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       In fact, 49% of fatal traffic accidents occur at night, even though we are only driving 25% of the time (1). Add rain to the mix, and the number of traffic accidents can increase by 57% (2).
       ”Rainy nighttime conditions are a dangerous and challenging time to drive,” said Adam Pike, principal investigator at the Texas A&M Transportation Institute (TTI).
       There are many factors that make driving on a rainy night difficult. One study found that drivers can only see about 5% of the objects they can see during the day at night(3). It takes about 200 feet for a vehicle traveling at 60 mph to stop, and standard low-beam headlights only illuminate 160 to 250 feet in front of the vehicle, making it difficult for drivers to see and react to objects on the road. Rain reduces the transparency of the windshield and increases glare from oncoming headlights, further reducing visibility. Additionally, water on the road reduces traction and makes the road slippery.
       Perhaps the biggest challenge for drivers in wet nighttime conditions is the ability to see road markings that indicate lanes and are critical to safe driving. In wet nighttime conditions, standard road markings are barely visible. “Most people with licenses have driven at night in a rainstorm and probably forgot about the road markings. It’s a pretty alarming situation,” said Ethan Peterson, a road marking and collision avoidance engineer for the Minnesota Department of Transportation (MnDOT).
       To understand why this happens, let’s first look at the science behind why road markings are visible.
       Road planners have long recognized the importance of clear and consistent lane markings. To improve the visibility of road markings and enhance road safety, industry engineers have worked for years to improve road marking technology, from simple white lines to the highly visible reflective road markings we often see on roads today.
       Standard reflective road markings consist of spherical glass beads (optical elements) embedded in a pigment binder (usually yellow or white) to improve visibility at night. Here’s how they work:
       The amount of light reflected back to the driver depends on several factors, including the refractive index (RI) of the optics and the air surrounding them. Most standard reflective road signs contain optics with a refractive index of 1.5. This is sufficient under normal, dry nighttime conditions. However, the refractive index of water is higher than that of air. As a result, light falling on wet road markings tends to be reflected back in a wider, weaker cone, meaning that much less light is returned from the car’s headlights to the driver’s eyes, significantly reducing the visibility of the road markings.
       As road designers, traffic safety experts and transport agencies increasingly realise that accidents occur with unusual frequency on rainy nights, they try to find out why. They soon discovered that the main factor in such conditions is the visibility of road markings.
       Based on this knowledge, manufacturers, engineers, researchers and safety experts have worked together to develop road markings that provide excellent visibility in most conditions – day or night, rain or shine. The latest road marking technology – reflective wet road markings – is made using ultra-high visibility reflective optics with a refractive index of 2.4. These high-performance optics reflect a narrower, more focused cone of light to compensate for the effects of water and provide brighter reflections in wet conditions. When combined with reflective optics with an RI of 1.5–1.9, it provides excellent visibility in most conditions.
       Wet reflective road markings are an important infrastructure for improving road safety, and studies over the past several years have confirmed the significant safety benefits of this technology. A 2015 study by the Federal Highway Administration (FHWA) evaluated the effects of wet reflective road markings before and after their use in Minnesota, North Carolina, and Wisconsin and found significant reductions in crashes on highways and multi-lane roadways(4). The Texas A&M Transportation Institute (TTI) is also interested in the safety benefits of wet reflective road technology. A 2019 TTI study found that in the Atlanta TxDOT region, wet reflective road markings reduced nighttime wet crashes by 30 percent and nighttime wet fatalities by 50 percent.
       In another TTI study sponsored by MnDOT and the Minnesota Local Roads Research Council in 2019, the institute conducted a comprehensive literature review, human factors study, and retroreflective light measurements to help determine initial and operational performance levels of reflective markings on wet pavement surfaces.
       “This project focused on determining minimum requirements for road marking visibility to people in wet evening weather. Previous studies have not fully addressed this specific objective,” said Pike. “We know that driving in the wet at night is less safe for a number of reasons. Most importantly, when road markings are wet, they are generally less visible than in dry conditions. If we can determine a minimum value for residual reflectivity in the wet, we can improve the visibility of road markings, making them easier for drivers to follow. Hopefully, with better lane control in these conditions, lane maintenance will improve, and sudden maneuvers and off-road crashes will be reduced.”
       In a literature review, TTI found studies showing that reflective wet pavement markings can reduce fatal and injury crashes in wet night driving conditions by 60 percent. Based on the human factors study results, the researchers recommend that where wet reflective road markings are required, they be installed with an initial continuous wet reflectivity of at least 200 mcd/m2/lux (based on an expected four-year service life) and a minimum continuous wet reflectivity of 50 mcd/m2/lux (5).
       As we have already discussed, driving in the rain at night can be very challenging. Visibility is poor, there are many distractions, and road conditions are poor. In such situations, drivers need clear and consistent lane guidance to help them navigate.
       Pike hopes that the Federal Highway Administration and state departments of transportation can use the results of their research to develop future pavement marking standards and specifications. “We identified minimum visibility requirements needed for people to see pavement markings in wet, nighttime conditions. We also recommended minimum initial reflectivity values. This can help states monitor initial pavement marking installations to ensure they are getting what they paid for at the performance levels they set. It also provides states with maintenance metrics to consider when replacing pavement markings.”
       Peterson believes that reflective markings that are highly visible on wet road surfaces improve driver comfort and are an added benefit. “Reflective markings on wet road surfaces allow drivers to see changing road conditions such as curves, intersections and turn lanes. This gives drivers more confidence, allowing them to stay in their lane or even off the road.”
       Setting minimum levels of wet reflectivity for road markings can help us build safer roads by providing drivers with clear, visible lane guidance when they need it most. This, in turn, helps reduce accidents, save lives and get families home safely. “Our goal is to ensure that road markings are visible 365 days a year,” Peterson said. “In Minnesota, that means working in challenging conditions. Ensuring that drivers can see road markings in wet weather is one of the most challenging tasks, but we are working hard to make it happen.”