Research – Pavement Markings

Chapter 5: Wet Pavement Marking Retroreflectivity

The interest in pavement marking performance during wet nighttime conditions has increased in recent years. Research on wet retroreflective pavement marking performance was practically nonexistent a decade ago, but more recently a number of efforts have sought to better understand wet retroreflective pavement marking performance. Much of the research is related to visibility gains in wet nighttime conditions and the durability of marking performance in wet nighttime conditions. The potential safety benefits of wet pavement marking retroreflectivity is a topic of recent interest but to date there has not been an established relationship. There are also concerns that increased visibility of markings in wet conditions could potentially result in more drivers going too fast for the wet conditions.

Operational Performance

In 2013, research documented findings related to speed and lateral placement in active highway work zones, with pavement markings designed to have high-performing wet nighttime retroreflectivity levels. Five test sites were selected in North Carolina and Ohio. The researchers used the vehicle travel speed, rate of lane encroachment, and linear lane displacement to evaluate the effectiveness of the markings. The initial retroreflectivity of the prototype markings was higher than that of comparable markings, but not consistently higher. These findings produced inconclusive results, similar to the results from dry retroreflectivity studies.(42)

Visibility

Early studies of wet nighttime pavement marking performance were conducted with the retroreflectivity measurements based on ASTM E2176, which required almost 10 inches of water per hour on the markings during measurements. The more recent studies summarized here have used the superseding test method, ASTM E2832, which is based on 2 inches of water per hour during measurements.

Several significant studies have involved the performance and visibility of pavement markings in wet conditions. One of the initial key studies was conducted in Texas in 2007. This study included visibility measurements of 18 different pavement markings in an intensity-controlled rain tunnel, where participants drove through simulated rain looking for randomly positioned lane line markings.(12) The results show that wet retroreflectivity cannot be predicted by dry retroreflectivity. In general, dry retroreflectivity is typically higher than wet retroreflectivity. The study of retroreflectivity, and particularly wet retroreflectivity, is very dependent on the characteristics of the pavement marking materials.

The researchers formatted the results in terms of preview times provided by various pavement markings (and RRPMs). The study produced results showing that pavement markings, even those specifically designed for wet nighttime performance, are not a replacement for RRPMs in terms of visibility and cost of visibility per mile. The costs of the RRPMs were based on surface-applied RRPMs, which are mostly used in areas with no or very little snow plow operations. This study did not include any aspect of durability. All of the markings used were performing as if they were newly installed.

In 2009, researchers reported an evaluation of pavement marking performance using specially designed optics for wet nighttime visibility. This study included three prototype optics-on-paint marking systems employing high-refractive-index, dual-optics, drop-on elements.(43) These markings were evaluated at night under dry, wet-recovery, and continuous wetting conditions. Two commercially available marking systems (one glass beads-on-paint system and one wet-reflective removable tape) were also evaluated as industry benchmarks. Thirty participants driving through simulated work zones on a closed course viewed all of the marking types at night under all three weather conditions. Each driver's task was to identify the direction of work zone lane shift tapers delineated by the markings. In wet recovery, all three prototype marking systems and the wet-reflective tape sustained 60 to 80 percent of their dry average detection distances. In rain, they sustained 50 to 70 percent of their dry average detection distances. In contrast, the average wet-recovery and rain detection distances for the conventional glass beads-on-paint benchmark system dropped to 28 and 17 percent of the dry detection distance, respectively. In addition, participants failed to detect the conventional glass beads-on-paint benchmark system in nearly half of the observations in the rain condition.

A 2012 Virginia study evaluated the detection distance of several pavement marking types in wet conditions.(22) The markings were surface applied, recessed, and installed over a rumble strip (often called a rumble stripe). While there were differences among the pavement marking products studied, the wet nighttime performance of the markings degraded to 100 mcd/m²/lx or less after the first year. The research team was able to derive a minimum required level for wet nighttime retroreflectivity of 150 mcd/m²/lx. This level was based on their data and assumptions about how much preview time is needed for safe nighttime driving.

Durability

As noted in the previous section, the durability of wet nighttime pavement marking performance has been reported to be relatively short, at least compared to dry pavement marking performance. This is an area where research is just starting to emerge, although several evaluations are currently underway and more results are expected soon.

In Iowa, research was performed in 2011 on 16 variations of pavement markings over a two-year period to assess their performance and durability in wet conditions.(44) The study included yellow edge line and white lane line markings that were both surface applied and recessed. They were measured under dry and wet conditions over a period including two winters. The research found that:

• For the yellow edge line markings, the initial wet retroreflectivity measurements showed that only 7 of the 16 sections measured above 100 mcd/m²/lx. After one winter, only three sections measured above 100 mcd/m²/lx. (Again, the performance of these sections is very dependent on the materials used and the application techniques.) After two winters, only two sections measured above 100 mcd/m²/lx.
• For the white lane line markings, the initial wet retroreflectivity measurements showed that 13 of the 16 sections measured above 100 mcd/m²/lx. After one winter, six sections measured above 100 mcd/m²/lx (and all of these were grooved). After two winters, only one section measured above 100 mcd/m²/lx.
• The results indicate that placing markings in a groove can protect them from plowing operations.

Test Methods and Specifications

In the United States, there are two test methods for measuring wet retroreflectivity:

• ASTM E2177. This test method measures pavement markings in a condition of wetness but not actually being wetted. This test method represents how quickly a pavement marking can recover from the rain.
• ASTM E2832. This test method measures how well markings can perform under a standard condition of wetting (i.e., representing rain conditions). This supersedes ASTM E2176.

Both of these test methods were designed around handheld measurement equipment. Both test methods include a precision and bias statement to provide users an indication of the errors associated with such measurements. There are still some concerns about the repeatability of the test method for continuous wetting.