Semisequicentennial Transportation Conference Proceedings
May 1996, Iowa State University, Ames, Iowa
Evaluation of All-Weather Pavement Markings: Report on Two Years of Progress
Joseph K. Fish
Graham-Migletz Enterprises, Inc.,
P.O. Box 348,
Independence, Missouri 64050.
The Federal Highway Administration (FHWA) is sponsoring an effort to determine the visibility, safety, and durability performance of all-weather pavement markings. FHWA has contracted with Graham-Migletz Enterprises, Inc. (GME) to use the Laserlux retrometer to collect data at a variety of all-weather pavement marking sites in various sites. Presently, 19 states participate in the FHWA effort. In the study, GME will evaluate the service life, safety, and cost-effectiveness of the all-weather pavement markings. GME's subcontractor, Midwest Research Institute (MRI), will perform the statistical analysis of the retroreflectivity, accident, cost, and weather data. Each site is visited by the research team and an initial inspection will be conducted to verify information submitted by states and to record unique characteristics such as interfering light from non-roadway sources, signing, traffic patterns, etc. of each site during the day and night. The research team will also meet with the participating states to discuss the plan for states to collect and provide information on retroreflectivity measurements using the Mirolux portable retroreflectometer, submission of pavement marking, cost, and other site-specific data, conduct of 24-hr traffic volume counts four times per year, submission of before and after traffic accident data, and collection of data about snow removal activities, such as the number of snowplow passes, equipment, and materials used. After new pavement markings have been installed at all test sites in each year, a working paper describing site characteristics will be submitted to the FHWA.
Research studies and investigations have established that nighttime guidance problems exist on America's roadways. Additionally, data suggests that these problems are aggravated by rainy weather. In particular, run-off-the-road type accidents seem to occur frequently at night on wet pavements. It would seem that the roadway delineation system intended to protect and guide the driver often fails under these inclement wet/night conditions. An average motorist could describe this oft-studied phenomenon more succinctly: roadway lines often disappear at night.PROJECT BACKGROUND
In order to understand this problem, one must understand how it is that markings are made visible to drivers. Retroreflection is that phenomenon that occurs when "light rays strike a surface and are redirected directly back to the source of light." In pavement markings, small beads embedded in the marking interact with incoming light and the pigment in the pavement marking binder to reflect light back approximately in the direction from which the light struck the marking.
There now exists a need to evaluate the wide variety of approaches that have arisen for the creation of "all-weather pavement markings." All-weather pavement markings (AWPMs) are defined as markings that are visible at night under dry conditions and also under rainy conditions up to 0.25 inches (0.635 centimeters) per hour of rainfall. Many types of materials qualify as all-weather pavement markings, including paint, thermoplastic, epoxy, raised/recessed pavement markers, and profiled markings. Standard and large beads and a mixture of both types are included.Legislative Requirements
In response to the need to evaluate all-weather pavement markings, Congress ratified Section 6005(a) of the Intermodal Surface Transportation Efficiency Act (ISTEA), which mandates the evaluation of AWPMs and provides funds to accomplish the evaluation.Retroreflection Measurement
In the past, performance of an all-weather pavement marking evaluation has still presented many logistics problems. Difficulties with conventional retroreflection-measuring instruments have persisted. Creation of a large data base of retroreflection measurements for all-weather pavement markings using conventional instruments is time-consuming and labor-intensive. Recently, however, a device was developed that resolves the majority of these difficulties with conventional technologies.
The Laserlux mobile pavement marking retroreflectometer makes use of a specific wavelength of laser light and a narrow band-pass filter to block reception by the photoreceptor of all other wavelengths of light. Thus, it makes possible day/night, wet/dry, retroreflectivity measurements. It was also designed to replicate the observation and illumination angles of a "typical" driver from the driver's eye to the marking on the road.Project Overview
The Federal Highway Administration (FHWA) is sponsoring the effort to determine the visibility, safety, and durability performance of all-weather pavement markings. FHWA has contracted with Graham-Migletz Enterprises, Inc. (GME) to use the Laserlux retrometer to collect data at a variety of all-weather pavement marking sites in various sites. Presently, 19 states participate in the FHWA effort. In the study, GME will evaluate the service life, safety, and cost-effectiveness of the all-weather pavement markings. GME's subcontractor, Midwest Research Institute (MRI), will perform the statistical analysis of the retroreflectivity, accident, cost, and weather data.
Each site is visited by the research team and an initial inspection will be conducted to verify information submitted by states and to record unique characteristics such as interfering light from non-roadway sources, signing, traffic patterns, etc. of each site during the day and night. The research team will also meet with the participating states to discuss the plan for states to collect and provide information on retroreflectivity measurements using the Mirolux portable retroreflectometer, submission of pavement marking, cost, and other site-specific data, conduct of 24-hr traffic volume counts four times per year, submission of before and after traffic accident data, and collection of data about snow removal activities, such as the number of snowplow passes, equipment, and materials used. After new pavement markings have been installed at all test sites in each year, a working paper describing site characteristics will be submitted to the FHWA.Service Life Evaluation
Service life is defined as time required for a pavement marking to become ineffective due to its having lost its luster, lost its retroreflectivity, or having been worn completely from the pavement. The service life evaluation will determine the change in pavement marking retroreflectivity as a function of time and traffic passages, time to reach minimum acceptable retroreflectivity, time to failure of durability and color/contrast, and service life of specific marking materials.
