Abstract
White and yellow traffic markings, used for the demarcation of traffic, are a common sight on most roads. In North America the majority of traffic markings are applied as paint to the road surface.

Before traffic marking paint can be approved for use it must be submitted for evaluation to ensure that it conforms to federal and/or state specifications. Testing includes the application of transverse traffic marking to test decks exposed to a high volume of vehicular traffic and the elements, as part of the National Transportation Product Evaluation Program (NTPEP), and may last as long as three years.

In an effort to reduce the time and cost associated with developing higher performance traffic marking paints, abrasion resistance has been proposed as an accelerated method for evaluating the durability of traffic markings. It is a simple and inexpensive technique which yields highly reproducible results in a short period of time. These results correlate well with available NTPEP test deck retroreflectivity and durability ratings.

Abrasion resistance testing suggests that traffic markings with 3 year durability could be achieved with a 15 wet mil line if a high durability latex binder was employed in the traffic marking paint formulation.

White and yellow traffic markings, used for the demarcation of traffic, are a common sight on almost all roads. These markings enhance driver safety under varying weather and lighting conditions by continuously providing drivers with information concerning vehicle speed, lane delineation, road direction, warnings about upcoming conditions, and indications of where passing is allowed.

The daytime visibility of traffic markings relies on the color contrast between the marking and the road deck. Color contrast alone is inadequate to ensure acceptable night time visibility. Instead, embedded glass beads function as retroreflective elements reflecting light from vehicle headlights. Over time, mechanical abrasion by vehicles reduces traffic marking performance by damaging or dislodging beads and ultimately removing the markings from the road deck.

In North America the majority of traffic markings are applied as paint to the road surface. Before traffic marking paint can be approved for use it must be submitted for evaluation to ensure that it conforms to federal and/or state specifications. Specifications for pavement markings usually consist of some combination of chemical composition and performance requirements.  The cost and availability of some chemical components used in paint manufacture vary dramatically and, as a result, detailed composition specifications favored by highway agencies in the past are being replaced by performance specifications.  In some cases, a combination performance-composition specification is used which indicates the percentage by weight of each ingredient, by generic classification, without specifying a brand name or chemical formula.  Performance testing involves the application of transverse traffic marking to test decks exposed to a high volume of vehicular traffic and the elements, as part of the National Transportation Product Evaluation Program (NTPEP), and testing may last as long as three years.

Once a traffic marking has demonstrated its conformance with the specification, it is included on federal and state approved product lists (APLs).  Traffic marking manufacturers with products on an APL can then bid on projects.

This testing process was designed to ensure that all traffic markings meet a minimum quality level and has been very effective in achieving this objective, but it has also had several unintended consequences. The relatively high cost associated with sample submission has resulted in only a limited number of test paints being placed on the test deck every year. Compositional restrictions have limited the paints to a very narrow formulation window which, when coupled with the long testing cycle, has substantially constrained the development of new technology. The net effect is that overall product performance has not dramatically increased and potential enhancements for driver safety have failed to be developed.

In an effort to reduce the time and cost associated with developing higher performance traffic marking paints, other methods for assessing product performance and numerous accelerated methodologies have been proposed for evaluating the durability of traffic markings, and indeed transverse striping on the NTPEP test deck is itself a form of accelerated testing.  No matter the technique a correlation must be established between accelerated test results and real world performance. An ideal accelerated testing protocol would be highly correlated to real world performance, but also must be inexpensive, reproducible, accessible and significantly shorten the time interval to yield results.

Of all the accelerated methods proposed, Abrasion Resistance, as described by ASTM D2486-96, most closely meets this set of criterion. It is probably the least expensive and most common technique for evaluating the wear resistance of coatings. The technique is highly reproducible and results for dozens of different traffic markings can be obtained in a week’s time. The use of abrasive scrub media simulates the abrading action of sand and salt embedded in tires.  One obvious concern is that traffic markings are not usually evaluated with embedded traffic beads and so the question may arise as to whether a correlation exists between abrasive wear and the loss of retroreflectivity.  Retroreflectivity is defined as the light that returns to the light source.

In an attempt to establish a correlation between abrasion resistance, durability and retroreflectivity, data from NTPEP trials were compared to laboratory test results.

Experimental Paint Formulations

Traffic marking paints utilized in this work were prepared using the formulas shown in Table 1. Three latexes (A, B and C) with varying abrasion resistance were included in the study.

NTPEP Test Deck

Transverse traffic paint pavement markings were applied to test decks as a part of the NTPEP; paints were tested on both asphalt and concrete.

Test decks were located on straight, flat roadways that were not likely to have areas of excessive wear caused by braking or turning, were fully exposed to sunlight during daylight hours, and had good drainage. Average daily traffic (ADT) was approximately 15,000.

All lines were applied in the transverse direction, or perpendicular to the flow of traffic, using a walk-behind striping machine with traction drive and spray guns similar to those used on commercial striping trucks. Lines measured four inches in width and either 15 ± 1 or 30 ± 1 wet mil in thickness. To calibrate the spray application of each paint, the four-inch wide stripe was sprayed onto a test panel of known dimensions, which was then weighed. To calculate the line’s thickness, that weight was then divided by both the paint’s density and the area of the panel covered by paint.

Glass beads conforming to AASHTO M 247, Type II and IV from Potters Industries, Inc., were applied to the painted lines of interest at a rate of 6 lbs and 12 lbs of beads per gal of paint, respectively. Beads were applied from a dispensing gun immediately following the corresponding paint gun.  Glass bead dosages were determined by weighing a panel striped with a bead embedded paint film and subtracting out the already calibrated amount of paint on the panel.

