Since motorized vehicles were created, nighttime and rainy-day driving conditions have caused motorists to land in the ditch or crash into each other. White paint striping was an improvement over unpainted lanes, but the lines were still hard to see and faded rapidly with time, wear and erosion.
In the 1930s, glass-bead-infused road striping replaced white paint striping due to its ability to wear better. In fact, the Virginia DOT (PDF, 711.16 KB) notes that in the early 1940s, glass bead road striping demonstrated its worth during World War II when reflective beaded lines were used to designate highway lanes during blackouts.1
While glass beads still have lane-marking advantages, they are problematic during rain or when under water because they sometimes seem to disappear from the driver’s vantage point. The answer to the question about why pavement markings disappear in the rain is found in the physics of retroreflectivity and the index of refraction—as it relates to pavement markings, how light is bent when entering and exiting retroreflective media.
When properly embedded in a striping binder, glass beads under dry conditions redirect light back to its source, e.g. the driver with his or her headlights on. This is called retroreflection. The driver can see that the light appears to be coming from the striping. During rainy or foggy weather, the glass beads are covered in water, which makes non-ideal refractive index layers. This is like putting on the wrong pair of glasses and seeing things out of focus. Headlights bounce off the water surface and from the beads in many directions other than back to the driver, so the driver cannot see the striping. Consider how a straw in a water glass appears to bend to one side.
Nighttime color of pavement markings is also important. Standard glass beads are clear, so they depend on a colored binder to perform adequately. When using a yellow binder, they can appear washed-out at night.
Pavement markings that use glass beads with a lower refractive index (RI) seem to “disappear” when wet because water presence causes the light to focus differently. Refractive Index (RI) interfaces define how much light bends (refracts.)
Glass bead durability also affects reflectivity. Once a bead is broken, it no longer reflects. Wear from tires, snow plows and road oils can wreak havoc on bead surfaces, creating pocks and other deformities that can negatively affect reflective performance.
Microcrystalline ceramic bead technology makes for long-lasting, highly reflective road markings– day or night and on wet or dry pavement. This technology helps markings focus light correctly even when under water.
Combining microcrystalline ceramic beads with a pigmented binder creates a high refractive index product with excellent performance during dry as well as rainy or foggy conditions. In fact, microcrystalline ceramic reflective beads with a refractive index of 2.4, similar to that of diamond, are much more effective at returning light to the driver when wet than 1.5 and 1.9 refractive index beads
To improve reflected color and visibility in yellow pavement markings, yellow microcrystalline ceramic beads (PDF, 813 KB) also are encased in yellow elements, helping to make the markings conspicuous at any time of day or night.
Microcrystalline ceramic beads can also outperform glass beads in terms of durability. Small crystals are engineered into each bead to help stop cracks from starting and propagating, resulting in a material that’s tougher than glass beads, so it lasts longer on the road.
Glass bead reflective technology has its place in highway marking and is an effective choice for certain road markings in certain environments and conditions. When conditions like rain call for higher reflectivity and more durable construction, ceramic reflective element technology can make even wet roads more visible, helping to make them safer and easier to navigate.
As the transportation industry moves to connected autonomous vehicles, the ability to detect pavement markings in all weather conditions, dry and wet, will become even more important. Microcrystalline beads will continue to play a vital role in the future of transportation safety.