GME visits the test sites at six-month intervals to collect the retroreflectivity data using two Laserlux retroreflectometer vans furnished by FHWA and to collect pavement marking durability data. Retroreflectivity data at test sites that have raised pavement markers on edgelines is collected using two Model 1200 C retroreflectometer units furnished by the government. GME also collected retroreflectivity data using Mirolux and Retrolux retrometers for data validation purposes.
The durability (wear) of the test pavement markings is evaluated on overall appearance which includes percentage of marking material remaining (estimated by observation with the unaided eye); the contrast of the marking in relation to the pavement surrounding it (also estimated by observation with the unaided eye); and color tolerance of the marking (in comparison to standard color tolerance chips).
GME also collects retroreflectivity data under wet conditions on the test sites at six-month intervals. The methodology for making these measurements is to apply water to the markings using a paint roller and to take measurements of the reflectivity continuously for a period of five minutes.
Photographic measurements of bead retention and marking condition are taken by GME at each test site at intervals of approximately six months. A 35-mm camera outfitted with a macro-lens is used to take pictures of the pavement marking beads to show bead distribution and embedment. The pictures taken show a view similar to the view seen through use of a 30-power pocket microscope. Photographs taken with a regular lens will document the durability and visibility of markings.Safety Evaluation
The safety evaluation will determine the effectiveness of specific all-weather pavement marking materials in reducing accident rates at the test sites. This evaluation will focus on the types of accidents that are susceptible to reduction by improved longitudinal pavement markings. These include single-vehicle, run-off-road accidents; multiple-vehicle, same-direction, sideswipe accidents; opposite-direction, sideswipe accidents; and head-on accidents at non-intersection locations. The safety evaluation will determine the extent to which specific pavement marking materials are effective in reducing these types of accidents.
At the end of the project, GME and MRI will analyze the before-and-after accident experience of the test sites and also determine the safety performance of the different all-weather pavement marking materials. Independent evaluations of each test pavement marking material will be conducted, as well as a comparative analysis between the raised/recessed pavement markers (RPMs) and the other group of pavement marking materials (paint, thermoplastic, epoxy, etc.). Daytime accident rates at the test sites will serve as the comparison against which nighttime accident rates are measured, i.e., it will assume that daytime accidents are unaffected by the type of pavement markings installed.
GME and MRI will compute and statistically compare before-and-after period, wet weather, nighttime accident rates for the test sites. Dry weather, nighttime accident rates at the test sites will serve as the comparison against which wet weather, nighttime accident rates are measured. Detailed hourly weather data for test sites will be obtained from the National Climatic Data Center (NCDC) in Asheville, North Carolina, to provide exposure (time periods when pavements are wet) data for determining dry- and wet-pavement accident rates.
A benefit/cost analysis of the all-weather pavement markings will be conducted. GME and MRI will calculate the benefits (reduction in accident costs) and the total installation, maintenance, and replacement costs of the test pavement markings. The accident costs used will be FHWA's most recent estimates of the societal costs of motor vehicle accidents. The test pavement markings will be ranked from highest to lowest based on a benefit/cost ratio or net present value.PROGRESS TO DATE
There are 19 states in the FHWA AWPM program. Fourteen of these states now have markings installed. The majority of the remainder of these states are scheduled for initial installations and initial research team visits in spring 1996. The 19 states currently have plans for a total of 83 AWPM sites. As of January 1996, 52 marking sites were installed. GME currently has measured all 52 sites, and all of these sites were measured at least once in 1995; a subset of the 52 sites were measured in 1994. A total of 550 Laserlux runs have been made since the initial measurements in August 1994.Laserlux Acceptance Testing
Recent progress has been made in the Laserlux hardware itself. As part of the ongoing pavement marking efforts, FHWA purchased 5 additional Laserlux vans to make available to the states for demonstration. GME evaluated these vans and made recommendations to FHWA on the vans' reliability and acceptability.
The Laser unit is manufactured by Advanced Retro-Technology of Spring Valley, CA, a recently-purchased subsidiary of Gamma Scientific. The Laserlux system is built by Roadware Corporation of Paris, Ontario, the same company that manufactures the ARAN van.
The new Laserlux van's user-friendliness has been greatly improved. The Laserlux no longer needs periodic recalibration to adjust for changing lighting conditions. The Laserlux crew can now just mount the unit, adjust its height to the proper level, and then aim the laser beam to a point on the pavement 10 meters from the unit; the unit is then ready to measure pavement marking retroreflectivity.
