By now the tools exist to effectively quantify the influence of weather, wear and chemicals on road surface quality and to model how often those surfaces need maintenance or replacement. Less familiar to highway engineers are the tools used to measure and model the lifetime of the retroreflective elements painted or embedded along roads to help guide drivers at night.
The science of electronically measuring retroreflective pavement markings is still in its early days, but quickly evolving. Visual analysis of retroreflective elements by human observers is giving way to portable and vehicle-mounted measurements that provide reproducible data more compatible with data management tools, such as computer modeling. Despite this, the nature and measurement of retroreflectivity remains an unfamiliar topic to many highway contractors and maintenance authorities. But it will not remain so for long.
Accident data from the Highway Safety Information System (HSIS), a multistate database, shows that although only 25% of travel occurs during the nighttime, about 55% of fatal accidents occur at night. At least part of this discrepancy stems from inadequate and poorly maintained signs and markings.
Such statistics help explain why the Manual on Uniform Traffic Control Devices (MUTCD) specifies that highway markings required to be visible at night should be retroreflective. That is, they should reflect light back toward the general direction of its source. Retroreflectivity is simply a measure of how efficiently a marking achieves this.
Retroreflectance measurement helps ensure new highways are marked according to established standards, which improves safety. It also helps engineers estimate when to replace retroreflective roadside elements. Replacing these elements too soon increases maintenance costs; replacing them too late compromises safety, driving comfort and tort exposure.
Also, many states and municipalities that contract pavement marking have begun to offer bonus payments based on the quality of retroreflective pavement markings immediately after installation and for several years afterwards. Roadway striping contractors, therefore, stand to benefit if they can provide hard data that shows their markings continue to meet specified retroreflectance values.
Typical retroreflectors, such as cube corners or transparent microspheres, have structures that produce multiple reflections. Whatever its mechanism, a perfect retroreflector would reflect all light directly back to its source. Fortunately, such efficiency is not only impossible for highway applications, it’s impractical. In order for retroreflection to be useful, the driver would need to see it—putting the ideal location for the head lamp on a driver’s forehead between the eyes, which is not only impractical, it would likely meet some market acceptance resistance.
In reality, headlamps are limited to a non-optimum location for drivers to view retroreflected light. So, standard illumination and viewing geometries have been defined to objectively evaluate performance of different materials and installation methods. A key factor in this evaluation is an understanding of the concept of luminous intensity of a light source.
The main goal of replacing and repainting highway signs and delineations is to maintain their visibility at night, but those who fund or perform highway maintenance can appreciate the value of predicting the service life of retroreflective elements.
It is intuitive to measure retroreflection as a ratio of the intensity of retroreflected light against the intensity of its origin. This ratio would provide a scale for retroreflection that consisted of a similitude (dimensionless number) between 0 and 1. This, however, requires a system of units to define light flux, intensity and other optical quantities.
Most important is the geometrical unit of a solid angle, which resembles an ice cream cone. The tip of the cone is the apex, the distance from the apex to the open end is the radius (r) and the open end has some defined surface area (S). Solid angles are measured in steradians (?), units that represent the ratio of the area of the open end of the cone over the radius squared (r2). In strict math terms, the area of the opening is a spherical surface, like the half-sphere of ice cream fitting into a cone’s open end.
Widening the apex of this cone until it formed a half-sphere would represent a solid angle measuring 2? steradians. So a complete sphere, such as one surrounding a light source, would have a total of 4? steradians.
In Figure 1, the solid angle subtended by the area “ABCD” is equal to the area of “ABCD” divided by the total area of the concentric sphere multiplied by the total number of steradians in the sphere. The equation looks like:
1m2 x 4? (w)
————— = 1 steradian
Roadway markings and signs are measured using different but related units of measure. Retroreflective measurement of roadway markings most commonly relies on the coefficient of retroreflected luminance (RL), which is the ratio of the luminance (L) of a surface to the normal illuminance (E) on the surface.
Normal illuminance, in this case, is the illuminance of a car’s headlights on the marking measured on a plane perpendicular to the direction of the headlight beams. For the car shown in Figure 2, the observation and illumination angles are fixed and, let’s assume, the headlights direct light of a specific intensity along the illumination axis. Let’s also assume that point B represents the single point on the marking where RL will be examined. This allows a precise definition of the illumination axis, directed along line AB.
By the time the light reaches point B, it has spread out and has established a specific illuminance. If a plane is placed at point B with an area of one square meter and a normal vector in the same direction as line A, then the value of illuminance at B will equal the amount of light that would fall on this plane if it were illuminated entirely by the same intensity of light as that directed at point B.
The light will be reflected back in a cone shape around the direction of the headlight and it will have a specific luminous intensity in the observation direction along line BC.
These two values enable calculation of the coefficient of luminous intensity. Missing is the luminous intensity’s per unit area, which must be calculated to provide an appropriate area to use as a divisor. Up to this point, all the quantities have been directional, dealing with infinitesimal areas. The challenge is to accurately illuminate the sample at the proper angle, 88.76°, and collect the light at the proper angle, 1.05°, from the illumination axis. These angles simulate the illumination of the pavement marker by a car with headlights 0.5 m above the pavement and 30 m in front of the car and the driver’s eye 1.5 m above the pavement. Precisely setting this measurement geometry minimizes the largest error-contributing factor because the projected measurement area changes a great deal with a small change in the 88.76° illumination angle.
Commercial retroreflective materials are generally measured in a laboratory and provide material samples that can serve as transfer standards outside the lab. These measurements include the ratio of illumination at a retroreflector’s sample position, its retroreflected light and the projected area of the sample. All this provides a direct determination of the coefficient of retroreflection that can then be applied in the field as a calibration standard to accurately set the intensity scale of a portable measurement instrument.
