Pavement markings serve a number of vital functions in traffic operations, and most are “retro-reflective” so they are visible at night. Various “external factors” can reduce this retro-reflectivity. For example, ultraviolet light breaks down the “chemical bonds of resins and pigment components,” and “fluctuations in temperatures cause both the pavement and the marking materials to expand and contract.” In addition, “salt, sand, snowplows, studded tires, and chains damage pavement marking materials through abrasive interaction.”

The “useful life” of a pavement marking is also a function of the material from which it is made. A 1996 study in Utah compared the performance and cost of three types of pavement marking materials on two different surfaces: “solvent-based Paint, Epoxy resin, and pre-formed pavement marking Tape” on Portland Cement Concrete (PCC) and Asphalt Concrete (AC). The authors of a report on this study–Joseph Perrin, Jr., Peter T. Martin, and Blake G. Hansen–also compared their results with previous research by J. M. Dale (1988) and discussed the comparisons in “A Comparative Analysis of Pavement Marking Materials.” In general, their results showed a longer useful life for the pavement marking materials (compared with Dale’s results). Tape has a longer useful life than either Paint or Epoxy, but is far more costly.

METHODS AND BACKGROUND

The roads included in the study were all “arterials or freeways” in the Salt Lake area. Data gathered included “pavement marking material type, location and date of application, measured retro-reflectivity, initial retro-reflectivity, pavement type, highway lane geometry and AADT [average annual daily traffic] level.” The UDOT (Utah Department of Transportation) provided dates of application and material types for each road. Information on Epoxy and Paint was available for both pavement types (PCC and AC), but information on Tape was available for AC only. Researchers discounted the effects of weather, because the roads were in the same area (with similar snowfall and snow removal). The study also did not include environmental factors such as those mentioned earlier, because they are generally “more important on low volume roads where intervals between pavement re-marking are longer.”

A “mobile retro-reflectometer” called Laserlux measured lane geometry and retro-reflectivity. It records retro-reflectivity “as an average of all of the valid retro-reflectivity values measured in each 80 meter (264 ft) interval.” Average values were then grouped for each 80-meter section of highway with the same characteristics (pavement type, number of lanes, etc.). To arrive at “an average retro-reflectivity value for each of these highway segments, each of the 80 meter interval values [were] averaged with respect to the number of valid measurements in that 80 meter interval.” Figure 1 plots “the average retro-reflectivity value, R, for each highway section with respect to age for the three materials.”

Determining a marking material’s deterioration rate and useful life required “both a starting point (initial retro-reflectivity) and a finishing point (minimum acceptable retro-reflectivity).” Dale’s research did not include values on initial or minimum acceptable retro-reflectivity; however, the Connecticut DOT established a minimum acceptable retro-reflectivity value of 100 mcd/m2/lx (millicandelas per square meter per lux) based on 15-meter geometry. The Utah study used 30-meter geometry, for which no minimum value was available. For the study’s purposes, the authors selected the 100 mcd/m2/lx value as the minimum acceptable retro-reflectivity. They also calculated average deterioration rates for each highway segment (either PCC or AC) “and then plotted [them] against AADT.” For both PCC and AC, the cost analysis comparing the three material types (Table 1) assumed an AADT of 10,000.

RESULTS

The authors compared their results with Dale’s in four areas: “the range of AADT values for which useful life is estimated, relative performance of Paint and Epoxy on AC and PCC, the relationship between AADT and useful life, and the magnitude of useful life estimates.” Dale’s results on AADT were within a range of 0-16,000; the Utah study’s results ranged between 5,000-15,000. Dale “represent[ed] the useful life-AADT relationship on both pavement types” with “a single, linear curve,” but the authors of the Utah study found “the useful life curves . . . better represented by a hyperbolic curve,” and “the useful life on PCC and AC appear[ed] . . . better represented by two separate curves.”

Figure 2 shows “the estimated useful life of Epoxy,” and Figure 3 compares results “of the estimated useful life of Paint related to AADT on PCC and AC.” Dale estimated the useful life of Paint on both PCC and AC was less than six months maximum, but the Utah results showed a useful life “4 to 5 times longer.” Dale also found “that Paint has a longer useful life on AC than PCC,” but the Utah results “indicate[d] the opposite for a given AADT.”

Figure 4 illustrates “the range for Tape’s useful life on AC depending on the assumed initial retro-reflectivity of the material.” Dale found Tape had “a maximum useful life of 3 years” on both PCC and AC, but the Utah study showed a useful life ranging from six to ten years.

Comparisons among the three material types showed “that Epoxy has an improved useful life of between 18% and 45% over Paint on PCC pavement types. On AC, Paint and Epoxy produce[d] similar useful life ranges.” Results “further indicate[d] that Tape has between a 150% and 350% longer useful life on AC pavement than both Paint and Epoxy for a given AADT level.” In terms of longevity, Epoxy lasts between 100 and 120 percent longer on PCC than on AC, while Paint lasts about 40 to 80 percent longer on PCC than on AC. The authors noted the “hyperbolic relationship between AADT and useful life” is valid “only in the surveyed AADT range of 5,000 to 20,000 AADT per lane.” Although the Utah study did not include AADT results between 0 and 5,000, the authors “hypothesized that other external factors [ultra-violet light, temperature fluctuations, abrasion, etc.] have a much greater impact on useful life of the material.”

CONCLUSIONS

With the exception of his conclusions on Epoxy, Dale’s results on the useful life of the three pavement marking materials were “generally lower” than those of the Utah study. Results also differed on how the materials performed on the two pavement types (PCC and AC) and on the relationship between useful life and AADT (linear in Dale’s research and hyperbolic in the Utah study). Authors of the Utah study emphasized their intent was not “to replace or challenge the work performed by Dale, but simply to use his prior work as a reference point in which to compare the useful life of the three pavement marking materials investigated.”