What Happens to the Road Markings at Night When It Rains?

To understand why pavement markings can be so hard to see in dark, rainy conditions, it's helpful to know a little bit about what makes pavement markings visible. In general, pavement markings are embedded with retroreflective glass beads (optics) that are designed to reflect a vehicle’s headlights back to the driver. This makes the pavement striping visible at night to the driver traveling down the road.

Different retroreflective optics are better at reflecting light under different conditions, however, and it’s not possible for a single bead to be optimized for both dry and wet conditions. The most common types of pavement markings use retroreflective optics with a refractive index (RI) of 1.5—designed, but not optimized, for dry conditions.

When it’s raining, the optics become surrounded by water instead of air, and water has a higher RI than air. As a result, the light from the headlight is spread out in a much broader, weaker cone, and very little light is reflected back toward the vehicle. This substantially reduces the visibility of the road striping.

In the last decade, pavement markings have been developed to deliver exceptional visibility in all conditions—day or night, rain or shine. These wet reflective pavement markings use retroreflective optics with an RI of 2.4. These higher-RI optics counteract the effect of water by creating a narrower cone of reflected light—thereby reflecting more light back to the driver’s eyes and making them visible in wet conditions where other pavement markings would be difficult to see. According to recent research from the US DOT, wet reflective markings support up to a 46 percent reduction in run-off-the-road crashes on multi-lane roads in wet conditions.

Wet reflective pavement markings also include advanced 1.9 RI optics that deliver peak retroreflective efficiency and maximize visibility in dry conditions. This is beneficial for human drivers and helps improve the performance of vehicles equipped with Advanced Driver Assistance Systems (ADAS) like Lane Departure Warning (LDW) and Lane Keep Assist (LKA).

Vehicles equipped with these features are becoming increasingly common. According to SBD Automotive, “In the U.S. market, 60 percent of vehicle models sold from MY2016-MY2017 offered lane departure warning systems as a standard or optional safety feature.” ⁵ The Highway Loss Data Institute estimated that by 2025, over 40 percent of all registered vehicles will be equipped with standard or optional LDW.⁶

These ADAS-equipped vehicles use cameras to help keep the vehicle in its lane. Pavement markings with improved visibility and increased wet retroreflectivity help these systems more reliably detect lanes in a range of conditions. This, in turn, helps keep the vehicle in its lane.

Pavement markings play an important role in road safety, by helping both human drivers and vehicles with camera systems stay in their lanes and on the road. But for pavement markings to be even more effective, they need to be visible in all conditions. As we’ve seen, nighttime rainy conditions make it more difficult for drivers to see standard pavement markings, potentially increasing the risk of crashes, injuries and fatalities.

Reimagine yourself driving down the same dark, rainy rural two-lane road that you were on in the beginning of this article. But now, the lanes are marked with advanced wet reflective pavement markings. These markings use retroreflective optics that counteract the effects of rain and water to deliver consistently high visibility—day or night, rain or shine. Now, you can see the lines, stay in your lane and get home safely.

This scenario is well within our reach. By installing wet reflective pavement markings, we can help build safer roads for the present and the future.

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¹Forbes. Most Dangerous Times to Drive, Jan., 2009.
²DOT/FHWA. Ten-year averages from 2005 to 2014 analyzed by Booz Allen Hamilton, based on NHTSA data, 2005-2014).
³BAAC (French road traffic database, government open data).
⁴NHTSA Fatality Analysis Reporting System (FARS), 2017
⁵SBD Automotive — ADAS Tracker — AUT 534. June 2016.
⁶Predicted availability and fitment of safety features on registered vehicles — 2017. Institute, Highway Loss Data. Vol. 34. No 28, September 2017.