Design Criteria for Adaptive Roadway Lighting

CHAPTER 3. LIGHTING LEVEL SELECTION METHODOLOGY

The system that provides the most flexibility in terms of adaptive lighting is the system proposed by the CIE.⁽¹¹⁾ This system provides both a methodology to select the lighting design level and a method of adapting the lighting level based on specified criteria for individual roadways.

During the course of this project, the parameters of the lighting system were established based on the criteria of the system. The collected data were explored for aspects of the lighting system that are statistically significant to the crash rate and the NTDCRR. As a result of evaluating these classification systems and the data from this research, a new methodology has been developed based on an analysis of vehicle crashes and lighting level.

The system proposed here is a more complete classification system, which is needed to fully apply the benefits of adaptive lighting for roadways. The methods presented here are based on the CIE method documented in CIE 115.⁽¹¹⁾ However, the CIE did not provide parameters to define each of the classification levels, and these have been added to the tables. Additional metrics were added, such as links to the IES requirements and considerations of traffic volume, geometric design, and pedestrian volumes. There was also a reduction in the number of lighting classes to reduce the possibility of over-lighting and adding additional complexity.

Three different selection criteria are used for the lighting level based on the type of facility being designed. The IES separates design criteria into roadways, streets, and residential or pedestrian facilities as follows:

• Roadway lighting is provided for freeways, expressways, limited access roadways, and roads on which pedestrians, cyclists, and parked vehicles are generally not present. The primary purpose of roadway lighting is to help the motorist remain on the roadway and help with the detection of obstacles within and beyond the range of the vehicle's headlamps.
• Street lighting is provided for major, collector, and local roads where pedestrians and cyclists are generally present. The primary purpose of street lighting is to help the motorist identify obstacles, provide adequate visibility of pedestrians and cyclists, and assist in visual search tasks, both on and adjacent to the roadway.
• Residential/pedestrian area lighting is provided primarily for the safety and security of pedestrians and not specifically for the driver. These facilities typically have driving speeds less that 25 mi/h (40 km/h), at which speed vehicle headlamps provide adequate lighting for the driver.⁽¹⁷⁾ It should be noted that the recommended levels in this lighting category are measured in vertical illuminance rather than luminance on the roadway. Previous research has shown that vertical illuminance is the best metric for pedestrian and object visibility.⁽¹⁵⁾

Once the facility type has been selected, the characteristics of the facility are used as weighting functions to determine the requirements of the lighting system. The sum of these weighting values is then subtracted from a base value. This result determines the lighting class. The lighting level is then determined from the lighting class. The equation for the lighting design class is shown in figure 27.

Figure 27 . Equation. Lighting class.

It is important to note that the base value changes based on the facility type. In this calculation, if the result is not a whole number, the next lower positive whole number is used (e.g., an H3.5 would use the H3 values). Negative numbers would result in applying the highest lighting level class. Similarly, if the calculated class number is higher than the highest class number, the highest class number is used.

Tables for the weighting parameters for roadway, street, and residential/pedestrian facility types are shown in table 21, table 23, and table 25, respectively. Similarly, lighting design levels based on the lighting class for roadway, street, and residential/pedestrian facilities are shown in table 22, table 24, and table 26, respectively. The base values for each of the facility types are also provided below.

For an adaptive lighting system in which the lighting level is changed based on the conditions of the roadway, the weighting factors are changed as the roadway conditions change, which determines a different lighting class and therefore a different required design level.

SELECTION CRITERIA

The following sections describe the criteria. The minimum values shown in the results of the data collection and analysis provide the basis for this system; additional lighting levels may be required to meet the requirements of the selection methodology.

Speed

Previous research has shown that drivers' visual behavior changes based on the roadway conditions.⁽¹⁸⁾ This research modeled the eye glance behavior of drivers in various roadway conditions. The results showed that as the roadway speed increases, the driver's eye glance pattern becomes restricted to a narrower range of glances. This results in blurring the peripheral field of view and limiting the visual field of the driver at higher speeds. As a result, it is recommended that a higher lighting level be used for roadways with higher speeds.

The literature shows that there is a tradeoff.⁽¹⁹⁾ Drivers have a tendency to increase their driving speed when lighting is present. However, drivers moving at higher speeds will likely benefit from higher lighting levels.

Traffic Volume

In the data analysis, the roadway ADT was not found to be significantly linked to the night-to-day crash rate ratio. In fact, in some of the analyses, the lighting level actually showed less impact at higher traffic volumes. This relationship is actually predictable. At high volumes, a cooperative safety effect is evident.⁽²⁰⁾ With increasing traffic volume, there is a reduction in crashes until the point where the traffic volume becomes saturated and the crash rate begins to increase. These data indicate that at certain volumes, the vehicles in front of a driver provide a clear path and limit the potential for a crash.

The impact of ADT for highway and street conditions is shown in figure 28 and figure 29. (Note that these figures are using the AADT as a surrogate for ADT.) In these figures, the blue line represents the lowest ADT but also the highest crash rate. The lighting level has an impact on reducing crash risk for the roadways, but no specific improvement level can be determined. As a result, it is recommended that the lighting level be increased on roads with high ADT and reduced on roadways with low ADT.

Figure 28 . Graph. Relationship between mean lighting level and weighted night-to-day crash rate ratio for highways by AADT.

Figure 29 . Graph. Relationship between mean lighting levels and weighted night-to-day crash rate ratio for streets by AADT.

