Development of a Pavement Marking Condition Index from Retroreflectivity and Presence Measurements

Transcription

1 Development of a Pavement Marking Condition Index from Retroreflectivity and Presence Measurements Carmine E. Dwyer, P.E. (Corresponding Author) Senior Engineer 100 Trade Centre Drive, Suite 200 Champaign, IL Ph: (217) , Fax: (217) cdwyer@ara.com William R. Vavrik, PhD., P.E. Vice President and Associate Division Manager wvavrik@ara.com Michael J. Harrell, P.E. Senior Engineer mharrell@ara.com Rachel L. Reinicke Staff Engineer rreinicke@ara.com ABSTRACT To prepare for the impending amendment to the Manual on Uniform Traffic Control Devices (MUTCD) to set minimum retroreflectivity levels for pavement markings, many transportation agencies have been monitoring marking retroreflectivity performance to determine marking service life and therefore life cycle costs. Another important metric when evaluating marking performance is the marking s presence, or how well the marking resists abrasion and remains bonded to the pavement surface. ASTM D Standard Practice for Evaluating Degree of Traffic Paint Line Wear describes a subjective method for comparing photographic references to estimate marking presence (1). However, a more quantitative method of monitoring marking presence has been accomplished by collecting photos of markings and then processing the photos through an image analysis program that reports the marking s presence as a calculated percentage. Retroreflectivity and presence data can be evaluated together for developing life cycle performance curves or for determining annual re-striping efforts. However, combining the two metrics can be subjective if there isn t a standard process for combining them. To standardize the process and eliminate subjectivity, (ARA) developed a Marking Condition Index (MCI). This paper briefly describes the two performance measures and details the development of the MCI. SECTION 1. Overview read from graphs of performance coefficient of retroreflected luminance, RL, which is defined by the 1.1. Background measurement versus deduct value. ARA has performed pavement The development of the MCI is the American Society for Testing and marking retroreflectivity data collection and analysis since comprehensive evaluation of pave- luminance of a surface to the normal beginning of a process to provide a Materials (ASTM) as the ratio of the Recognizing the importance of ment marking performance and illuminance on the surface (3). For monitoring a marking s ability to resist abrasion and provide longevity, improve roadway safety. the retroreflectivity of pavement markings, this is the reflected light ARA developed an image analysis 1.2. Performance Measurements from the pavement marking (seen program for quantifying the percentage of markings that remain The two metrics ARA frequently by the driver) versus the source uses for evaluating the performance light (from the vehicle headlamps) bonded to pavements. Data collection on research test sections was of pavement markings are retroreflectivity and presence. RL is given in units of millican- directed at the pavement marking. conducted by ARA using hand-held delas/m devices, but network-wide retroreflectivity and presence data have 2 /lux. ASTM standards Retroreflectivity require 30-meter geometry for Retroreflectivity indicates how visible markings are to drivers at night. 1 depicts the components of R measuring retroreflectivity. Figure been collected by a mobile (vehiclemounted) service provider. Later, L. Retroreflectivity is reported as the when the two metrics (retroreflectivity and presence measurements) were being used together to provide a holistic view of marking performance, a need arose to standardize the process for evaluating the measurements together, and a Marking Condition Index (MCI) was developed. Much like the calculation for a Pavement Condition Index (PCI), calculating a MCI begins with determining deduct values for each performance measurement (2). The MCI begins with a value of 100 Figure 1. Depiction of roadway retroreflection and then a combined deduct value is subtracted. Deduct values are Continued on page 52 March/April 2014 Page 51

