retroreflectivity Degradation Trend of Preformed Patterned Tape under Dry,
Transcription
retroreflectivity Degradation Trend of Preformed Patterned Tape under Dry,
Retroreflectivity Degradation Trend of Preformed Patterned Tape under Dry, Wet and Simulated Rain Conditions To aid drivers in maintaining the proper position of the vehicle on the road, longitudinal pavement markings need to be visible under all driving conditions. To enhance nighttime visibility, markings are retroreflective. The objective of this research is to evaluate and analyze the retroreflectivity characteristics of preformed patterned tapes under dry, wet, and simulated rain conditions. By David B. Clarke, Ph.D. P.E. and Xuedong Yan, Ph.D. ITE Journal on the web / December 2009 INTRODUCTION Longitudinal pavement markings provide a visual reference to aid drivers in maintaining the proper position of the vehicle on the road. Because guidance is critical to safe performance of driving task, markings need to be visible under all weather conditions. To improve nighttime visibility, pavement markings are retroreflective. The markings incorporate tiny transparent beads that reflect a portion of the light from the vehicle headlamps back to the driver’s eye. The more light the marking reflects back to the driver, the greater its nighttime visibility. A marking’s retroreflectivity, defined in units of millicandelas/m2/lux (mcd·m2·lx-1), is an indicator of its nighttime visibility. Specialized retroreflectometers enable the measurement of pavement marking retroreflectivity. The American Society of Testing and Materials (ASTM) publishes a number of standards related to pavement marking retroreflectivity, its measurement, and retroreflective materials. Presently, there are no established criteria for the minimum retroreflectivity of pavement marking, but 100 mcd·m2·lx-1 is a target generally supported by the literature. The visibility of markings in wet weather conditions is especially crucial for driving at nighttime. However, a film of water covering the beads will change the refractive properties, reducing the retroreflectivity.1 The depth of submersion is a key factor. Rain conditions that allow water to completely submerse the marking can virtually preclude any retroreflectivity. Wet conditions, where the marking surface is not submerged, but retains a thin water film, may permit a reduced level of ret- roreflectivity versus dry conditions. Suppliers offer several marking products that advertise improved rain and/ or wet retroreflectivity. One product– preformed patterned tape–is textured with raised sections to keep portions of the retroreflective surface above the water layer (see Figure 1). Schnell et al. reported that the preformed patterned tape incorporating 1.75 ceramic beads can provide acceptable retroreflectivity under wet conditions, but under simulated rain conditions its retroreflectivity is as low as 15 mcd·m-2·lx-1—not better than flat marking tape’s performance.2 Aktan and Schnell compared and evaluated the visibility of flat painted markings with large-beads, patterned tapes with high-index beads, and another patterned tape with mixed high index beads.3 Under wet conditions, the patterned tape with mixed-index beads performed best. The retroreflectivity of the paint and large beads was slightly lower on average than the patterned tape with high index beads. In simulated rain conditions, the paint and large beads and the patterned tape with high-index beads both yielded unsatisfactory retroreflectivity (lower than 50 mcd·m-2·lx-1), while the patterned tape with mixed-index beads gave the best retroreflectivity (156.8 mcd·m-2·lx-1). The objective of this research was to evaluate and analyze the in-service retroreflectivity degradation trend of preformed patterned tapes under dry, wet, and simulated rain conditions. Pavement marking retroreflectivity generally degrades over time because of abrasion by traffic, sun and heat exposure, application methods, and other factors.4 Since patterned tape is still a relatively new product, there are few published studies reporting its retroreflectivity degradation trend. All the 65 a. White marking b. Yellow Marking Figure 1. Patterned tape and marked sample locations. previous studies on nighttime visibility of patterned tape were conducted in experimental fields to measure and evaluate the initial retroreflectivity after marking application. It is not clear how long patterned tape can provide acceptable retroreflectivity under wet conditions as traffic exposure accumulates. Preformed tape has shown excellent durability in retroreflective property studies.5, 6 It is suggested for use in heavy traffic urban areas with continuous roadway lighting. METHOD Site Selection Between December 2005 and July 2007, the authors studied the retroreflectivity performance of 18 sites with patterned tape longitudinal markings. Those sites were distributed on roads in 5 counties in Tennessee. The sites varied by pavement marking color (9 for white and 9 for yellow), roadway type (8 for interstate, 4 for off-ramp, and 6 for state road), and Average Daily Traffic (ADT) (8 for lower ADT and 10 for higher ADT). Initial retroreflectivity measurements were taken soon after the application of the pavement markings. At each site, yellow centerlines or white/yellow edge lines were selected for evaluation. Apparatus and Data Measurement Retroreflectivity of the in-service marking materials was