Guidelines for The Implementation of Reduced Lighting on Roadways

Chapter 1 Roadway Lighting Level Selection

Regardless of the adaptability of a lighting system, the selection of the appropriate lighting level for the given roadway is the key component for the implementation of the system. This document provides a new method for the selection of a lighting level based on the relationship between lighting and safety.

A more complete classification system than that of the IESNA and AASHTO is needed to obtain the benefits of adaptive lighting for all types of roadways. The AASHTO and IESNA lighting guides do not provide methods to calculate changes to the recommended light levels for high-speed roads in response to changes in the driving environment. The classification methods used by the ILE and CIE evaluate such factors as traffic volume and geometry in addition to pedestrian volumes. After evaluating these classification systems and the data from the current research, a new methodology has been developed based on an analysis of vehicle crashes and lighting levels. The proposed methodology is a modification of the method currently recorded in CIE Document 115.⁽²⁾

This proposed approach has been developed from the results of an analysis of crashes and lighting levels on the roadway. For this analysis, lighting data were measured in situ on a variety of roadways in seven different states. Crash data were then used to determine the relationship of the lighting data to the crash rate. From these results, a variety of criteria were determined to be significant to the lighting–safety relationship, including traffic volume and roadway type. Other criteria were added to the selection process based on design approaches and relevant literature. An accompanying document, Design Criteria for Adaptive Roadway Lighting, provides more detail on the criteria listed here.⁽⁶⁾

For the proposed methodology, three different selection criteria based on the IESNA approach are used to determine the lighting level. The IESNA separates design criteria for the following facilities: roadways, streets, and residential/pedestrian:

• 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 aid detection of obstacles within and beyond the range of vehicle headlamps (figure 2).
  
  

  

  Figure 2. Photo. Roadway lighting.

• Street lighting is provided for major, collector, and local roads on which pedestrians and cyclists are generally present. The primary purpose of street lighting is to help motorists identify obstacles, provide adequate visibility of pedestrians and cyclists, and assist road users in visual search tasks both on and adjacent to the roadway (figure 3).
  
  

  

  Figure 3. Photo. Street lighting.

• Residential/pedestrian area lighting is provided primarily for the safety and security of pedestrians, not specifically for drivers. These facilities typically have driving speeds slower than 25 mi/h (40 km/h), where vehicle headlights provide adequate lighting for drivers (figure 4).
  
  

  

  Figure 4. Photo. Residential/pedestrian area lighting.

Because these three facility types—roadway, street, and residential/pedestrian —form the basis for the lighting requirement selection methodology, each is characterized as an H (roadway), S (street), or P (residential/pedestrian) class. Each of these classes has a specific set of criteria for the selection of the lighting requirements.

As with the method in CIE Document 115, once the facility type has been selected and the class identified, the characteristics of the facility are used as weighting factors to determine the requirements of the lighting system. The equation for the lighting design class is shown in figure 5.

Figure 5. Equation. Lighting class.

The base value changes depending on the facility type; the sum of the weighting values is then subtracted from this base value. This result determines the lighting class. The lighting level is then determined from the lighting class. If the result is not a whole number, the next lower positive whole number should be used (e.g., an H3.5 would use the H3 value). Negative numbers call for the highest lighting level class (i.e., H1, S1, and P1 are the highest classes because they have the highest lighting requirements). Similarly, numbers resulting in a class lower than the lowest class would default to the lowest class (e.g., a lighting class of H6 would use the H4 value).

For an adaptive lighting system, the lighting level requirements change based on the roadway conditions. In response, the current approach calls for changes in the corresponding weighting factors as the roadway conditions change, which determines a different lighting class and, therefore, a different required design level.

Parameters

The parameters for each of the weighting factors are defined below. Each of these parameters has been determined to be an important aspect of the driving environment based on its relationship to vehicular crashes.

Speed

The speed parameter is the posted speed of the roadway, as opposed to the design speed of the roadway. For an active adaptive system, the 85th percentile speed, or other measured speed of vehicles, can be used instead.

