4.1 Warrants

Lighting warrants assist in evaluating locations where lighting will maximize benefit based on defined conditions or rating systems. Meeting these warrants does not obligate the state or other agencies to provide lighting. Conversely, using engineering judgment in addition to warrants, considering things such as roadway geometry, high crash rates, or frequent occurrences of poor weather conditions such as rainfog, ice, or snow, may influence a decision on whether to install lighting.

Warrants indicate where lighting may be beneficial, but should not be interpreted as an absolute indication of whether or not lighting is required. The need for lighting should be determined by sound engineering judgment and rests with the agency having jurisdiction over the roadway.

Warrants do not represent a requirement to light, only an indication of situations where lighting should be investigated

4.2 AASHTO Warranting System

Warrants for highways, freeways, interchanges and bridges may be undertaken using the AASHTO Roadway Lighting Design Guide Warranting System. AASHTO defines warrants for Continuous Freeway Lighting (CFL), Complete Interchange Lighting (CIL) and Partial Interchange Lighting (PIL) based on warrant conditions including:

• Traffic volumes
• Spacing of freeway interchanges
• Lighting in adjacent areas
• Night-to-day crash ratio

AASHTO believes it is desirable to provide lighting on long bridges in urban and suburban areas even if the approaches are not lighted. On bridges without full shoulders, lighting can enhance both safety and utility of the bridges, and is therefore recommended. Where bridges are provided with sidewalks for pedestrian movements, lighting is recommended for pedestrian safety and guidance.

4.3 Warranting Method Example for Collector/Major/Local Streets

The warrant system presented is based on the Transportation Association of Canada (TAC) Guide for the Design of Roadway Lighting (27) which was based on the 1978 Roadway Lighting Handbook published by the U.S. Department of Transportation.

The warrant system is based on factors grouped into geometric, operational, environmental, and crash factors. For each factor a numeric rating (R) from 1 to 5 corresponding to the defined criterion is defined. Each criterion is assigned a weight (W) to indicate its relative importance. The rating value (R) is multiplied by the weight (W) to obtain a point-score (R x W) for each criterion characteristic, indicating its relative significance. The overall point-score for all items indicates the need for lighting, as well as the relative risk on that road compared with other roadways.

When undertaking a warrant analysis, the length of roadway segment being analyzed should be as long as possible, and should take into account future development. Where the roadway classification or roadway land use classification changes, a separate warrant analysis should be considered for each roadway section. Where classifications are relatively constant along the segment of roadway under consideration, a single warrant analysis may be undertaken.

Classification factors listed on the warrant sheets are defined as follows:

4.3.1 Geometric Factors

Includes key geometric factors listed for the length of roadway to which the warrant is being applied. These include:

• Number of lanes
• Lane width
• Number of median openings per kilometer
• Driveways and entrances per kilometer
• Horizontal curve radius
• Vertical grade
• Sight distance
• Parking

The worst-case rating factors (R) shall apply for the entire length of road being considered. The weighted value is very high for sharp horizontal curve radii.

4.3.2 Operational Factors

Includes operational factors for the entire length of roadway to which the warrant is being applied. These include:

• Signalized intersections
• Left turn lanes
• Median width
• Operating or posted speed
• Pedestrian activity (conflict) levels (ref to IESNA RP-8 for definition of high, medium or low activity)

The worst-case rating factors (R) shall apply for the entire length of road being considered. The weighted value is high for pedestrian activity level.

4.3.3 Environmental Factors

Includes environmental factors for the entire length of road to which the warrant is being applied. These include:

• Percentage of development adjacent to the roadway. Adjacent development must be a reasonable distance from the roadway and must tie into the roadway for which the warrant is being undertaken via a driveway or intersection which generates a reasonable amount of traffic. Determining the amount of ambient lighting present in an area depends on the judgment of the individual performing the warrant analysis. As a general guide, the following ambient lighting definitions may be applied:
  • Sparse - Would typically include rural freeways and highways with little or no development outside of city boundaries.
  • Moderate - Would typically include rural or urban roads with some building lighting and development outside of commercial areas. Areas with residential and industrial development will typically have moderate ambient lighting.
  • Distracting - Would typically be downtown commercial areas with well-lighted building exteriors adjacent to the roadway. Distracting lighting can also include that from fuel stations, automotive sales lots and other commercial development where lighting is used to attract attention to businesses.
  • Intense: Would typically be areas with large advertising signs, sports lighting, and other intense light sources adjacent to the roadway. Intense sources can be found in both rural and urban areas.
• Area classification
• Distance from development to roadway
• Ambient Lighting
• Raised median curb

The worst-case rating factors (R) shall apply for the entire length of road being considered. The weighted value is high for ambient lighting.

