FAQ about reflection and retroreflection
Understand the importance of reflection and retroreflection.
All your questions about RoadSensors instruments answered:
What creates reflection and retroreflection?
Reflection and retroreflection are created by the road markings and road traffic signs when being illuminated by sunlight, overhead streetlight or vehicle headlights. But a light source alone is insufficient if markings and signs are not provided with suitable properties. Markings must contain small glass beads, and the sign film material must contain glass beads or microprisms to enhance the reflection or retroreflection to a level which can assist drivers.
What are reflection and retroreflection?
Reflection comes in three forms assisting drivers when driving on roads:
Diffuse reflection
This is the situation driving in the daytime with the sun as the light source. The light from the sun hits e.g., road markings and is reflected in all directions, including the eyes of the driver.
The diffuse reflection is what is being used for measuring:
- Qd = the retroreflection of the marking
- Daylight Contrast (DC) = the ratio in illumination between the road surface and the marking
Retroreflection
This is the situation when driving at night. The vehicle headlights provide the light; the light hits the marking or sign and is reflected back into the eyes of the driver.
Retroreflection is where we can measure the visibility of markings driving at night. The highest retroreflection level is obtained on dry markings, while wetness will reduce the performance of the markings.
Mirror reflection
This is the situation where markings are wet due to rain or water condensation during high humidity on markings and signs. The water will cover part or all of a marking and reduces or blocks lights being retroreflected.
Type 1 markings (flat lines) may have a water layer on top of the markings creating a mirror reflection.
Type 2 markings (profiled lines) have an improved capability to drain water and improve retroreflection.
This may particularly be an issue when driving at night when visibility is already reduced and limited to light coming from vehicle headlights. If water wholly or partly covers the marking or sign surface, it may act as a mirror resulting in light being reflected away from the eyes of the driver, thus reducing or eliminating visibility.
What do reflection and retroreflection have to do with safe driving conditions?
“Inadequate and poorly maintained signs and markings are often cited as the contributing factor to accidents. While only 25% of travel occurs at night, about 55% of fatal accidents occur at night.”
US Federal Highways Administration
Safe traffic conditions depend on adequately performing traffic signs and road markings. At night on dark roads, the retroreflection created by traffic signs and road markings is essential to guiding drivers in keeping vehicles safely positioned on the roads, thereby avoiding or at least reducing the number of traffic accidents.
During wet and rainy weather, even high-performing traffic signs and road markings present a challenge, as water often partly or fully covers the reflective surfaces, reducing or eliminating retroreflectivity.
In short, retroreflection is a crucial parameter in understanding how road visibility can be improved. It is about how we maximise the light from vehicle headlights being returned back along the same path it came from, maximising the driver’s visibility.