Traffic Volume

The traffic volume parameter typically used in the selection of a roadway lighting level is average daily traffic (ADT). While this is an effective parameter for selection of a basic lighting level, it is not practical for application of adaptive lighting. Actual traffic volumes vary by day of the week and hour of the day, which limits the applicability of ADT to driver needs at any particular time.

The hourly traffic volume of a roadway is a recommended parameter for the application of adaptive lighting because it is indicative of current roadway conditions. For hourly traffic volume parameters, the level of service (LOS), as defined by the Highway Capacity Manual, is used to determine the traffic flow level criteria for the adaptive lighting level.⁽⁷⁾

The capacity of a roadway is determined by the roadway speed, the number of lanes, the percentage of trucks, and a factor for the geometry of the roadway. This capacity is then characterized by an LOS grouping (i.e., Levels A through F, with A being low-traffic volume and F being high-traffic volume). The LOS of the roadway is determined by the density of the vehicles per lane. The Highway Capacity Manual should be referenced for additional information.⁽⁷⁾ For the three roadway types used for these guidelines (i.e., roadway, street, and residential/pedestrian), the LOS calculations were based on the assumptions in table 5.

Table 5. Assumptions made for the LOS calculation.

¹This value accounts for speed and capacity changes resulting from the impact of hills and curves.

The resulting calculations for the threshold traffic volume based on the LOS are shown in table 6.

Table 6 . Threshold traffic volume based on LOS.

N/A = not applicable.

As a result of these calculations, the criteria to be used for an hourly adjustment of the lighting level based on traffic volume are shown for roadways, streets, and residential/pedestrian areas in table 7, table 8, and table 9, 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).⁽⁸⁾ Note that the traffic volume values listed in the tables are for single-direction travel. Thus, the values would have to be doubled when applied to undivided roads.

Table 7 . Hourly traffic flow criteria for roadways.

Table 8 . Hourly traffic flow criteria for streets.

Table 9 . Hourly traffic flow criteria for residential/pedestrian roads.

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

Median

The median parameter defines the presence of a median barrier. Typically, a median is present on large roadways to separate the two directions of travel. The median must have a barrier or be designed such that the light from opposing headlamps is limited and not visible to drivers approaching each other. The AASHTO Roadside Design Guide defines a median width of 49.2 ft (15 m) where a barrier is not required in a roadway, and this median width is suitable to limit glare between vehicles.⁽⁹⁾ Median widths between 32.8 to 49.2 ft(10 to 15 m) require a design review with engineering judgment.

Intersection/Interchange Density

The intersection/interchange density parameter refers to the number of intersections and entrances into the roadway per mi or km. This parameter represents the possibility of vehicles interacting in the roadway. It includes not only other roadways but driveways and other entrance areas.

Ambient Luminance

The brightness and amount of light in the surrounding area affects the lighting requirements for the roadway and is accounted for in the lighting level selection. To differentiate lighting and ambient zones, the IESNA has developed Lighting Zones (LZ) describing different ambient lighting conditions.⁽¹⁰⁾

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 considered less important than nature in this zone. The vision of human residents and users is adapted to 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

The guidance parameter refers to the presence and quality of the other non-lighting-related visibility and guidance tools on the roadway. In particular, the quality of the pavement markings has been shown to interact with the lighting in terms of driver performance. The criterion presented here for guidance is the retroreflectivity of the pavement markings in millicandela/meter squared lux (mcd/m² lx).

Pedestrian/Bicycle Interaction

The pedestrian/bicycle interaction parameter refers to the number of pedestrians and bicycles present in the roadway, either crossing or walking parallel to the roadway.

Parked Vehicles

The parked vehicles parameter refers to the presence of parked vehicles along the side of the roadway.

