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Publication Number:  FHWA-HRT-14-051    Date:  July 2014
Publication Number: FHWA-HRT-14-051
Date: July 2014


Design Criteria for Adaptive Roadway Lighting


As mentioned, adaptive lighting is the adjustment of lighting based on the current conditions of the roadway. The following parameters can change through the night and can influence the lighting level required to maintain safety:

As these conditions change, the roadway class may be reevaluated and the lighting requirements adjusted. For an adaptive lighting solution to produce a financial benefit, the lighting system must have the following components:

The recommended technological approach to adaptive lighting is dimming. In the past, reduced lighting on roadways was typically accomplished through switching or "half-code" lighting, in which every other luminaire or the luminaires on one side of the roadway are turned off or removed. This is not considered an acceptable solution, because it is not possible to meet design criteria for uniformity and glare control using half-code lighting. In contrast, dimming a luminaire allows the light level to be adjusted without upsetting the other design criteria. Dimming luminaires are typically capable of dimming from 100-percent output to anywhere between 50 and 10 percent of maximum output, depending on the light source technology.

Monitoring the lighting system and the current lighting level is an integral part of the lighting control system implementation. Linking a monitoring system to the control system provides continuous feedback on the current condition of the lighting system. These types of control systems allow control of the lighting level as an asset.

The impact of weather was not included in this project. While the number of crashes in adverse conditions can be determined, exposure of the driver to weather conditions cannot be calculated. Accordingly, crash rates have not been established for various weather conditions. Other studies that have considered crashes on wet roads found that they significantly affected object visibility in the presence of adaptive lighting.(16) At present, it is recommended that during periods of adverse weather, the lighting level should be at the design level.

Using the light level selection methodology, when one of the parameters changes in the table, the lighting class would be recalculated and the lighting level could be changed. Obviously, not all criteria will change; some may change nightly (Pedestrian Usage), and some may change over a long period of time (Guidance).


The traffic volume parameter in the selection of the lighting level is ADT. While this is an effective parameter for warranting lighting, it is not practical for the application of adaptive lighting. ADT does not provide insight into the change in traffic levels over the course of the day. An analysis of the hourly traffic volumes was undertaken to provide data for the selection of the lighting level based on an immediate condition rather than the ADT.

In this analysis, the hourly traffic volume and the hourly crash rates were used to identify the relationship between traffic volume and the lighting system. The results are shown in figure 30. As before, it was determined that there was no overall relationship with the lighting level. There was some variation, but in general, the lighting level did not affect safety. However, it is noteworthy that the general safety of the roadway is affected by the hourly traffic volume, with the lowest volume road having a higher relative crash rate than higher volume roads. The ADT metric produced a similar result. Again, it is believed that this represents a form of cooperative safety on roadways with significant vehicle volume, meaning that as the traffic volume increases, the presence of other vehicles reduces the crash rate until the roadway is saturated, at which point the crash rate increases.(20)

This figure is a line graph with night-to-day crash ratio on the y-axis, ranging from 0 to 3.5, and illuminance range on the x-axis, ranging from 0-0.5 to more than 18 lux, for roads with 4 different hourly traffic volumes: 0-500 vehicles per h, 500-1,500 vehicles per h, 1,500-3,000 vehicles per h, and more than 3,000 vehicles per h. The graph has four traces, one for each hourly traffic rate. The trace for the lowest hourly traffic volume, 0-500 vehicles per h, has the highest night-to-day crash rate ratio, between approximately 1.75 and 3.5, for all illuminance ranges. The highest night-to-day crash rate ratio was approximately 3.25, occurring at an illuminance range of more than 18 lux. The trace for the next-lowest hourly traffic volumes, 500-1,500 vehicles per h, had the next highest night-to-day crash rate ratios, between 1 and just over 1.5 for all illuminance ranges. There was little variation in night-to-day crash rate ratio for this hourly traffic volume. The traces for the two highest hourly traffic volumes, 1,500-3,000 vehicles per h and more than 3,000 vehicles per h, showed very similar night-to-day crash rate ratios of approximately 1 between illuminance ranges of 0-0.5 and 12-15 lux. At illuminance ranges of 15-18 lux and more than 18 lux, the day-to-night crash rate ratio for roads with 1,500-3,000 vehicles per h was more than 1. For the same illuminance ranges, the day-to-night crash rate ratio for roads with more than 3,000 vehicles per h reached a minimum of below 0.5 at more than 18 lux.
Figure 30 . Graph. Relationship between mean hourly traffic flow and weighted night-to-day crash rate ratio.

Because of the lack of relationship between lighting level and hourly traffic volume, there is flexibility in how the lighting level may be changed as traffic volume changes. There is no one performance measure that defines the complexity of the operation of the traffic stream, but the level of service (LOS) is commonly used to translate this complexity into a simple categorical system from A to F.(22) LOS A describes primarily free flow operations where vehicles are only dependent on their ability to maneuver within the traffic stream. As the LOS deteriorates, the ability to maneuver within the traffic stream becomes more restrictive, and drivers experience a reduced comfort level until reaching LOS F, which describes breakdown or unstable flows.