Retroflectivity and Durability Measurements

Within seven days of striping the lines and approximately monthly thereafter, for a period of 24 months,  retroreflectivity was measured on each marking in the skip-line and wheel track areas. Retroreflectivity was measured using a LTL2000 retroreflectometer (Delta Light and Optics, Denmark), LTL-X retroreflectometer (Delta Light and Optics), or other approved equivalent device exhibiting the same geometric design criteria. The ratings were averaged for replicate lines.

Durability assessments were made near the skip-line and in the wheel tracks. In the wheel tracks, an area covering nine inches on either side of the wheel track midpoint was examined and rated. The durability rating was an assessment of the percent of the marking remaining divided by 10. For example, a line without any notable wear receives a 10. Ratings were averaged for replicate lines.

Abrasion Resistance

The abrasion resistance of the traffic marking paints was assessed following ASTM D2486-96. Paint films were prepared on a Leneta Form P-121-10N plastic chart utilizing a 7, 15 or 30 mil drawdown bar.  Two paints were cast side-by-side with one functioning as the control and the other as the test film. After drying for one week at 25˚ C and 50% relative humidity the paint films were scrubbed utilizing a Gardner Straight Line Washability and Wear Abrasion machine equipped with three brushes. The average number of cycles to failure was reported.

To calculate the line’s thickness, the weight of paint applied to the Leneta chart was divided by both the paint’s density and the area of the panel covered by paint.

Discussion

NTPEP Trials

The durability of the traffic marking paints was assessed at numerous times throughout the two year trial. Figure 1 illustrates the impact of 10 months of road wear on the appearance of traffic marking paints applied at 15 ± 1 wet mils line thickness. These paints differed only in the latex used in their formulation. Differences in line durability are already apparent even after this short period of time.

Figure 2 illustrates the impact of 24 months of road wear on lines applied at 15 ± 1 and 30 ± 1 wet mils line thickness. The thinner line is quite worn and would be considered in need of re-striping. The thicker lines are more intact, but a clear difference can be seen between different latex types.

The durability rating of the traffic marking paints over the 24 month testing interval is depicted in Figure 3. The rate of degradation was found to be a function of line thickness and latex type. Unless otherwise specified, traffic markings with a durability rating less than 4 (less 40% remaining) are considered in need of replacement.
Figure 4 illustrates the skip-line retroreflectivity of the traffic markings. The retroreflectivity of the traffic marking can be expected to decrease as beads are lost or damaged.  Below 100-150 millicandales/m2/lux markings are not easily seen by drivers. The higher initial retroreflectivity of the 30-mil lines is a result of the use of larger Type IV beads and the higher application rate of 12 lbs/gal of paint. White lines also generally have higher initial retroreflecitvity than yellow. The rate of degradation of retroreflectivity was also found to be a function of line thickness and latex type.

It was found that the degradation in the skip-line retroreflectivity was highly correlated to the wheel-track durability rating with a correlation coefficient of 0.94. This finding suggests that durability and retroreflectivity degrade in a similar manner and higher durability markings will tend to retain an acceptable level of retroreflectivity longer. Figure 5 outlines the skip-line retroreflectivity predictive formula developed with data from the NTPEP trials, showing the correlation to wheel track durability.
From this data it is clear that the life expectancy of a traffic marking can be enhanced simply by applying a thicker line.

Abrasion Resistance Testing

Standard durability traffic marking paints are applied at 15 wet mils and durable markings at 30 wet mils. Abrasion resistance testing protocol specifies films be applied with a 7-mil drawdown applicator which, depending on paint rheology, yields a film only 4-5 mils thick. Thus abrasion testing does not directly provide information on the durability of thicker films. To establish the impact of film thickness on abrasion resistance, a series of films of varying thicknesses were cast and their performance tested.
Figure 6 illustrates the scrub performance of the traffic marking paints applied to the NTPEP test deck. Clearly film thickness has a dramatic impact on abrasion resistance. This finding is not surprising since the test involves the removal of material with an abrasive media, so it would be expected that doubling the film thickness would double the number of abrasive cycles required to remove the paint film.

Since both paints passed 1 year durability testing when applied at 15 ± 1 wet mils and 2 year durability testing when applied at 30 ± 1 wet mils on the NTPEP deck, a correlation between abrasion resistance and durability can be inferred.  It would appear that to achieve 1 year durability on the NTPEP test deck a minimum abrasion resistance of 5,000 cycles is required. It follows that 2 year durability would require a minimum abrasion resistance of 10,000 cycles. Further validating the approach, the slight difference in performance between the two traffic marking paints noted on the test deck was also observed in the abrasion resistance testing.

The use of abrasion resistance testing as a tool to guide new product development promises not only to shorten the product development cycle, but also suggests significant improvement in the durability of traffic marking paint is possible. It has long been recognized that the ratio of latex binder to coating pigmentation has a significant impact on abrasion resistance; this fact is illustrated in Figure 7.

The drawback to this approach is that formulating a higher latex binder level would increase the cost of the paint formulation, placing the paint manufacturer at a disadvantage in the competitive bid process.

Alternatively, higher abrasion resistant latex binders could dramatically improve durability. Figure 8 illustrates the performance of a traffic marking paint based on a high abrasion resistance latex (Latex C).  This approach would enable multi-year performance without requiring the application of thicker lines. Indeed the abrasion resistance data suggests that 3 year durability could be achieved with a 15 wet mil line if a high durability latex binder was employed in the traffic marking paint formulation.

Conclusions

Abrasion resistance appears to be a promising tool in the development of new traffic marking paints.  It is a simple and inexpensive technique which yields highly reproducible results in a short period of time. These results correlate well with available NTPEP test deck retroreflectivity and durability ratings.

Abrasion resistance testing suggests that traffic markings with 3 year durability could be achieved with a 15 wet mil line if a high durability latex binder was employed in the traffic marking paint formulation.