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       Of all the standard driver safety features—seat belts, airbags, anti-lock brakes—there’s one thing many of us take for granted: road markings. They help maintain a safe distance between vehicles on both straight, narrow roads and winding, dangerous sections.
       Those familiar bright white or yellow road markings are made from coatings that must balance two important factors: drying time and durability. Of course, reducing drying time is crucial to minimizing traffic disruptions. Durability, on the other hand, is essential to ensure the markings can withstand various conditions, including exposure to sunlight (especially UV radiation), temperature fluctuations, road surface deformation, and vehicle traffic. David S. Entrekin, technical director of Future Labs, a company specializing in road safety testing, explains…
       Road marking paints fall into three categories, all using similar pigments. White markings typically contain titanium dioxide. Yellow lane markings were previously painted with lead chromate, but this pigment was gradually phased out due to its toxicity. Fred Gelfant, vice president of research and development at Epoplex, stated that one of the main challenges for road marking manufacturers is developing pigments that can effectively mimic the color and stability of lead chromate, allowing it to withstand high temperatures and daily exposure to ultraviolet radiation. Therefore, companies in the road marking industry typically do not disclose the formulas of their yellow paints, only stating that the color is derived from organic molecules.
       Pigments must be applied to the pavement using a quick-drying liquid medium, and the layer must be as thin as possible—usually less than 1 millimeter—while still being durable. Latex paint is the least durable, lasting only one to two years. Like latex paint used for exterior buildings, pavement coatings also contain pigments, polymer resins (usually acrylic), and water as a solvent. As the water evaporates, the coating dries, and the polymer coagulates.
       The second type of road marking is a so-called multi-component material, consisting of two separate components: a polymer resin and a catalyst; a common combination is epoxy resin and an amine catalyst. When using multi-component markings, the materials must be heated to approximately 40–60 degrees Celsius, mixed to initiate a cross-linking reaction, and then sprayed onto the road surface. The cross-linking reaction begins immediately upon contact with the road surface and hardens within 15–30 minutes.
       Another multi-component composite material is polyurea, which is made by mixing amine resins and isocyanate catalysts. Polyurea cures very quickly—in just two to three minutes—and can last for about four years. However, a disadvantage is that road markings require a clean surface, which is especially important for polyurea, says Jerry Britt, Technical Service Manager at Ennis Traffic Safety Solutions.
       Britt stated that the third main category of road markings are thermoplastic markings, which were originally developed during World War II to address solvent shortages. Thermoplastic resins are typically based on modified gum or tallow esters, or on aliphatic synthetic C5 hydrocarbons. Thermoplastic markings must be heated to above 200 degrees Celsius before application to the road and can be sprayed onto the road surface or extruded into strips. They harden almost instantly, causing minimal impact on traffic. Thermoplastic markings 1 mm thick can last approximately three years. Markings up to 3 mm thick take longer to dry but can last up to five years.
       In addition to spray-on or extruded coatings, preformed thermoplastic or polymer tapes can also be used for lane markings, pedestrian crossings, or turn signals. Thermoplastic tapes require heating with a propane spray gun to melt and bond to the road surface, while polymer tapes have a pressure-sensitive adhesive backing. Some tapes are designed for temporary use, while others can last for five years or more.
       Neither paint, nor multi-component materials, nor thermoplastic road markings themselves emit a bright glow under car headlights at night. Reflectivity—or, more accurately, retroreflectivity, the ability to reflect light back to the vicinity of the light source with minimal scattering—is achieved by adding glass microspheres to the road marking surface after application but before drying. These microspheres, made of sodium silicate glass, typically range in diameter from 180 to 850 micrometers. In thicker thermoplastic materials, microspheres are also incorporated into the material, so that new microspheres are exposed as the coating wears over several years.
       The so-called “rainy night problem” refers to the fact that road markings are least visible at night, creating additional challenges for road marking designers. One solution is to apply jagged, rather than smooth, lines to thermoplastic material so that drivers can notice noise or vibration if they drift out of their lane. Another approach involves adding small protruding spikes to the surface that protrude above the water’s surface to better reflect headlights. 
       The choice of road marking materials depends on a number of factors: weather, traffic volume, the impact of road closures, road repair schedules, and the ability of snowplows to easily remove thick layers of pavement. For example, a durable but slow-curing epoxy resin might be suitable for paving 100 miles of highways in Montana, but impractical for busy Los Angeles highways. Similarly, if an expensive, durable thermoplastic is easily removed by snowplows, or if the road will be repaired in a year or two, there’s no point in investing in that material. These are just a few of the many factors to consider when applying road markings, and most drivers tend to overlook them.