Those white and yellow lines on the roadway delineate lanes and orient and guide roadway users, enhancing traffic efficiency, reducing conflict and making navigating easier and more comfortable for travelers.
Most importantly, by helping roadway users navigate properly, those lines help reduce collisions and run-off-the-road crashes. Pavement markings are critical tools for enhancing the safety of motorists, bicyclists and pedestrians.
Looking acceptable
The effectiveness of pavement markings depends on their visibility. To the motoring public, acceptable pavement markings are usually defined by two simple criteria: “How well can I see them during the day?” and “How well can I see them at night?” Among technical people, these attributes are described as “daytime presence,” which is largely determined by how much marking material remains on the surface, and “nighttime retroreflectivity,” which is related to the amount of light that markings reflect back to the driver’s eye.
Compared with many roadway assets, pavement markings have a relatively short life; that is, their visibility characteristics—presence and retroreflectivity—deteriorate fairly quickly. How quickly depends on several factors:
Type of marking material;
Quality control of application process;
Damage due to wear from traffic; and
Winter-road-maintenance activities (snowplows can severely damage markings, especially the reflective beads embedded in markings to enhance their retroreflectivity).
Maintaining marking visibility at an acceptable level is therefore a major element of agencies’ roadway maintenance and safety programs. Yet the question remains: What is an “acceptable” level? Historically, agencies have had no way to quantify the point at which marking visibility is no longer adequate for safety purposes. Lacking performance benchmarks, traditionally many agencies have simply refreshed all markings in their system on a cyclical basis or on a less formal, as-needed basis. Neither method allows agencies to systematically maintain acceptable marking visibility—and thus the related safety benefits—across their networks while maximizing cost efficiency.
In recent years, the convenience and accuracy/reliability of retroreflectometer technologies have improved significantly, allowing the relationship between retroreflectivity measurements and driver safety to be studied. The Center for Transportation Research and Education (CTRE) at Iowa State University recently completed an analysis of five years of pavement-marking retroreflectivity data and corresponding crash and traffic data from all primary roads in Iowa for the Iowa Highway Research Board (TR-580). Logistic regression analyses were completed to find statistical relationships between crash occurrence probability and several variables, including marking retroreflectivity levels, marking/line type, road type and traffic (vehicle-miles traveled).
Retroreflectivity was found to have a significant effect on crash occurrence probability for all line types, with 90% confidence level for white edge lines, 95% for yellow edge lines and 99% for yellow centerlines. For white edge lines and yellow centerlines, crash occurrence probability increased as retroreflectivity values decreased.
Although the study did not determine the point at which retroreflectivity values are “acceptable” from a safety perspective, it seems feasible that such a determination can be made with additional research.
Come up with a plan
Almost concurrently with the publication of CTRE’s findings, the Federal Highway Administration (FHWA) published a notice of proposed amendment to the Manual on Uniform Traffic Control Devices (MUTCD) that establishes a uniform minimum level of pavement-marking retroreflectivity. The proposal includes a four-year phased compliance period (through fall 2014) for agencies to achieve and maintain pavement-marking retroreflectivity at or above the minimum level within their jurisdictions.
Assuming the amendment will be adopted in some form, maintaining minimum marking retroreflectivity performance levels across their networks will require many agencies to enhance their marking-management methods. Agencies will need to develop a pavement-marking-management plan outlining how they will bring their markings into compliance and maintain compliance systemwide with available financial resources.
The positive result is that using a well-conceived management program will help agencies ensure they are maximizing the safety benefits of pavement markings in their jurisdictions, while focusing their pavement-marking resources most efficiently and effectively.
How marks are tracked
The Iowa Department of Transportation (Iowa DOT) has been a leader in exploiting the power of management systems to enhance its highway network, and CTRE has been the agency’s technical partner in developing and implementing statewide pavement, bridge, sign and safety-management systems/programs. The center also developed the scope and implementation plan for the Iowa DOT’s pavement-marking-management system. The system focuses on the use of tools to track marking performance statewide, and a matrix that aligns material selection for short- and long-term marking maintenance with material selection for new construction. As local agencies have grasped the significance of the FHWA’s proposed amendment to the MUTCD, CTRE also has assisted city and county street and road agencies in developing and implementing pavement-marking-management programs.
From these efforts come the following general concepts and elements of a pavement-marking-management program:
Management system
A management system (electronic or manual) is needed that can, as clearly and simply as possible, track pavement-marking conditions (presence and/or retroreflectivity) across the network; compliance or noncompliance to appropriate benchmark(s); improvement actions selected, scheduled and completed; and costs. Storing data within a GIS database allows for easier review and decision-making and for providing data in graphical forms that support communication efforts within the agency and with the public.
Assessment
An initial assessment of the marking network conditions is necessary to establish a baseline inventory in the management system and to prioritize improvement activities. Condition data should be tied to location, material and installation method, line type (e.g., longitudinal, transverse, center, edge, solid, broken, etc.) and pavement type. The assessment can be phased over several months, and agencies can choose from various assessment options:
Visual (subjective) inspection of durable markings only;
Visual (subjective) inspection, daytime;
Visual (subjective) inspection, nighttime;
Measuring retroreflectivity; and
Some combination of the above.
Selecting an assessment approach is neither straightforward nor without consequences. For example, one county in Minnesota experienced a 23% reduction in striping needs after starting to use nighttime surveys to evaluate marking conditions. Whatever approach is used, systemwide assessments should be repeated at regular intervals, generally every 12 months, to develop a history of pavement-marking performance that will help guide material-selection decisions in the future.
