Q&A: Reflection and retroreflection

Understand the importance of reflection and retroreflection

The challenge of safe driving conditions

Safe traffic conditions are highly dependent on properly performing traffic signs and road markings. At nighttime on dark roads, the retroreflection created by traffic signs and road markings are essential in 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 covers partly or fully the reflective surfaces of the elements partly thus reducing or eliminating retroreflectivity.

To put this challenge into perspective we use a statement by the US Federal Highways Administration: “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 in the night.” In this context retroreflection is a very important parameter in understanding how visibility can be improved on roads. To cut it short – this is about, how we maximize the light from vehicle headlights being returned back along the same path it came from maximizing the visibility of the driver.

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 not enough if markings and signs are not provided the right 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 is able to assist drivers.

What is reflection and retroreflection?

Reflection comes in three forms assisting drivers when driving on roads:

 

Diffuse reflection

This is the situation driving at day-time with the sun as the light source. The light from the sun hits e.g. a road markings and is being reflected in all directions including in the direction of 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 nighttime. The light is provided by the vehicle headlights, the light hits the marking or sign and is being reflected back to the eye of the driver.

The retroreflection is where we are able to measure the visibility of markings driving at nighttime. The highest retroreflection level is obtained on dry markings while wetness will reduce the performance of the markings, see below under mirror reflection.

 

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 this way reduce or block for lights being retroreflected.

Type 1 markings (flat lines) may be keeping a water layer on top of the markings creating mirror reflection

Type 2 markings (profiled lines) have an improved capability to drain of water and improve retroreflection

This may in particular be an issue driving at nighttime where visibility is already reduced and limited to light coming from the vehicle headlights. If water fully or partly covers the marking or sign surface they may act as a mirror resulting in light being reflected away from the eyes of the driver, thus loosing visibility.

The result can be seen in this animated graphic.

How do we ensure safe driving conditions?

High levels of reflection during daytime and retroreflection at nighttime are very important parameters to ensure that drivers safely reach their destinations. Markings of high standard with sufficient quality glass beads and traffic signs with proper retroreflective sheeting material are the way to reach this goal.

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