There’s more to the stripes on the road than you might think. An entire subculture of researchers and industry experts spend their time and energy exploring the benefits of pavement markings, human factors, new materials, application techniques, measurement methods, specification protocols and management techniques in order to provide long-lasting, high-visibility and low-cost pavement markings.
The work is never-ending as advanced research techniques are developed, innovative products become available and traffic loads increase. Researchers at the Texas Transportation Institute (TTI) work closely with government sponsors and industry leaders to examine the effectiveness of pavement markings and how they can be improved to make roadways safer for all drivers in all conditions.
Effective when wet
Nighttime crash rates are about three times higher than daytime crash rates. Driving at night and under wet conditions is perhaps one of the most difficult driving conditions. Traditionally, pavement markings have reduced effectiveness under wet night conditions. Industry leaders are now providing specially designed pavement markings with structured surfaces and enhanced retroreflective optics to be effective under both dry and wet conditions. TTI researchers built a 1,600-ft rain tunnel to evaluate pavement marking visibility under nighttime rainy conditions and compared the results to nighttime dry conditions (http://tti.tamu.edu/documents/0-5008-2.pdf). One of the key findings was that the current ASTM test method (ASTM E2176) for assessing wet pavement marking performance needs to be modified or replaced. As a result, TTI researchers are working to develop an ASTM continuous wetting test measurement that would provide more repeatable and equitable results for all pavement markings under wet conditions.
Regular checkups
TTI researchers also have been working to identify maintenance retroreflectivity levels for pavement markings. Currently, most agencies in the U.S. specify an initial level of retroreflectivity as a performance criterion. Some agencies have additional performance criteria at six or 12 months after installation. However, there have only been a few agencies that have specified maintenance retroreflectivity levels such that they schedule their markings for rehab as they approach or reach the maintenance levels.
A recent report with pavement marking maintenance retroreflectivity levels has been published by FHWA and used at the FHWA 2007 Pavement Marking workshops, which were conducted to solicit agency input regarding the introduction of maintenance retroreflectivity levels for pavement markings in the Manual on Uniform Traffic Control Devices (MUTCD).
One of the key factors to implementing maintenance levels of pavement markings is being able to consistently and repeatably measure retroreflectivity. TTI researchers work in various ways to support and develop measurement protocols. For instance, TTI works with industry and state leaders to enhance the measurements from mobile retroreflectometers. TTI also is working with world leaders to develop an ASTM standard practice for measuring pavement marking retroreflectivity with handheld devices.
One of the most elusive questions related to pavement markings has been their cost-effectiveness. TTI is leading durability test decks around the country to develop a set of robust durability data. The pavement markings on these durability test decks are installed in both a transverse and longitudinal orientation. TTI works with industry leaders to install the conventional and experimental pavement markings on the test deck and monitor their performance.
A healthy fattening
Research also is being conducted to address the safety of pavement markings and the safety of pavement marking characteristics such as width and retroreflectivity. The findings from these safety studies will complement the durability research such that cost-effectiveness levels can be established for pavement markings and various characteristics of pavement markings.
As an example of a pavement marking safety study currently under way, TTI is conducting a national retrospective crash study to determine if wider longitudinal pavement markings are safer than normal-width lines (4 in.). The research team surveyed the states to find out if they used wider edge lines. If they did, researchers wanted to know where and when they were applied. For those states that were able to provide this data, the research team obtained before-and-after crash data.
Several states have been able to provide the necessary data the research team needs in order to conduct the appropriate statistical analyses. The types of crashes being analyzed are total, fatal and injury, PDO, nighttime, daytime, wet, dry, wet-night, older-driver (55-plus), nighttime older-driver, run-off-road and opposite-direction (head-on and sideswipe opposite direction) crashes. Among these crash types, run-off-road and opposite-direction crashes that occur at night or under wet conditions are considered as most relevant crashes on which wider lines could make a significant safety impact.
To date, Illinois and Michigan data have been analyzed and the preliminary analyses show a positive safety effect of wider edge lines for most of the crash types under consideration. The analyses are continuing with Kansas data and possibly other states able to provide the necessary information. The analyses are being segregated by facility, for example two-lane rural roads, urban/suburban streets, multilane highways and freeways, because it is suspected that the magnitude of the safety effect of wider lines might be different for different roadway types. Specific safety numbers are being held until all the data are analyzed.