2 Continued from page 51 During ARA s field investigations, retroreflectivity measurements are collected with a hand-held 30-meter geometry retroreflectometer, shown in Figure 2. Figure 2. Hand-held 30-meter geometry retroreflectometer Presence Presence, the second metric for evaluating pavement marking performance, is an indication of a marking s durability, how well it remains bonded to the pavement and in place. In the fall of 2008, ARA developed a binary image analysis program, MarkAnalysis, specifically for measuring a marking s presence. Photos of the markings are taken under standardized lighting conditions and are then processed with the image analysis software. MarkAnalysis converts the image to pixels of two different colors and then counts the pixels of each color. Results from the software report presence as the percent of material missing from the pavement marking. Figure 3 is an example of a pavement marking photo viewed in the analysis program. Figure 3. Image analysis program, MarkAnalysis To standardize the lighting for each pavement marking photo, ARA devised a simple box that blocks all outside light and illuminates the marking with lights mounted inside the box. The camera mounts on top of the box. Figure 4 is a photo of the camera box placed over a pavement marking, and Figure 5 is a view of the interior of the box. Because some markings have anomalies such as latency on top of the marking or pavement cracks through the marking, the images of such markings are either edited or removed prior to processing through the image analysis program. Therefore, reported presence values (percent missing) are not affected by anomalies that are not related to the marking s durability performance. SECTION 2. Marking Condition Index Development Combining two performance measures to present one value that represents the overall condition of a pavement marking requires a standardized process so that comparisons can be made over time and between different types of markings. Calculating an MCI is performed much like the calculation for a PCI. The index begins with a value of 100. Then deduct values for each of the performance measures are read from graphs. Next, the deduct values are totaled, and a corrected total is read from another graph. Finally, the corrected total is subtracted from 100 to give the final index value (2). The following sections describe 1) the development of the retroreflectivity deduct curve, 2) the development of the presence (or percent missing) deduct curve, and 3) the final MCI calculation Retroreflectivity Deduct Curve The deduct curve for retroreflectivity sets the maximum deduct value to correspond with a minimum required retroreflectivity value, which can later be set to the new federal standard. Table 1 lists the current proposed minimum maintained retroreflectivity levels for longitudinal pavement markings (4). Page 52 Figure 4. Camera box Figure 5. Camera Box Interior Continued on page 53 IMSA Journal

3 Continued from page 52 Table 1. Proposed Table 3A-1 for the MUTCD (Minimum Maintained Retroreflectivity Levels1 for Longitudinal Pavement Markings) Since many of the markings that ARA evaluates fall into the category of All other roads and Speed Limit 55 mph, the maximum deduct value was set to correspond with a retroreflectivity of 100 mcd/m 2 /lux, as shown in Figure 6. However, additional deduct curves could be created easily for the other combinations of road type and speed limit. Figure 6. Deduct curve for retroreflectivity Since some markings can have high retroreflectivity values ( 600 mcd/m 2 /lux), an exponential formula was selected so the curve becomes asymptotic near the higher values. Therefore, retroreflectivity values of 300 or higher have a deduct value of approximately zero. The shape of the curve was created such that the largest deducts occur after retroreflectivity drops below 140 mcd/m 2 / lux. While the wide variety of available marking types can produce a large range of retroreflectivity values, having one deduct curve allows for comparisons among the various marking types. Continued on page 54 March/April 2014 Page 53