The new Laserlux vans were evaluated at the Roadware Corporation facilities in Kylertown, PA. Six one-mile test sites were selected. Two white edgelines, two white lane lines, and two yellow centerlines were measured. The vans lined up and made three laserlux runs over each test site.
Before testing, GME recommended to FHWA that the vans be capable of measuring pavement marking retroreflectivity with a precision of plus-or-minus 10 percent. That is, if the true value of the marking's retroreflectivity is 100 mcd/m2/lux, then the Laserlux should be capable of measuring retroreflectivity within the range of 90 to 110 mcd/m2/lux. The laserlux vans failed the acceptance test criteria at the 10 percent level of precision. If a 10 percent precision is the acceptance criteria, the vans should be rejected. However, the vans passed at the 15 percent level of precision. If a 15 percent precision is selected as the acceptable level of precision, the vans should be accepted.
Originally, it was believed that the Laserlux vans could measure pavement marking retroreflectivity within a 10 percent level of precision. However, after testing the vans, it is now believed that a 15 percent level of precision is the best that can be accomplished at this time.
As with any piece of measuring equipment comprised of many pieces of precision components, each final assembled unit performs individually with its own characteristics. The Laserlux unit (box of components) can be adjusted in the laboratory to perform in a manner comparable to other Laserlux units. However, when the units are installed into the Laserlux vans and are used to measure pavement marking retroreflectivity levels under actual highway conditions, the real-world differences among the vans are discovered. The differences among the Laserlux units, vans, roadway environment, weather, and drivers all contribute to the variability, or lack of precision, among the vans.
GME recommended to FHWA to request readjustment and testing of the Laserlux units. It was recommended that FHWA determine the level of precision desired in the Laserlux (either 10 percent or 15 percent), and to decide whether to accept or reject the vans based on that criteria.Minimum Visibility Study and Literature Review
A literature review is included as part of the AWPM study. The results of this review were submitted to FHWA in 1995. This literature review focused on identifying minimum acceptable levels of retroreflectivity. This effort was conducted in support of an effort to evaluate several candidate minimum levels of retroreflectivity.
The literature review revealed that a wide variety of factors affect what is subjectively considered by the average motorist to be a "minimum" level of retroreflectivity. These factors include, but are not limited to, marking color, highway type and condition, and driver age and visual acuity. In general, most research has come to a conclusion that a minimum acceptable level of retroreflectivity is around 100 to 120 mcd/m2/lux.
In addition to the literature review conducted by GME, MRI is analyzing retroreflectivity data on existing markings collected by Tonya, Inc. and the Minnesota DOT. These data were collected as a means of surveying existing markings and their condition as they exist in the field. The focus of this mini-study is to determine, based on several candidate minimum retroreflectivity levels 1) the number of miles of unacceptable markings, and 2) the cost to upgrade these unacceptable markings. This will be accomplished by extrapolating the collected data to determine nationwide costs and using what is now known about pavement marking and refurbishment costs.
In any case, these determinations of economic impact of various candidate levels of minimum retroreflectivity will be submitted to FHWA for review and comment. FHWA will make a final selection for a level of minimum retroreflectivity based on the results of this review, and other factors, as part of its rulemaking activities. The new standard is planned for inclusion in the next version of the FHWA Manual on Uniform Traffic Control Devices (MUTCD).Retroreflectivity Results
The final effort at the end of our second year was to start assembling retroreflective data to determine pavement marking service lives. Figure 1 shows the results of Laserlux runs on thermoplastic pavement markings at six-month intervals at six sites in Arkansas, Kansas, and New Hampshire. The graph shows large variations in retroreflectivity readings over time. This is due to a number of factors in addition to marking degradation.
As the Laserlux acceptance tests have shown, there is considerable variation in the Laserlux's capability to read markings consistently. For a 300-mcd marking, and +/-15 percent variation results in readings anywhere from 255 to 345. In addition, some sites have been dirty or largely covered with sand at the scheduled Laserlux measurement times. As shown in Figure 2, however, when the data are averaged, these elements of "noise" cancel each other out somewhat, and a clearer picture emerges.
Figure 2 presents data from Laserlux measurements on four different types of materials: pavement marking tape, thermoplastic, epoxy, and polyester. All of the runs taken at the sites shown were averaged together and then plotted.
Figure 2 appears to show that these materials belong to the same family of curves, but with different initial values of retroreflectivity. The intended use of these data is to be able to determine a marking's service life. That is, the length of time it takes a marking to reach the minimum level of acceptable retroreflectivity. Knowing a markings's service life will enable traffic engineers to moreefficiently plan and manage their pavement markings programs.
The time from marking installation to when the marking reached the minimum acceptable level of retroreflectivity is the service life of the marking. Service life, installation costs, and accident reduction benefits will be used to determine a marking's cost effectiveness. The project final report at the end of the six-year effort will make recommendations concerning the safety and costeffectiveness of all of the all-weather pavement markings investigated through the course of the project.