retroreflectors required standards for definitions, instrumentation, methods of measurement, and specifications. To achieve this, two ASTM committees with seemingly very different interests worked in concert to develop the necessary documents. In 1981, ASTM Committee E12 on Color and Appearance wrote the standard practices for describing retroreflection(1) and for measuring photometric characteristics of retroreflectors(3) and the colorimetric characteristics of retroreflectors under nighttime conditions.(4) The test methods for sheeting materials(5) and horizontal coatings(6) (pavement markings) followed with the exact instructions on how to measure the amount of light reflected from these materials and devices. Once these practices and test methods were in place, Committee D04 on Road and Paving Materials, with its Subcommittee D04.38 on Highway Traffic Control Materials, wrote specifications for all the different types of retroreflective devices.
Retroreflective materials used for transportation must have the same meaning during the night as well as day. Since the retroreflective properties are of negligible importance in daylight, other coding techniques must be utilized. One technique is shape and the other is color. For legible messages on traffic signs the contrast between the letters or symbols and their background is of prime importance. All retroreflective materials age with time and their effectiveness must be carefully monitored. If the red color of a stop sign fades toward pink then the retroreflectivity increases due to the underlining white material. The white material on the word STOP also degrades with time and thus the contrast between the legend and the background can approach zero and the word STOP is unreadable. This is unfortunately a common occurrence when traffic signs are neglected.
The colors of traffic signs and markings impart a message to the driver. Traffic signs and devices are color coded to have specific meanings. The color message must generate the same impression by night that it possessed during the day. The color appearance of an object consists of the combination of the light source and the inherent spectral (color) selectivity of the object. Change the light source and the color of the object changes. Sunlight is quite different from the light of a vehicles headlight. Fortunately, the human visual system adapts to the color of the illuminant and thus the appearance of the object elicits the same color impression from day to night.
ASTM E 8114 is the standard practice for determining the color of retroreflective materials at night. The color can be measured using the International System of Colorimetry (CIE) and used to establish boxes or gamuts in that system within which an acceptable color must lie throughout the materials service life. The standard practice E 308(7) provides the details as to how this computation is carried out. Subcommittee D04.38 and its various task groups are involved in defining the day- and night-time color boxes for all types of retroreflective materials.
The conspicuity of maintenance workers and pedestrians is very important at night as well as during the day. Two specifications(8, 9) set forth requirements for adequate retroreflection of garments. An additional specification(10) addresses the conspicuity during the day and especially at dusk and dawn by the use of fluorescent material. There is an increasing use of fluorescence in traffic signs and work zones as well.
All retroreflectors deteriorate with time. The rates of deterioration depend on the type of material, use, and exposure to the environment. Thus, the lifetime of a retroreflective device depends on its maintenance and monitoring. ASTM has four different test methods for using portable instruments for in-situ measurements of retroreflective materials and devices.
ASTM standards have helped improve the quality of the life-saving devices called retroreflectors. We believe that with the widespread use of all of these ASTM standards, together with professional training in their use, that transportation safety will continue to be greatly improved. //
(1) E 808, Standard Practice for Describing Retroreflection
Talk to the Editor: Maryann Gorman
|by Justin J. Rennilson
Justin J. Rennilson (Jay) instructs the ASTM Technical and Professional Training course on Retroreflective Materials used in Transportation. He has chaired Committee E12 and a D04.38 task group and international committees on this subject. Rennilson has written a retroreflection chapter in the Optical Society of Americas Handbook on Applied Photometry and numerous other papers.