4 Continued from page Presence Deduct Curve The deduct curve for presence was developed based on experienced analysis of presence data and observed increases in rate of bond degradation. The presence deduct curve is shown in Figure 7. Figure 7. Deduct curve for presence (percent missing) The shape of the presence deduct curve is opposite from that of the retroreflectivity deduct curve because as the percent missing increases, the deduct values for the condition index should increase. When a marking is 100 percent present, the deduct value should be zero, so the y-intercept was set to zero. Based on expert opinion of presence data analyzed to-date and on observation of presence degradation, the curve s initial slope is gradual up to 40 percent missing. However, beyond 40 percent missing the slope increases because degradation is more rapid past this point. All presence values at or above 55 percent missing will have a deduct value of 100. Markings with 55 percent or more missing material were considered to provide no value Page 54 to the traveling public. Presence deducts, as presented here, are only applicable to solid line markings. Structured markings which begin with coverage values close to 55 percent missing would obviously fail quickly. If the MCI were to be standardized for all markings, including structured markings, then a standard method for taking photos from an angle representing a driver s perspective would need to be developed. While a marking s presence is not dependent on retroreflectivity, retroreflectivity is partially dependent on a marking s ability to remain bonded to the pavement surface. A marking can be 100 percent present and have little or no retroreflectivity if there are no reflective media on it. However, if a marking material becomes debonded, then there is less surface area returning light back to the driver s eyes. Retroreflectivity also depends on other variables such as bead embedment, bead condition and retention, and type of reflective media, but the graph in Figure 8 clearly shows that there is a relationship between retroreflectivity and presence. This graph represents much of the retroreflectivity and presence data that ARA has collected to date. The data is from a variety of marking types and ages, and each point typically represents an average of 20 readings. The graph on page 55 also supports the points selected in the presence deduct curve. Continued on page 55 IMSA Journal

5 Continued from page MCI Calculation After the retroreflectivity and presence deduct values are read from their respective curves, the two values are added together, and then a Corrected Deduct Value (CDV) is read from the graph in Figure 9. The function of the CDV is to adjust the total deduct to a 100-point scale. The MCI is then calculated as follows: MCI = CDV Figure 8. Relationship between retroreflectivity and presence A condition index typically has a threshold for pass/fail, and the pass/ fail threshold for MCI was set at 60. Therefore, a CDV of 40 would produce a failure rating for a marking. Because it is possible to have a marking that is 100 percent present (presence deduct = 0) and that has a retroreflectivity value less than 100 mcd/m 2 / lux (retroreflectivity deduct = 100), the total deduct value of 100 must correspond with a CDV of 40 in order to fail a marking that doesn t meet the minimum retroreflectivity standard. Figure 9. CDV curve for calculation of MCI The MCI values for many of the points shown in Figure 8 were calculated and reviewed to determine if the final values were reasonable. As more data (particularly presence data), is collected, the points on the deduct curves as well as the shapes of the curves will be reviewed and revised. Continued on page 56 March/April 2014 Page 55

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7 Development of a Pavement Marking Condition Index... Continued from page 56 SECTION 3. Summary The measurement of a Measurement of both retroreflectivity and presence is important for monitoring the performance of pavement markings. Use of the Marking Condition Index provides a consistent, repeatable, and objective method for evaluating the two metrics together. The MCI will enhance an agency s holistic view of marking performance when developing life cycle performance curves and determining annual maintenance efforts. pavement marking s presence is relatively new. Therefore, as more presence data becomes available, the process for determining presence deduct values can be reviewed and refined. The retroreflectivity deducts also can be adjusted based on the Final Rule for minimum maintained retroreflectivity levels (4). References 1. ASTM D Standard Practice for Evaluating Degree of Traffic Paint Line Wear. 2. Shahin, M. Y. Pavement Management for Airports, Roads, and Parking Lots. Sec. 3.5 Calculating the PCI. pp ASTM E Standard Test Method for Measurement of Retroreflective Pavement Marking Materials with CEN-Prescribed Geometry Using a Portable Retroreflectometer. 4. Federal Register. Proposed Rules. (FHWA Docket No. FHWA ). National Standards for Traffic Control Devices; the Manual on Uniform Traffic Control Devices for Streets and Highways; Maintaining Minimum Retroreflectivity of Longitudinal Pavement Markings. Vol. 75, No. 77. April 22, RAI Products Traffic Control and ITS Solutions SAME DAY SHIPPING! PO Box Charlotte, NC PH (704) FX (704) Toll Free (888) info@raiproducts.com March/April 2014 LED Cabinet Test Displays Wire & Cable Loop Sealant Backer Rod Saw Blades Polyconcrete Pullboxes Stainless Steel Banding & Accessories Pole Line Hardware Page 57