measured under dry, 66 wet, and simulated rain conditions over time in accordance with the American Society of Testing and Materials (ASTM): •Dry condition: Measurements were conducted according to ASTM E-1710 (Standard Test Method for Measurement of Retroreflective Pavement Marking Materials with CEN-Prescribed Geometry Using a Portable Retroreflectometer, 2005); •Wet condition: Measurements were conducted using the wet recovery test method, which is the definition of condition of wetness according to ASTM E-2177 (Standard Test Method for Measuring the Coefficient of Retroreflected Luminance (RL) of Pavement Markings in a Standard Condition of Wetness, 2005); and •Simulated rain condition: Measurements were conducted using the continuous wetting test method according to ASTM E-2176 (Standard Test Method for Measuring the Coefficient of Retroreflected Luminance (RL) of Pavement Markings in a Standard Condition of Continuous Wetting, 2005). A hand held retroreflectometer using 30-meter geometry was used for the measurements. Because data collection on in-service markings required encroachment onto the traveled way, field personnel worked within a short-term work zone including, where necessary, protection by arrow displays or vehicle mounted attenuators. For safety reasons, all data was collected during daytime hours. Retroreflectometer tests showed no significant difference in daytime and nighttime readings. To better compare white and yellow marking retroreflectivity, eleven sample locations of yellow and white markings were spatially matched in each selected highway segment. At each sample site, the research team established eleven discrete measurement locations at five meter intervals along the marking. Following the initial site selection and measurement, the researchers visited the test sites at three to six month intervals to measure marking retroreflectivity. The eleven unique measurements taken during each site visit were recorded; the mean value represented the overall marking retroreflectivity for the site. Pavement marking materials typically do not exhibit homogeneous retroreflectivity, and only a few inches movement of the test instrument can produce significantly different readings. For consistency, the exact same areas of pavement marking material must be measured on each visit. In order to place the instrument with sufficient precision to ensure measurement repeatability, spray-painted outlines shaped like the retroreflectometer were used to mark all sample locations, as ITE Journal on the web / December 2009 a. Dry condition b. Wet condition c. Rain condition Figure 2. Degradation trend of pavement marking retroreflectivity. illustrated in Figure 1. During each site visit, field personnel also took digital photographs of the marking. These photos provided a visual assessment of the marking condition to support data analysis. Data Reduction Examination of the final dataset revealed some anomalous values. If such values were explainable, they were removed from the dataset. Recognized causes for anomalies include tire marks on the test sections, dirt or mud on the markings, and possible human errors associated with equipment operation or data recording. Photos helped to confirm such issues. The most typical time series pattern resulting from dirty markings is that after marking application, retroreflectivity increases, decreases, and increases again. The reason is that during a certain site visit the markings were relatively dirtier due to continuous dry weather conditions, and as a consequence, lower levels of retroreflectivity were observed. However, after that site visit, heavy rain cleaned the markings, resulting in large increases in measured retroreflectivity at the next visit. OBSERVATION RESULT AND DATA ANALYSIS Retroreflectivity Degradation Trend Figure 2 shows the retroreflectivity degradation trend of patterned tape based on a 200-day marking service time interval. The measurements reflect dry, wet, and simulated rain conditions for both white and yellow markings. Using 100 mcd·m2·lx-1 as the minimum threshold, both colors of patterned tape markings provide sufficient retroreflectivity under dry conditions during the observation period. ITE Journal on the web / December 2009 a. White marking (dry) b. Yellow marking (dry) c. White marking (wet) d. Yellow marking (wet) Figure 3. Retroreflectivity changing trends for different levels of ADT per travel lane. Under both dry and wet conditions, the average retroreflectivity of white markings is significantly higher than that of yellow markings, and the difference exists throughout the whole observation period. This finding is consistent with a number of previous observations for other marking materials.7–9 The natural difference in initial retroreflectivity between yellow and white markings indicates that their degradation trends should be analyzed separately. Under dry conditions, the white markings’ retroreflectivity decreased more rapidly over time than the yellow markings. In fact the yellow marking retroreflectivity even increased during the observation period (see Figure 2-a). One possible rea67 of ADT per lane (below 50 mcd·m-2·lx-1), as illustrated in Figure 3-d. a. White marking (dry) c. White marking (wet) b. Yellow marking (dry) d. Yellow marking (wet) Figure 4. Retroreflectivity changing trends for different highway types. son is that white markings experienced a higher level of traffic encroachment and