Facial Recognition

The facial recognition parameter refers to the requirement of a driver or pedestrian to recognize the facial characteristics of a person walking in the roadway or on the sidewalk (figure 6 and figure 7). This function is related to the feeling of safety and security of the roadway users. Typically, facial recognition can be expected to always be an important aspect of the roadway environment.

Figure 6. Photo. Facial recognition under low lighting.

Figure 7. Photo. Facial recognition under high lighting.

Conflict Areas

Although not a specific criterion for the selection of the luminance level, conflict areas are a consideration in the design process and may be affected by the adaptive lighting design. Lighting in these conflict areas, such as intersections and crosswalks, can also be adjusted in relation to the lighting levels of nearby roadways, streets, and residential/pedestrian areas. For example, the IESNA RP-8 recommended lighting level for intersections is the sum of the lighting levels of the intersecting roads. In an adaptive lighting design, if a change in use of the intersecting roads allows a reduction in lighting levels, then the lighting level of the intersection, in turn, will also be reduced.

Design Criteria and Luminance Selection

The parameters described above are used to determine the lighting level. Table 10, table 12, and table 14 show the weighting parameters, while table 11, table 13, and table 15 show the recommended lighting design levels (based on the lighting class) for roadway, street, and residential/pedestrian facilities, respectively. The base values for each of the facility types are also provided for each classification. It is important to note that the lighting design criteria for the residential/pedestrian areas are horizontal and vertical illuminance, not luminance. Road luminance is the criterion for both roadway and street facilities.

It is also important to note that, as the weighting values change, the lighting level can change. For example, the traffic volume or the number of pedestrians can change, which affects the weighting value of that parameter. This, in turn, may change the roadway lighting class;therefore, the lighting level requirements may change as well. This is particularly critical with respect to the traffic volume and pedestrian levels.

Design Criteria for Roadways (H-Class)

Base value for class: 5

Design Criteria for Streets (S-Class)

Base value for class: 6

Design Criteria for Residential/Pedestrian Areas (P Class)

Base Value for class: 6

Example of Lighting Design Criteria Selection

The following example illustrates how to select a roadway lighting class. The parameters are the following:

• Speed limit of 70 mi (113 km).
• Equivalent of 35,000 ADT.
• 39-ft (12-m) median between opposing directions with no barrier.
• An average of 2 mi between interchanges.
• Zoned as an LZ3 lighting area.
• Has brand-new pavement markings measuring at 425 mcd/m² lx.

The resulting weighting functions are shown in table 16.

The resulting road class is H1 (weighting value total of 4 subtracted from base value of 5), and the lighting design level has an average luminance of 1, maximum-to-average uniformity ratio of 3, maximum-to-minimum uniformity ratio of 5, and veiling luminance ratio of 0.3.

If this design was for an active adaptive system, the lighting design level would be changed based on the roadway conditions. For example, if the traffic volume of the roadway decreased from 35,000 to 15,000 ADT, the weighting value of traffic volume would decrease from 1 to 0, and the road class would change from H1 to H2, allowing for a decrease in the lighting level from 1.0 to 0.8 cd/m² (shown in table 10 and table 11).

An alternative method for defining the traffic volume is to use hourly traffic volume. For example, if the hourly traffic volume was 4,100 vehicles per h (vph), the weighting value would be 1 (table 7). This would still result in an H1 class, assuming the same variables listed in table 16 are used. If the hourly traffic volume dropped to 1,000 vph, the weighting value would be -1, and the roadway classification would drop to H3.

A similar example is provided for a street facility. The design level criteria are shown in table 17. Here, a change in the number of pedestrians per h could result in a change in the recommended lighting level.

The sum of the weighting values is 6 in this example, which would be subtracted from the base value of 6, resulting in a value of 0. This value would infer the use of the lighting class S1 (table 13). If the traffic volume changed to less than 5,000 vehicles per day and the pedestrian volume changed from more than 100 pedestrians per h to less than 10 per h, the sum would change from 6 to 4, allowing for a reduction in light levels at the S2 class.