Information recording
Information about pavement-marking installations, whether new or routine maintenance, should be recorded in the management system, with the data also tied to date, location, material, installation method and line type. Again, this information will contribute to the historical record of pavement performance.
Specification
Material specifications should be developed or used, along with guidelines for ensuring compliance.
Partnering
Partnering with other agencies and/or a private contractor, through a multiagency agreement, can save costs by taking advantage of quantity pricing and promote cross-jurisdiction consistency in materials and installation specifications.
In addition to helping agencies implement marking-management systems, CTRE has developed several related tools, including a presence measurement tool and a web-based marking material selection tool.
Have a presence
The key to a management system that supports effective decision-making is pavement-marking condition inputs that are objective, accurate and consistent. For measuring nighttime retroreflectivity, several retroreflectometers based on 30 m geometry are available, including laser-equipped mobile vans and a variety of handheld devices that, with little training, provide quick, objective, repeatable and quantifiable measurements in millicandela/square meter/lux-mcd (See Figure 1).
However, no tool has existed for measuring daytime presence as defined by the amount of marking material remaining on the pavement. As a result, marking presence typically continues to be determined subjectively through visual inspection, perhaps assisted by a calibration panel or photographs for comparison.
To support pavement-marking-management systems, CTRE has developed an innovative computer-based presence measurement tool that calculates percent of paint remaining based on digital images of pavement markings. The tool has been set up to accommodate a variety of pavement surface types and has been calibrated for different combinations of marking materials.
In addition to the presence tool, the only equipment needed is a digital camera. Working with color digital images, the tool’s calculations include three processes: image enhancement, clustering and analysis.
Image enhancement
First the digital image is enhanced to maximize the probability of distinguishing or separating the markings from the pavement. This process consists of applying filters, with empirically selected values, to the digital images; different filters are applied depending on whether an image contains white or yellow markings.
Clustering
Then “foreground” (markings) and “background” (pavement surface) areas are grouped, or clustered. In this process, the enhanced digital image is first converted from color to a greyscale image in which each pixel has a value of black from 0 to 255. Then, based on an empirically based pixel-value threshold, each pixel is labeled as either background or foreground, and adjacent pixels with the same label are grouped into a background or foreground component or cluster.
Analysis
The final analysis consists of collecting data for all the components (their number and size) and reporting the percentage of foreground to background, or markings to pavement surface.
The upper portion of the image on p 16 is the original digital image of a white paint marking on an asphalt surface; the lower portion is the processed image, showing contiguous paint by color, with a very close pattern recognition of paint material. (The color is used to provide feedback on contiguous areas of foreground material). It is unlikely that anyone could estimate, based on the upper image, that only 50% paint remains as revealed in the lower image. Such findings have shown that the tool greatly reduces subjectivity and provides much-needed repeatability and efficiency to the analysis of presence.
Turning to the matrix
The traveling public may think that pavement markings are “just painted on,” but types of marking materials and installation methods vary significantly, as do their costs. In addition to various kinds of paint, materials include more durable products like high-build waterborne paints, polyuria thermoplastics, preformed tape and epoxies. In addition, optical bead packages are a critical part of the installation. Street and road agencies are constantly trying to balance their resources by selecting wisely among traditional materials/installations and more durable (but generally more expensive) materials and installation practices.
A useful tool for selecting marking material is a selection matrix. Such a matrix can simplify decision making by outlining material options based on a variety of criteria. Criteria may include the following:
Marking type (longitudinal/transverse, centerline, edge line, walkway delineator, etc.). In recent demonstration projects funded by the Iowa Highway Research Board (TR-551), CTRE determined that marking performance should be monitored by line type, as variations in performance were notable;
Type of roadway and traffic level (rural/urban, number of lanes, interstate, AADT);
Pavement type (concrete, asphalt, chip seal);
Pavement condition or remaining service life; and
Level of winter-maintenance activities.
Depending on the agency, a selection matrix might be very basic. For low-volume roads, a conventional product such as paint may be the most cost-effective material. For roadways with a remaining service life of at least five years, higher traffic volumes and a history of markings disintegrating in less than 12 months, more durable pavement-marking materials may be considered. Grooving the markings in to extend their performance also is a possibility.
Whether simple or complex, a material-selection matrix should be informed by performance history, as tracked by a pavement-marking-management system. As future condition data are collected, matrices should be adjusted accordingly.
Taking the material selection matrix to another level, CTRE has developed a prototype web-based pavement-material-selection tool (PMST). Its calculations are based on performance prediction curves developed using four years of retroreflectivity data (generally collected quarterly) from nine different marking materials on two test decks in Tennessee.
Developed for the FHWA as part of a markings evaluation project, the interactive PMST allows users to input desired marking-performance characteristics (minimum retroreflectivity measurement; years of service up to six) and installation and operating conditions (roadway temperature at application, grooved or ungrooved, traffic level, level of winter maintenance activities). Radial buttons and drop-down choice boxes for all inputs provide an easy-to-use interface.
Based on user inputs and the prediction curves, the tool selects, with a check mark, products that are likely to meet the desired performance characteristics as long as the applied materials have a similar initial retroreflectivity level (as indicated by each product name).
The PMST input menu and output area appear on the same screen. This allows users to change the inputs for various “what-if” scenarios and immediately see the effect on the list of products. Note the upper left corner of the display page, Pavement Marking Products, which links to a page with a more detailed description of each product.
The link between traveler safety and pavement-marking visibility (as measured by retroreflectivity), along with the FHWA’s proposed minimum standards, supports increased investments in effective marking-management programs and tools. CTRE and other transportation research organizations are working hand in hand with street and road agencies to maximize the value of these investments. ST