resulting wear than yellow markings. Under wet conditions, the patterned tape appeared to provide acceptable retroreflectivity only during the first 200 application days. Afterwards, the average retroreflectivity fell to a maximum of 70 mcd·m-2·lx-1. Although the patterned tape illustrates better wet-night visibility during the initial application period than the flat tape, its special function of wet-night visibility rapidly decreases with time, possibly as the texture is worn by traffic.10 Under simulated rain conditions, the patterned tape’s retroreflectivity is very low (no more than 20 mcd·m-2·lx-1) throughout the measurement period. This finding is also similar to the previous research result done by Schnell et al.11 Thus, it is not meaningful to further analyze the patterned tape’s retroreflectivity degradation trend under rain conditions. 68 Effect of Traffic Abrasion The average daily traffic per lane represents the potential traffic abrasion exposure of the pavement markings. Because accurate ADT data are not available for each site, the ADT per lane was classified into 2 levels (level 1 = less than to equal to 6,000 veh/day; level 2 = more than 6,000 veh/day) to investigate the traffic abrasion effect. 6,000 veh/day was chosen as break point because it provides balanced observations between the lower and higher traffic abrasion exposures. Figure 3 depicts retroreflectivity trends for white and yellow markings under dry and wet conditions respectively. As expected, the higher volume generally resulted in a higher rate of retroreflectivity decay. Under the wet conditions during the first 200 application days, the yellow marking’s retroreflectivity is acceptable for the lower level of ADT per lane (over 130 mcd·m2·lx-1) but not sufficient for the higher level Effect of Highway Type Figure 4 compares retroreflectivity over time among markings by types of roadways: interstate, off-ramp, and state road. The marking retroreflectivity of interstates is constantly lower than other roadways at a given service time point, perhaps because traffic volumes of interstates are higher than those of off-ramps and most state roads, resulting in accelerated wear. This tendency is especially strong under wet conditions (see Figures 4-c and 4-d). Comparison of markings of off-ramps and state roads reveals no consistent trend, except that yellow markings on off-ramps show better wet condition visibility than similar markings on state roads (see Figure 4-d). Site-by-Site Retroreflectivity Decay Trend To better investigate the retroreflectivity decay patterns, a site by site analysis of marking retroreflectivity was conducted under dry conditions. The plots of retroreflectivity versus time for the selected sites revealed two basic patterns for dry conditions. Pattern #1, shown in Figures 5-a, is a parabolic trend. After marking application, reflectivity first increases, and once it peaks, it gradually reduces over time, presumably because the markings wear further. Pattern #2, seen in Figures 5-b, is a monotone decreasing trend: after marking application, reflectivity gradually decreases. Under wet conditions, only pattern #2 was observed. As explained before, the tape’s wet-night visibility declines as the texture is worn by traffic. Under dry conditions, patterns #1 and 2 were each observed at 9 sites. The occurrence likelihoods for both patterns are therefore equivalent. According to previous research results, retroreflectivity is dependent on the embedment depth of the bead in the pavement marking material.12 Either lower or higher embedment depth of the bead may affect the longevity of the beads and marking retroreflectivity. The optimum retroreflectivity occurs at 5060% bead embedment. Generally, a new marking has 70% of the beads completely buried in marking materials. So, for new tapes, reflectivity increases initially because the glass beads become exposed ITE Journal on the web / December 2009 a. Pattern #1 b. Pattern #2 Figure 5. Typical marking retroreflectivity changing trends under dry conditions. after some amount of traffic wear. Once the depth of the exposed beads reaches 40-50%, retroreflectivity peaks and then gradually reduces over time as the markings wear further. This process explains the mechanism of pattern #1. Several previous studies on marking retroreflectivity also reported this phenomenon.13–15 On the other hand, when traffic volume is quite heavy and vehicles frequently run over the markings, pattern #2 is more likely to be observed since the beads can be quickly abraded from the material. Figure 6 illustrates how the two patterns are associated with marking color, ADT per lane, and highway type. Yellow markings, lower ADT per lane, and offramps and state roads are more likely associated with the pattern #1, while white markings, higher traffic volume, and interstates are associated with pattern #2. CONCLUSIONS This study was focused on the retroreflectivity degradation trend of patterned tape markings under dry, wet, and simulated rain conditions during the markings’ early service life (800 days). Based on data observation and analysis, the following ITE Journal on the web / December 2009 conclusions can be drawn: •Under dry conditions, the patterned tape markings are durable materials and can continuously provide acceptable retroreflectivity, even in heavy traffic highways. •Under wet conditions, the initial retroreflectivity of the patterned tapes is acceptable, but