Presence of a Median

The presence of a median in the roadway is significant because it blocks the glare from the other side of the roadway. Oncoming vehicles cause drivers both discomfort glare and disability glare. Research has shown that lighting can overcome the impact of this glare.⁽¹⁵⁾ A high barrier or wide median can block or limit glare from oncoming vehicles. In these cases, the glare from the other vehicles is limited, so less light on the roadway is required. As a result, the weighting factor for the presence of a barrier allows the light level to be reduced if opposing glare is blocked.

Interchange Density

This factor considers the number of vehicles that enter and exit a roadway at legitimate intersections, driveways, and interchanges. Griffith showed that the impact of lighting only the interchanges for urban freeways is 12 percent less effective than lighting the entire roadway, whereas not lighting the roadway had a 50-percent impact on safety.⁽²⁾ This indicates that lighting has a much greater impact at interchanges than in the other segments of a roadway. As a result, a roadway with closely spaced interchanges requires a higher lighting level than a roadway with greater spacing between interchanges. The weighting parameter for interchange density allows a lower lighting level for roadways with greater spacing between interchanges and a higher lighting level for roadways with closely spaced interchanges. Because of a lack of references regarding the relationship between lighting, safety, and density of at-grade intersections, the weighting parameter for interchange density is also used for intersections.

Ambient Luminance

The ambient luminance refers to the amount of light around roadways. Ambient light tends to come from many directions and usually has a high vertical component. To limit the vertical-to-horizontal illuminance ratio, the lighting level on the roadway should be increased to account for the higher ambient lighting condition. The weighting function for this parameter is based on the IES Lighting Zones (LZ), defined as follows.⁽⁴⁾

LZ0: No ambient lighting

LZ0 represents areas where the natural environment will be seriously and adversely affected by lighting. Effects include disturbing the biological cycles of flora and fauna and detracting from human enjoyment and appreciation of the natural environment, although human activity is less important than nature. The vision of human residents and users is adapted to the total darkness, and they expect to see little or no lighting. When not needed, lighting should be extinguished, although lighting is not typically used in an LZ0 condition.

LZ1: Low ambient lighting

LZ1 represents areas where lighting might adversely affect flora and fauna or disturb the character of the area. The vision of human residents and users is adapted to low light levels. Lighting may be used for safety and convenience, but it is not necessarily uniform or continuous. After curfew, lighting may be extinguished or reduced as activity levels decline.

LZ2: Moderate ambient lighting

LZ2 represents areas of human activity where the vision of human residents and users is adapted to moderate light levels. Lighting may typically be used for safety and convenience, but it is not necessarily uniform or continuous. After curfew, lighting may be reduced as activity levels decline.

LZ3: Moderately high ambient lighting

LZ3 represents areas of human activity where the vision of human residents and users is adapted to moderately high light levels. Lighting is generally desired for safety, security, or convenience, and it is often uniform and/or continuous. After curfew, lighting may be reduced as activity levels decline.

LZ4: High ambient lighting

LZ4 represents areas of human activity where the vision of human residents and users is adapted to high light levels. Lighting is generally considered necessary for safety, security, or convenience, and it is mostly uniform and/or continuous. After curfew, lighting may be reduced in some areas as activity levels decline.

Guidance

This parameter refers to the quality of the signage and pavement markings in the area to be lighted. Minimum marking and signage levels have been determined for both dry and wet conditions in a variety of tests. One of these conditions included lighting as a parameter in the evaluation.⁽²¹⁾ This research showed that lighting increased the visibility of the pavement markings in all conditions. Lighting, therefore, is required when lane delineation and guidance are poor.

The weighting value for the guidance system allows a lower lighting level where marking and sign retroreflectivity are maintained at the Federal recommended minimums.

Pedestrian Presence

This parameter is applied only in the street and pedestrian roadway categories; highways are not expected to have pedestrians. The eye-tracking research mentioned earlier found that the eye behavior in areas where pedestrians are present expands to include a much wider area of the roadway and that the peripheral vision of the driver is more important. As with the interchange and intersection parameter, the potential for conflict increases with additional pedestrians. The lighting level is then increased based on the number of pedestrians along the roadway. The lighting levels should therefore be increased with additional pedestrian presence.

Presence of Parked Vehicles

Like pedestrians and the intersections, parked vehicles provide an additional potential form of conflict. Pedestrians and other vehicles can appear around parked vehicles, so additional lighting should be provided in these areas.

Facial Recognition

This final parameter should be used very carefully. To provide adequate lighting for the recognition of faces, additional lighting is required. Research has shown that light levels in excess of 30 vertical lux are required to adequately recognize faces. It is unclear when facial recognition would not be important in a residential or pedestrian (P-class) environment, but if an agency or authority determines that it is not important, the lighting level can be reduced.

DESIGN CRITERIA FOR ROADWAYS (H-CLASS)

Base Value for Class: 5

Table 21 . Roadway design level selection criteria.

Table 22 . H-class lighting design levels.

DESIGN CRITERIA FOR STREETS (S-CLASS)

Base Value for Class: 6

Table 23 . Street design level selection criteria.

Table 24 . S-Class lighting design levels.

DESIGN CRITERIA FOR RESIDENTIAL/PEDESTRIAN AREAS (P-CLASS)

Base Value for Class: 6

Table 25 . Residential/pedestrian design level selection criteria.

Table 26 : P-class lighting design levels.

Conflict Areas

Lighting for other conflict areas, such as intersections and crosswalks, can also be adjusted in relation to the roadway, street, and residential/pedestrian lighting levels. For example, the lighting level recommended for intersections included in ANSI/IES RP-8-00 is the sum of the lighting levels of the intersecting roads.⁽⁴⁾ If the intersecting roads have had a change in use, allowing a reduction in lighting levels, lighting at the intersection would also be reduced.