4.3.4 Crash Factors (Night and Day)

In the warranting forms crash factors are included using the night-to-day crash ratio for the given length of road to which the warrant is being applied. As the warrant point-score for this category is heavily based on night-to-day crash ratios, it is essential that detailed and well-defined crash data be applied. Where crash ratios are not known, engineering judgment should be applied using crash statistics from similar roads where data is available.

Where a low number of crashes have been recorded (i.e., two at night, and one during the day), lighting may meet the warrant crash ratio; however, due to the low numbers it may be of less benefit than for other areas with similar ratios and higher numbers.

Excerpt from the TAC Guide for the Design of Roadway lighting (27)

Lighting is warranted where a total point-score of 60 or more is achieved. If the night-to-day crash ratio is 2:1 or greater, lighting is automatically warranted regardless of the overall point-score.

Lighting may be prioritized solely on the basis of the point-scores, or in conjunction with a benefit/cost analysis. Benefits would typically be based on the potential reduction in crash frequency and severity. Depending on road authority practice, costs would typically include the initial cost of the lighting system, its ongoing (electricity) costs, and its maintenance costs. Initial costs may be substantial if a power source is not present.

4.4 Warranting Method for Intersections

The Transportation Association of Canada Guide for the Design of Roadway Lighting includes a warranting system for intersection lighting. The warranting system is based on geometric, operational, environmental and crash factors. The critical factors determining the need for illumination are traffic volumes and night-time crashes. The warrant point score indicates whether full intersection lighting, partial lighting or delineation lighting is needed. Full intersection lighting denotes illumination covering an intersection in a uniform manner over the traveled portion of the roadway. Partial lighting is the illumination of key decision areas, potential conflict points, and/or hazards in and on the approach to an intersection. The illumination of vehicles on a cross street or median crossing, or lighting that marks an intersection location for approaching traffic, is referred to as sentry or delineation lighting.

The critical factors used to determine the need for illumination include the following:

• Traffic volumes (particularly on the cross street).
• The presence of crosswalks.
• Nighttime crashes that may be attributed to the lack of illumination.
• The extent of raised medians.
• Several secondary factors are also considered in the warrant, but are given less weight in the overall point-score. In the warrant, traffic volumes and nighttime crashes are given greater weight than raised medians, which can be designed, marked, or modified to reduce the risk associated with its presence in the roadway.

The following terminology is used with respect to the amount of lighting, as determined by the warrant system:

• Full Lighting – Denotes lighting covering an intersection in a uniform manner over the traveled portion of the roadway.
• Partial Lighting – Denotes lighting of key decision areas, potential conflict points, and/or hazards in and on the approach to an intersection. Partial lighting may also guide a driver from one key point to the next, and (if sufficient luminaires are used) place the road user on a safe heading after leaving the lighted area.
• Delineation Lighting – Denotes lighting that marks an intersection location for approaching traffic, lights vehicles on a cross street or lights a median crossing.

Based on the warrant analysis (the warranting form can be found in the TAC Guide for the Design of Roadway Lighting Document (27)), the following conditions define the need for full, partial or delineation lighting:

• If the intersection is signalized, full lighting is warranted.
• If the intersection is not signalized, the need for and the amount of lighting is indicated by comparing the point-score obtained from the warrant form categories to the following criteria:
  • Full Lighting – Is warranted where a total point-score of 240 or more points.
  • Partial Lightning – Is warranted where the point-score is between 151 and 239 points.
  • Delineation Lighting – Is warranted where the point-score is between 120 and 150.
  • No Lighting – Generally, a point-score under 120 indicates that lighting is not warranted. This score indicates that neither the critical operational warranting factor (substantial traffic volumes) nor the critical crash warranting factor (repeated nighttime crashes) is present.

Lighting may be prioritized solely on the basis of the point-scores, or in conjunction with a benefit/cost analysis. Benefits would typically be based on the potential reduction in crash frequency and severity at the intersection. Depending on road authority practice, costs would typically include the initial cost of the lighting system, its ongoing (electricity) costs, and its maintenance costs. Initial costs may be substantial if a power source is not present at the intersection.

4.5 Other Examples of Intersection Warranting

Some authorities have looked at simple ways to prioritize lighting needs, particularly with rural intersections. Preston and Schoenecker (1999) (16) developed a system using traffic volumes on the major street by functional classification to give a priority to lighting intersections.

Figure 17 – Prioritization of Street Light Installations by Functional Class