It is important to differentiate between the different operational conditions of a road's uninterrupted flow facilities and interrupted flow facilities. Uninterrupted flow facilities have no fixed cause of delays. These types of operations include freeways and multilane highways (four to six lanes in each direction and posted speed limits between 40 and 55 mi/h (64 to 89 km/h)) and urban streets with two or more lanes in each direction that have traffic signals spaced an average of 2 mi or more apart. For this type of facility, LOS is defined by density (passenger cars (pc)/mi/lane) computed as the flow rate (pcphpl (passenger cars per hour per lane)) divided by the speed (mi/h). Several factors affect the LOS, including the free flow speed, terrain and road geometric characteristics, percentage of heavy vehicles, and driver population factors.

The resulting calculations for the volume for uninterrupted flow for highway and streets by the LOS are shown in table 27. These values are the threshold values where the LOS transitions from one level to another.

Table 27 . Threshold traffic volume per direction based on LOS transitions.


Threshold Density

Two Lanes

Two Lanes

A to B




B to C




C to D




D to E




E to F




        1Lane width 12 ft (3.7 m), 6-ft (1.8-m) lateral clearance,
5 percent trucks, level terrain, population factor 1,
     PHF 0.94, 3 ramps per mi, speed 65 mi (105 km/h).
        2Lane width 12 ft (3.7 m), 6-ft (1.8-m) lateral clearance,
3 percent trucks, level terrain, population factor 1,
             PHF 0.95, 20 access points per mi, speed 45 mi (72 km/h).

For real-time operations, the agency can compute density using loop detector data and create its own threshold values based on the loop detector data.

Contrary to uninterrupted flow, interrupted flow facilities have fixed causes of periodic delay, such as traffic signals that make the traffic stop periodically independent of the level of traffic. In this case, the traffic flow patterns are the results not only of vehicle interactions but also of the traffic control used at the intersections and the frequency of access points. Signal timing and the prevailing conditions affect the operations and observed volumes. The resulting calculations for the volume for interrupted flow for streets and residential is shown in table 28.

Table 28 . Threshold traffic volume per direction based on LOS transitions
for interrupted flow.


Two Lanes

One Lane













1Traffic cycle length 120 s, weighted average green
time g/C 0.45, 10 percent of left turns, 10 percent of
right turns,signal spacing 1,500 ft (457.2 m), number
of access points/mi 10, posted speed 45 mi/h
(72 km/h), saturation flow rate 1,900 pcphpl,
facility length 2 mi (3.2 km).
2Traffic cycle length 120 s, weighted average green
time g/C 0.45, 10 percent of left turns, 0 percent of
right turns, 1,050 ft (320 m) signal spacing, 20 access
points/mi, posted speed 30 mi/h (48 mi/h), saturation
flow rate 1,900 pcphpl, facility length 2 mi.
N/A = not applicable.

The criteria for hourly adjustment of the lighting level, as a result of the above calculations, based on traffic volume, are shown for roadways, streets, and residential areas in table 29, table 30, and table 31, respectively. These values are rounded values based on the transition from LOS B to C and the transition from LOS C to D. These levels were selected because they represent when the road reaches maximum free flow (B to C) and when crash rates begin to increase (C to D).(22) Thesevalues are recommended initial levels, and the agency is encouraged to produce new thresholds based on the specific conditions of the facility where the system will be implemented.

Table 29. Hourly traffic flow criteria for roadways.




Weighting Value

Traffic Volume


> 2,000 vehicles hourly



1,000-2,000 vehicles hourly



< 1,000 vehicles hourly


Table 30 . Hourly traffic flow criteria for streets.




Weighting Value

Traffic Volume


> 1,500 vehicles hourly



750-1,500 vehicles hourly



< 750 vehicles hourly


Table 31 . Hourly traffic flow criteria for residential/pedestrian roads.




Weighting Value

Traffic Volume


750 vehicles hourly



300-750 vehicles hourly



300 vehicles hourly


An agency may choose to recalculate these limits for its own specific roadway conditions.


In terms of the trigger for adapting the lighting system, two approaches are typically used. The first is curfews, in which the lighting system changes at a predetermined time, and the other is monitoring of the roadway environment.


Curfews are used to adapt lighting systems during defined time periods. These curfew times should be established based on an evaluation of the parameters of interest. An average of the traffic and pedestrian volumes can be evaluated on an hourly basis and used to determine the timing of the adaptive changes. The adaptive cycle should also be able to be overwritten to allow special events.

Roadway Monitoring

Active monitoring of the roadway through pedestrian counts and vehicle counts is an alternative to curfews. Active monitoring would require loop detectors or review of roadway video to determine the roadway criteria that would then drive the light level selection.

The resource requirements for a control and monitoring system may be significant, although they may become less demanding if connected vehicle and connected infrastructure technologies provide a new source of traffic and pedestrian volumes.


The size of the area of the lighting system to be adapted must be considered. Dimming of the roadway system can occur broadly over all the roadways within a given area, or selective dimming of sections of the roadway network can be implemented, based on an analysis of needs.

In general, dimming a large area will maintain a constant lighting level throughout the roadway so that drivers will not experience a bright lighting condition on one roadway and then turn onto a dark roadway and be forced to significantly transition their eye adaptation level, which can be uncomfortable and dangerous. However, dimming a large area may also cause some areas to be too dark.

It is recommended that each street be evaluated in terms of its lighting needs. However, the difference in lighting classes for streets in a given vicinity should be no greater than two. It is also recommended that residential areas be adapted to a single lighting level. For roadway facilities, each roadway should be assessed individually, but drivers should not experience greater than a two-level change in the lighting class. Transitions on roadways should be a maximum of 1 class per mi of travel.


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