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

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Imagine yourself driving down a rural two-lane road. It’s night. The road is poorly lit. The headlights of oncoming traffic are distracting, making it even more difficult to see the road in front of you. You’re doing everything you can to stay in your lane and on the road.

And then it starts to rain.

Most drivers have faced similar circumstances and understand how difficult it can be to see lane markings in these conditions. In fact:

  • Forty-nine percent of fatal crashes happen at night, even though most traffic is on the road during the day.¹
  • Seventy percent of weather-related crashes happen on wet pavement.²
  • Driving at night in light or heavy rain increases the risk of injury-related crashes by 7.7 times and the risk of fatal crashes by 10 times (compared to daytime driving).³
  • In 2017, 6,952 people died in crashes on U.S. roads when it was raining.⁴

In rainy, nighttime conditions, the pavement markings that define lanes can become difficult to see. And pavement marking visibility plays a major role in helping drivers navigate, stay in their lanes, and get home safely.

So why, exactly, are pavement markings more difficult to see at night and in the rain?

Pavement Markings vs. Rain
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.

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