Section 4: Lighting CurfewsAnchor: #i1002296
Curfews for lighting involve the use of modern controls to turn off or dim selected parts of lighting systems as permitted by reduced traffic flow, favorable weather conditions, and other local conditions. Lighting curfews represent the active “operation” of the system, allowing for energy savings, greater flexibility in resource allocation, and reduction of light-trespass. However, officials implementing such options should be aware of consequences and conduct meaningful studies of costs and benefits.Anchor: #i1002306
Studies on the effectiveness of roadway lighting have mixed results. It is generally accepted that, overall, continuous lighting reduces crashes by about 30 percent. The precise mechanism for the reduction of those crashes is not known. Currently it is not possible to translate surrogate measures, driver performance with targets, or other measures directly into a safety change.
Motor vehicle crash data for 2001 from the National Highway Traffic Safety Administration’s Fatality Analysis Reporting System (FARS) and the General Estimates System (GES) show that 90 percent of fatal and injury crashes that occur on the roadway, where lighting guidelines specify that light be placed, are multiple vehicle crashes. The number of overall crashes tapers off substantially after midnight on weekdays and after 4:00 a.m. on weekends. At these late hours, most of the crashes are single vehicle off roadway crashes for which lighting may not be likely to help, except possibly at decision making points such as ramp gores, intersections, and merge areas.
Warrants for lighting are empirically derived and based, among other things, on traffic volume. For lighting that has been installed based on traffic volumes, it may be reasonable to cut back the operation of the lighting system to complete interchange lighting or to partial interchange lighting when traffic volumes subside.
Studies show that crash rates can increase where systems are turned off or where every other luminaire is turned off. Alternate luminaire operation results in poor uniformity ratios. The issue of driver needs and safety versus conservation efforts should be closely examined when considering curfews. Some poorly conceived conservation efforts may contribute to excessive increases in traffic crashes and operational problems. These problems may actually result in higher overall costs.Anchor: #i1002333
Reasons for Curfews
Government entities around the world are considering lighting curfews for the following reasons:
- Low late-hours traffic volumes. AASHTO warrants for highway lighting are based, among other things, on traffic volume. When the primary reason that lighting is (or was) installed was due to high traffic volumes and high usage, which drops off in later hours of the night, it is reasonable to turn off or reduce the lighting after such drop-offs.
- To free up resources for greater overall safety. Lowering the operational costs of lighting systems by reducing electrical and maintenance costs through curfews may allow more lighting systems or other crash countermeasure to be installed, thereby reducing the overall nighttime crash rate within the jurisdiction of the master lighting plan.
- Technology now practical. Modern controls technology now allows control of individual luminaires or systems of luminaires at reasonable costs.
- Positive study results. Recent studies show that light dimming and turn-off curfews are viable options for management of public lighting systems, including roadway lighting. Although past studies showed unfavorable increases in traffic crash rates because of turning off lighting, these studies were performed on systems where the lighting was turned off or partially turned off for the entire nighttime period.
- Energy savings. Energy costs are high.
- Sky glow issues. Sky glow issues are of increasing importance to citizens.
Considerations Before Implementation
Special events, weather, and other local considerations should be included in the decision to implement lighting curfews. Implementation of curfews should occur through traffic management centers and should be monitored to gain experience as to the best operational procedures.
The following excerpts are from FHWA/RD-86/018: “Reduced Lighting During Periods of Low Traffic Density.” When considering lighting curfews, the complete report should be read, while realizing that it dates from August 1985. Modern crash data should be analyzed.
In part, FHWA/RD-86/018 states:
Over 50 percent of all motor vehicle fatalities occur in darkness even though only 25 percent of all travel occurs at night. This over representation has been used as a justification for installing fixed roadway lighting on many highways. However, research that has attempted to determine the effect of such fixed lighting on frequency and severity of night accidents appears to be mixed, such frequencies and severities being dependent on a host of geometric and traffic factors including the volume of traffic utilizing the road, how such volume is related to the road’s capacity, and the complexity of the driver’s visual search task.
During the past decade, several highway agencies have switched off roadway lighting during periods of energy shortages to reduce maintenance and operating costs. However, quite often such lighting was restored when nighttime accidents increased. One fundamental problem with these light reduction techniques was that lighting was reduced or eliminated during the entire nighttime period, rather than only when traffic volume was low.
By providing full lighting during periods when volumes are high and the roadway operates near capacity and providing reduced lighting as the traffic decreases, the potential exists for realizing considerable energy savings while still providing the benefits of full lighting at locations (e.g., interchanges) and at times (i.e., high volumes) where driver decision-making is the most critical and the greatest visibility is required.
From a safety standpoint there is a definite reduction in (simulated) hazard detection performance, which theoretically implies some reduction in safety. This implied reduction in safety is statistically significant for all off and one side only lighting tactics, but not statistically significant for the dimmed tactics and the every other off tactic. Unfortunately, it is not possible at this time to quantify the exact decrease in safety in terms of the frequency of nighttime accidents, the night accident rate, or the night-to day accident ratio. Only an evaluation of long term installations can address this issue (see following discussion of further research needs).
Reduced freeway lighting tactics normally should not be implemented before about 11:00 p.m. in most urban areas, since traffic density typically remains relatively high until that time. Regularly scheduled sports events and other large traffic generators could change this time to a later hour, while cities with little or no evening activity might allow an earlier light reduction.