wet retroreflectivity falls below acceptable levels after the first 200 application days, perhaps as the texture is worn out by traffic. •Under simulated rain conditions, patterned tape retroreflectivity is too low and to provide acceptable visibility. Therefore, patterned tapes are helpful but not essential solution for improving marking visibility in heavy rain areas. •The initial retroreflectivity of white markings is significantly higher than that of yellow markings. However, the white markings illustrated a more apparent degredation trend in retroreflectivity compared to the yellow markings. Presumably, the white markings experienced a higher level of traffic encroachment and wear than the yellow markings during the service time. Figure 6. Association of retroreflectivity changing trends with marking color, traffic volume and highway type. •Patterned tapes are sensitive to the level of ADT per lane. When applied at heavy traffic highways such as interstates, their retroreflectivity is apparently lower than those applied at relatively light traffic roadways. ADT per lane appears a useful independent variable to predict patterned tape’s service life under dry conditions. •Patterned tape markings exhibit two types of retroreflectivity degradation trends: parabolic and monotone decreasing. The form of the two trends appears correlated with the intensity of traffic exposure and with marking characteristics such as bead size, embedment depth, and distribution. The patterned tape’s production quality is much better controlled than that of thermoplastic markings which retroreflectivity quality could be affected by many application factors. Further studies are suggested to explore the relationship between production factors and the retroreflectivity life of patterned tape. 69 ACKNOWLEDGMENT The authors would like to acknowledge the Tennessee Department of Transportation (TDOT) for its sponsorship of this research project. Appreciation is also extended to TDOT staff for providing assistance with data collection. The recommendations of this study are those of the authors, and do not represent the views of TDOT. n References 1. Schnell, T., Aktan, F. and Lee, Y.C., 2003. Nighttime Visibility and Retroreflectance of Pavement Markings in Dry, Wet, and Rainy Conditions. Transportation Research Record 1824, pp. 144–155. 2. Ibid. 3. Aktan, F. and Schnell, T., 2004. Performance Evaluation of Pavement Markings under Dry, Wet, and Rainy Conditions in the Field. Transportation Research Record 1877, pp. 38–49. 4. Migletz, J., Graham, J.L., Harwood, D.W., and Bauer, K.M., 2001. Service Life of Durable Pavement Markings. Transportation Research Record 1749, pp. 13–21. 5. Attaway, R.W., 1989. In-Service Evaluation of Thermoplastic and Tape Pavement Markings Using a Portable Retroreflectometer. Transportation Research Record 1230, pp. 45–55. 6. Lee, J., T.L. Maleck, and W.C. Taylor, 1999. Pavement Marking Material Evaluation Study in Michigan. ITE Journal, vol. 69, no. 7, pp. 44–51. 70 7. Scheuer, M., Maleck, T.L., and Lighthizer, D.R., 1997. Paint-Line Retroreflectivity over Time. Transportation Research Record 1585, D.C., 1997, pp. 53–63. 8. Zwahlen, H.T. and Schnell, T., 1995. Visibility of New Pavement Markings at Night Under Low-Beam Illumination. Transportation Research Record 1495, pp. 117–127. 9. Zwahlen, H. T. and Schnell, T., 1997. Visibility of New Centerline and Edge Line Pavement Markings. Transportation Research Record 1605, pp. 49–61. 10. Schnell 2003 11. Ibid. 12. VDOT, 2008. www.vdot.virginia.gov/ business/resources/bu-mat-PaveMarkCh2.pdf, Access date: April 9, 2008. 13. Gates, T.J., Hawkins, H.G., and Rose, E.R., 2003. Effective Pavement Marking Practices for Sealcoat and Hot-Mix Asphalt Pavements. Research Report No. 0-4150-4. Texas Transportation Institute, the Texas A&M University. 14. Kopf, J., 2004. Reflectivity of Pavement Markings: Analysis of Retroreflectivity Degradation Curves. Report WA-RD 592.1. Washington State Transportation Center, University of Washington, Seattle. 15. Thamizharasan, A., Sarasua, W.A., Clarke, D.B., and Davis, W.J., 2003. A Methodology for Estimating the Lifecycle of Interstate Highway Pavement Marking Retroreflectivity. Paper presented at the 83rd Annual Meeting of the Transportation Research Board. David B. Clarke, Ph.D., P.E., presently serves as Director of the University of Tennessee Center for Transportation Research. He also directs the Tennessee Transportation Assistance Program (TTAP), a federally funded Local Technical Assistance Program center providing training, technical assistance, and technology transfer services to local highway agencies throughout Tennessee. Dr. Clarke’s research focus areas include highway and railroad safety, materials performance, transportation facility operations, and the development of planning and analytical models. Xuedong Yan, Ph.D., achieved his Ph.D. in civil engineering at the University of Central Florida. Dr. Yan serves as a research assistant professor in the Civil & Environmental Engineering Department at the University of Tennessee and also serves as a research director in the Southeastern Transportation Center (STC) to technically coordinate the expanding comprehensive transportation safety research efforts of the STC. Dr. Yan’s research focus areas include database analysis, highway design and operation, driving simulation, and intelligent transportation systems. ITE Journal on the web / December 2009