As a complement to direct safety studies such as the one previously described, TTI researchers also are studying the benefits of wider-than-normal edge lines on horizontal curves on rural two-lane highways using an indirect safety approach. Last summer, a team of researchers collected data for two months on vehicles negotiating curves with 4-in. edge lines. This summer the same curves will be re-striped with 6-in. edge lines, and the research team will repeat the data collection process and compare the results. In this study the researchers will be looking at operational surrogates for safety such as braking, lane placement and deceleration, which are all characteristics of vehicles negotiating two-lane horizontal curves. The goal is to determine if wider edge lines change the way drivers negotiate rural two-lane horizontal curves, which are over-represented in national crash statistics.
Keeping the lead out
The recent national and even global environmental emphasis also has been felt in pavement marking practices. Revised federal regulations have led to changes in industry that might affect the color appearance of the markings. For instance, the standard pigment in yellow thermoplastic pavement markings has been encapsulated lead chromate. While encapsulated lead chromate pigments pass EPA testing, they may pose health risks for workers on the road and in the thermoplastic manufacturing facilities. A concern about lead-free yellow thermoplastic is the ability to retain the yellow color as the marking ages. If it begins to look white, the marking configurations may confuse drivers. TTI researchers are evaluating several field applications of newly formulated lead-free thermoplastic pavement markings. The researchers are specifically focused on the color appearance of these markings. They are monitoring the long-term retroreflectivity and color appearance using a variety of geometries and illuminants. These evaluations will help better understand how the color of the lead-free markings changes over time, as well as providing insight into potential recommendations for modifications to the chromaticity coordinates that define yellow for daytime and nighttime pavement marking appearance.
Stay in front
The use of pavement markings as horizontal signs has been increasing with the support of TTI-led research. Drivers tend to spend most of their time focusing on the roadway in front of them, and any object or sign that appears in this region will be more likely to be observed than a sign that would appear in their peripheral vision. In addition, roadside signs can be missed by drivers due to visual clutter (billboards, etc.) or other traffic (heavy trucks, etc.). A redundant method of information dissemination would increase the likelihood of the critical information getting to drivers.
A recent TTI study found that drivers understood the guidance provided by route markers applied to the pavement but in some situations were confused when they were used for optional exits. This study also tested words and arrows as speed-reduction warnings in advance of rural curves. On-road measures of speed were used to evaluate horizontal signing (both symbols and text) intended to reduce speed at entries to horizontal curves on rural roadways and expressways. Treatments that included a specific advisory speed produced clearer speed reduction results than treatments that simply warned of an upcoming curve through words and transverse lines. In addition, the use of a curve arrow provided drivers with information about the direction of the curve.
Another application tested was the placement of directional arrows on the pavement to reduce wrong-way movements on a two-way frontage road. This study showed an overwhelming reduction in wrong-way driving at the one site tested. The study also assessed durability of pre-formed thermoplastic materials on concrete, asphalt and chip-seal pavements.
Ghost busters
Pavement markings are particularly useful in helping drivers maintain their lane position and navigate the roadway. In work zones, pavement markings are particularly needed to maintain safety and mobility when pavement markings are frequently removed and re-striped due to temporary changes in roadway alignment (lane shifts and crossovers). TTI researchers have been working to identify and test innovative products that meet the special needs of work zones. For instance, they evaluated an experimental “removable” marking that uses a patented liquid remover to wash away the markings instead of traditional methods that often damage the roadway surface leaving “ghost” markings, which can make it difficult for drivers to decipher the correct travel path.
Recent TTI research also has focused on how to select the appropriate pavement-marking material for work zones. On one hand, it is desirable that the material selected be durable enough to last for the duration of the temporary change in alignment for that particular project or project phase. On the other hand, since the application is temporary and will eventually be removed or covered, it is desirable to use as inexpensive a material as possible with an anticipated service life for that particular application that simply exceeds the temporary duration that will be needed.
TTI researchers developed an objective methodology for selecting the most appropriate pavement-marking material for work-zone situations based on the duration of the project or project phase for which the marking is needed, type of pavement surface the marking will be placed on and durability of the various marking materials available for use in work-zone situations.
Everything is being done
The research that goes into pavement markings is paying off in many ways. The pavement markings you see on the road are made with long-lasting materials, installed with state-of-the-art techniques and being measured with appropriate test methods that have evolved with time. TTI’s facilities and breadth and depth of relevant expertise have been key elements to the progress seen in the pavement marking world.
