|Reflecting on Highway Safety
by Wendy Ealding
Highway signs and pavement markings provide information and guidance to motorists and are thus an important contribution to safety on the road. It is critical that this information be available both day and night, regardless of the availability of overhead lighting, under a variety of weather conditions, and against backgrounds of varying complexity. The driving environment is becoming increasingly complex at the same time that new headlight designs and the aging of the driver population are challenging the adequacy of this component of the transportation infrastructure. It is the mission of ASTM Subcommittee D04.38 on Highway Traffic Control Materials, part of Committee D04 on Road and Paving Materials, to provide relevant standards for materials such as sign sheeting, pavement marking materials and pavement markers.
Pavement Marking Color
Pavement markings guide and warn drivers as well as regulate traffic. In the United States, yellow center lines indicate two-way traffic, flowing in opposite directions. Yellow lines also mark the left edges of divided highways and one-way roads. White lines separate lanes of traffic going in the same direction. The white and yellow colors need to be clearly distinguishable from one another, under both daytime and nighttime conditions. An example of a standard that addresses the color requirements for pavement markings is D 6628, Specification for Color of Pavement Marking Materials. This specification covers the daytime and nighttime color of pavement marking materials used for traffic control lane markings on road surfaces. It is intended that the materials meet the specification throughout their service lives. D 6628 is a performance specification because the requirements are independent of the type and composition of the pavement marking material. This is an important feature because there are a wide variety of marking materials.
There is a need for consistency so that a motorist traveling on roads maintained by a state department of transportation (DOT), a city or county department of public works, or a federal agency such as the National Park Service, sees the same color. With this in mind, in 2002, the Federal Highway Administration (FHWA) issued regulations for the daytime and nighttime color of retroreflective sign and marking materials in the Manual on Uniform Traffic Control Devices. (1) The color specifications for pavement marking materials are essentially similar to those in D 6628, the main difference being that the MUTCD requirements are for newly applied markings while the D 6628 requirements are for material in-service.
There are four components to the color appearance of pavement markings: the nature of the material, the illumination geometry, the viewing geometry, and the nature of the illumination (e.g., daylight vs. headlight). During the day, the markings are diffusely illuminated from the sky and the viewing geometry is the position of the drivers eye relative to the markings. In the nighttime driving scenario, the illumination geometry is the position of the headlights relative to the markings, and the viewing geometry is the position of the drivers eye relative to the headlights and the markings. Hence daytime and nighttime colors are specified differently in D 6628.
The importance of consistency between daytime and nighttime color was demonstrated a number of years ago, when a state DOT changed its specification to require all pavement marking materials to be lead-free as part of a commitment to the environment and worker safety and health. Soon after the change was adopted, the department received a number of complaints from its own safety inspectors that certain types of yellow pavement markings appeared white at night, with the potential for driver confusion and liability to the department.
At the time, like many DOTs, the department had a quality control specification for the daytime color of material without its retroreflective elements, but no specification for either the daytime or nighttime color as it would appear to the motorist. Nighttime color is totally dependent on the presence of the retroreflective elements (typically small glass spheres). A number of state DOTs that had mandated lead-free markings also experienced fading of the daytime color of the yellow markings shortly after installation, due to UV exposure from sunlight.
The existence of the D 6628 standard enables agencies to ensure that pavement marking materials can achieve satisfactory performance throughout their service life. Being a performance specification independent of material composition also provides a level playing field for industry to supply a wide variety of materials and not be limited to a particular type of technology.
High Visibility Signs
Highway construction work zones are hazardous to both motorists and workers. In 2002 there were almost 1,200 fatalities associated with motor vehicle crashes in work zones in the United States. (2) Work zones can be a source of distraction to motorists due to such factors as encroachment of the work zone into the travel lane, detours that may be unfamiliar, and sudden reductions in speed. Advance warning by means of highly visible signs can give drivers a chance to prepare for the challenges of driving in a work zone. A number of state DOTs have adopted the use of high visibility fluorescent orange signage in their work zones in an attempt to reduce accidents. (2, 3)
Fluorescent signage materials have been available for a number of years, and a draft ASTM Work Item, WK143, Specification for Color of High Visibility Fluorescent Retroreflective Sheeting is under development. The scope of this proposed specification covers the instrumental color requirements of high visibility fluorescent retroreflective sheeting for use in traffic control signs, delineators, barricades, and other work zone related devices, throughout their service life. The unique high visibility appearance of these materials is particularly apparent under low-light conditions experienced at dawn and dusk, and in foggy conditions.