However, the sample area of portable instruments differs from one manufacturer to another, which introduces the possibility of non-uniform retroreflection of samples on the road-pavement stripes, for example. It is not unusual for the center of a 4-in.-wide retroreflective stripe to have a 2-in.-wide section down the middle with a coefficient of retroreflection value higher than those to either side.
The unit used in roadway measurements is millicandelas per lux per square meter (mcd/lx/m2). This is equal to 1/1000 of the basic unit, which was given before as candelas per lux per square meter (cd/lx/m2). Retroreflective measurement of road signs relies on a different standard, the coefficient of retroreflection (RA). A description of RA is available in ASTM Standard E808-91, which defines it as the coefficient of luminous intensity (RI) of a flat retroreflecting surface to its area. The metric unit for RI is candelas per lux per square meter (cd/lx/m2) and it is the ratio of the luminous intensity of the retroreflector in the direction of observation (I) to the illuminance at the retroreflector on a plane perpendicular to the direction of the incident light.
After taking into account all of the units and other considerations, RA is conceptually identical to the coefficient of retroreflected luminance (RL) used to measure roadway markings. Like RL, it is still a ratio of returned intensity to incident illumination divided by the area of the retroreflector.
RA, however, is simpler to apply to measure the retroreflectivity of signs because signs have a fixed area. Also, the measured geometry is arranged so that the plane of the sign is more perpendicular to car headlights, so its area does not change as fast as near horizontal angles. This makes the measurement much simpler and more accurate.
The retroreflectometry ride
The American Society for Testing and Materials (ASTM) has established controlled procedures for measuring retroreflective materials in the laboratory. These procedures require the use of a tungsten lamp operated at a correlated color temperature of 2,855.6°K. The spectral power distribution of a tungsten lamp operated at this color temperature approaches the ideal CIE Illuminant A, an internationally agreed upon standard type of illumination used for comparison and specification of colors. Photoreceptor measuring illuminated retroreflective surfaces must match the CIE 1931 human eye response function—also called the photometric response function—to within a tolerance of 3% defined by the f1' (f-one-prime) analysis method given in CIE publication 69.
This level of precision in the measurement instruments yields a precision and bias of about 6% between well-maintained and well-staffed laboratories. It is not practical to apply these very rigorous standards in the field.
The ASTM also has developed standards for portable measurements of retroreflectivity in the field. Like those developed for the lab, these procedures apply tungsten lamps although they also can use flashed xenon sources. They also permit compensating the detector’s response by the amount the light source deviates from 2,855.6°K. Field and laboratory measurements taken over many years have shown that the combination of the spectral power distribution of a light source and the detector spectral sensitivity must match the combination of the CIE Illuminant and photometric response functions. Filters can compensate for temperature differences and provide the required match to the CIE photometric response function.
Commercial retroreflectometers for the field can either be hand-held or vehicle-mounted. Each type has advantages and drawbacks stemming from the initial price, maintenance costs, the manpower required to operate it, the accuracy of its data, its reliability and its compliance with testing standards. Ultimately, selection of the appropriate technology depends on what type of monitoring program an agency plans to implement as well as the staff and resources available to implement it.
The purchase cost of a mobile retroreflectometer instrument including all necessary equipment can be over 10 times the initial cost of a hand-held instrument. The operating costs of mobile instruments, such as maintenance, fuel, depreciation and technical support, also are higher.
Hand-held instruments require only one person to take a reading. But if that person is measuring lane and centerlines, which is often the case, their work will require a crew for traffic control. This imposes its own expenses as well as safety issues. In 2002 alone, 1,181 people were killed in construction or maintenance zones, according to the National Center for Statistics and Analysis’ Fatality Analysis Reporting System.
Mobile instruments use lasers for the illumination source—a retroreflectometry method for which ASTM is still developing standards. However, studies performed or funded by the Federal Highway Administration (FHWA), Ontario Ministry of Transportation and others concluded that there was a very good correlation between laser- and tungsten-based instruments.
Mobile instruments can measure any and all retroreflective roadway elements at highway speeds, eliminating the need for traffic control and reducing the number of personnel to two: one to drive and the other to operate the equipment. Although hand-held retroreflectometers provide an accurate sample-based series of point locations, mobile retroreflectometers, by their nature, collect more data, more quickly. This can render transportation authorities more in-depth yet extended representations of their roadway system’s retroreflective health.
In fact, mobile technology is raising interest among state and federal transportation authorities because of its potential to advance retroreflective research and modeling technology. For example, nine western states in conjunction with the FHWA are using volumes of hard data collected by mobile retroreflectometers to develop performance standards and degradation curves for retroreflective elements on their highway systems. The data will help model where and when retroreflective maintenance is needed with greater precision. The study also has raised speculation about combining mobile measurements with global positioning satellite technology to provide the status of markings and gauge when and where they will need to be replaced.
Besides its voluminous sampling, the rapidity of mobile measurements makes repetitive runs more practical, which provides a better average and standard deviation. This could aid comparative research of different retroreflective materials or designs under different conditions.
Compared to many roadway maintenance tools, retroreflectometry is a relatively new but rapidly evolving technology. Retroreflectance measurement helps maintain visibility standards, improving safety and comfort for drivers. It also can help predict where and when retroreflective roadside elements need replacing.
Transportation agencies generally inspect new pavement markings within a month. After that, there is little agreement over how often inspection is required. The accuracy, repeatability and reliability of both hand-held and mobile retroreflectometers is helping to change this. Their use will continue to grow, fueled by administrative and operational transportation authorities’ desire to efficiently review and model retroreflectivity data independent of differences in marking materials, roadway conditions or altitude.
Retroreflectivity measurements go mobile with new instruments traffic safety