Guidelines for The Implementation of Reduced Lighting on Roadways

Chapter 2. Adaptive Lighting Application

The approach taken for adaptive lighting affects where the lighting system should be controlled and when it should be controlled. This section applies specifically to active adaptive systems.

Where to Adapt Lighting

It is believed that adaptive lighting can be used in most roadway scenarios. However, in certain areas, it is not advisable to implement active adaptive lighting systems, such as in critical visibility areas where it is vital to see objects and vehicles in the roadway. Responsible designers of adaptive policies must evaluate areas of critical visibility, such as roadways that have a significant number of curves with short visibility distances or locations where traffic and pedestrian volume are consistent throughout the night (e.g., a hospital or other service facility). It is also important that adaptive policies not be used to replace other responsible lighting activities, such as luminaire maintenance and tree trimming.

Another consideration in implementing adaptive lighting is the size of the area covered by the lighting system. Dimming a roadway lighting system can occur broadly over all of the roadways in the area, or it can occur section by section on each of the roadways being dimmed, depending on nighttime use and driver needs.

In general, dimming a large area maintains a constant lighting level such that drivers do not experience a high lighting condition on one roadway and then turn onto a dark roadway, requiring significant adaptation between the lighting levels. Depending on the range of light level changes, the abruptness of the change, and the age of the driver, this transition can be uncomfortable and dangerous. However, dimming a large area without consideration of differences in road usage at night may cause some sections to be too dark.

To control for varying lighting levels, the following recommendations are made for each of the road facility types:

• For roadway facilities, each roadway should be assessed individually, but drivers should not experience greater than a one-level change in the lighting class per mi of travel.

• For streets, each street should be evaluated in terms of the lighting needs. However, the difference in lighting classes for streets in a given vicinity should be no greater than two.

• Residential/pedestrian areas should be adapted to a single lighting level.

When to Adapt Lighting

The optimal approach to selecting the timing of the adaptive lighting is to continually monitor the roadway and the environment. As an example, ITSs can provide traffic and pedestrian counts as inputs to an algorithm that establishes the lighting level in real time.

When ITSs are not available, such as on smaller streets and residential/pedestrian areas, curfews are typically established to determine when the lighting system can be dimmed. The following criteria can be used to establish a curfew:⁽¹¹⁾

• Changes in vehicular traffic level sampled over a period of time.
• Typical closing hours of surrounding businesses.
• Changes in the transportation schedule.
• Changes in parking regulations.
• Sampled pedestrian activity level.

It is important that exceptions to the curfew (e.g., for sporting or entertainment events) be considered, providing an agency with the ability to override the adaptive lighting program on demand.

It is not advisable to adapt the lighting system during periods of adverse weather. The impact of dimming lighting during fog, snow, and rain is not clear. Some research has shown that visibility on a wet roadway is negatively affected by dimming of luminaires.⁽¹²⁾ Further investigations are under way.