In addition to the now widespread use of fluorescent orange signs in work zones, FHWA has adopted the color fluorescent yellow-green for use with pedestrian, school and bicycle crossing warning signs. (1) The development of this standard will assist agencies in specifying materials that have desirable high visibility properties, and will provide needed standardization for the industry. Progress in developing the standard has been hindered by a lack of consensus on the proper way to measure and specify the high visibility properties. Subcommittee E12.05 on Fluorescence, part of Committee E12 on Appearance, has recently published E 2301,Test Method for Daytime Colorimetric Properties of Fluorescent Retroreflective Sheeting and Marking Materials for High Visibility Traffic Control and Personal Safety Applications.
Wet Reflective Pavement Markings
One of the leading complaints from drivers is the inability to see pavement markings under wet night conditions. This is a major source of dissatisfaction in state DOT customer satisfaction surveys. Driving under wet night conditions is stressful and fatiguing for all drivers, but particularly so for elderly ones, who are becoming an increasing percentage of the driving population.
Traditional pavement markings such as traffic paint are made retroreflective by the addition of small round glass beads that direct the light from the headlights back to the drivers eye (see Figure 1). Under wet conditions, the beads become covered with water and are unable to perform their retroreflective function (see Figure 2).
In recent years a number of technologies have been developed in an attempt to overcome this problem. Larger beads may overcome some of the effect of flooding of the markings, and may be effective under lighter rain conditions (see Figures 3 and 4). Entire portions of profile markings are elevated above their base, in the form of waffles, or ridges of extruded material (see Figures 5 and 6). Enclosed lens optics, like those used in some forms of sign sheeting, are not substantially affected by the presence of water, and may be most effective in heavy rain (see Figure 7).
A draft Work Item WK 144, Specification for Retroreflective Performance of In-Use Pavement Marking Materials When Wet, is under development. The scope of this proposed standard is the retroreflective performance requirements of pavement marking materials used for traffic control lane markings under prescribed conditions of wetness. Progress in drafting this standard has been dependent on the development of suitable test methods.
In 2001, Subcommittee E12.10 on Retroreflection developed two test methods. E 2176, Test Method for Measuring the Coefficient of Retroreflected Luminance (RL) of Pavement Markings in a Standard Condition of Continuous Wetting, is popularly known as the spray method. This method calls for the retroreflectivity to be measured while the marking is being sprayed with water under defined conditions. There is evidence that the performance level obtained as measured by E 2176 correlates with the visual performance experienced by drivers under both wet conditions and during rain events.
E 2177, Test Method for Measuring the Coefficient of Retroreflected Luminance (RL) of Pavement Markings in a Standard Condition of Wetness, also known as the bucket method, is based on a test method developed by the European Committee for Standardization (CEN). In this test, a defined volume of water is poured over the markings, allowed to drain for a specified period, and retroreflectivity is measured. The performance level obtained as measured by E 2177 predicts visual performance under wet conditions, but it may not predict visual performance during rain events, depending on the duration or intensity of the rain.
These tests will be cited in the proposed wet reflective specification in that markings will be classified according to their performance in the tests. Users will also specify an expected performance life for the markings. Markings will be specified as Type I if tested in accordance with E 2176, or Type II if tested in accordance with E 2177. Actual measured values of coefficient of retroreflected luminance are higher for a given material when measured by E 2177 compared with E 2176. The proposed specification will allow users to choose among a variety of performance levels according to their particular needs, such as local rain conditions.
For example, a user in a location such as Florida that experiences frequent heavy downpours, and with a significant elderly driver population, may want to use one of the higher performance levels of a Type I specification. In a desert climate, one of the lower performance levels of a Type II specification may be adequate. The development of this standard will also allow the opportunity for innovation on the part of the industry because they will not be tied in to a particular method of construction.
Subcommittee D04.38 has jurisdiction over a number of standards that impact the safety of the traveling public, and add to the quality of the driving experience. The subcommittee is blessed in the diversity and expertise of its many participants from industry, the public sector, and the consultant community. Through the activities of Subcommittee D04.38, ASTM is making an important contribution to the safety of the highway infrastructure.
(1) Federal Register 67 FR 70161 and 67 FR 49569. Federal Highway Administration 23 CFR Part 655 Traffic Control Devices on Federal-Aid and Other Streets and Highways; Color Specifications for Retroreflective Sign and Pavement Marking Materials
(2) National Work Zone Safety Information Clearinghouse, Fatalities in Motor Vehicle Traffic Crashes by State and Construction/
Maintenance Zone (2002) http://wzsafety.tamu.edu/files/2002_ fatality. stm
(3) TxDOT News Release, State Focusing on Work Zone Safety, April 1, 2003, www.dot.state.tx.us/txdotnews/034-2003.htm
Copyright 2003, ASTM