TDOT Lighting Manual

Chapter 1: Introduction

The manual provides design guidelines and best practices for light-emitting diode (LED) illumination systems in Tennessee.
It adheres to the Tennessee Department of Transportation (TDOT) Standard Specifications for Road and Bridge Construction and TDOT Standard Drawings.
The manual aligns with current industry practices and is based on the American Association of State Highway and Transportation Officials (AASHTO) Roadway Lighting Design Guide.
Illumination needs should be justified through warrants for existing and proposed projects.
Meeting warrants does not obligate TDOT to design and install illumination systems; other considerations may be used.
The manual is for professional engineers and designers.
U.S. customary units (inch-foot-mile) are used.

1.1 Guideline Overview

The primary objective is to enhance roadway safety.
Properly designed lighting should enable motorists and pedestrians to quickly discern significant roadway details like:
- Roadway alignment
- Surrounding environment
- Obstacles on or near the roadway
- Vehicles, people, or animals that are about to enter the roadway
Lighting functions:
- Enables the driver to determine the geometry and condition of the roadway at extended distances
- Promotes safety at night by enhancing visibility for drivers and pedestrians
- Delineates the roadway and its surroundings, alerting motorists to potential obstructions and hazards
- Assists motorists in orienting themselves to the roadway’s geometry
- Illuminates long underpasses and tunnels during the day
- Discourages crime at night or in dark situations
- Enhances commercial and other activity zones to attract users

1.2 Engineering Requirements

Design and construction documents must be signed and sealed by a registered professional engineer experienced in roadway lighting and associated electrical systems.
Expertise required for TDOT lighting designs includes:
- Lamp types and characteristics, including depreciation factors
- Fixture mechanical characteristics lens types
- Photometric performance of luminaires and factors impacting such performance
- Fixture mounting types
- Pole mechanical and electrical characteristics
- Breakaway device options and their appropriate use
- Clear zone criteria
- Pole types, mounting options, and loading considerations
- Foundation and support details
- Pavement reflection factors
- Mounting height and spacing options
- Light trespass and sky glow (light pollution) issues, including laws and ordinances
- Lighting quality requirements, such as illuminance, luminance, veiling luminance, and visibility
- Lighting controls
- Electrical system requirements such as circuitry, voltage drop, and equipment sizing
- Maintenance considerations for individual components and the lighting system as a whole
- Energy and life cycle costs
- Coordination with master lighting plans
Designers should exercise engineering judgment when balancing all factors.

1.3 Funding Requirements

TDOT includes roadway lighting in state highway projects when certain conditions are met to improve safety.
Interstate Highway System:
- TDOT will typically prepare plans to install new roadway lighting as part of the related interstate highway construction project when the following conditions are met:
  - Freeway lighting is warranted by the Traffic Design Office and as prescribed by the Illuminating Engineering Society (IES), the AASHTO Roadway Lighting Design Guide, and the FHWA Lighting Handbook and these references’ latest revisions.
  - Roadway construction requires the replacement or relocation of the existing lighting, and the local governing agency agrees to maintain the installation.
Interstate Interchange Lighting:
- Interchanges not under construction or not eligible for other funding may be approved and lighting installed, provided that the local governing agency submits a request for interchange lighting to the TDOT Commissioner in writing.
- The local governing agency must also submit funding to cover 50 percent of the costs for interchange lighting to TDOT when the project is programmed.
Non-interstate Highways:
- TDOT generally does not replace or install new lighting on state routes and other non-interstate system highways.
- Installation or relocation of lighting on non-interstate system highways or related projects occurs only under the following specific circumstances:
  - Replacement of existing lighting impacted by construction on a state roadway project shall first be considered a utility relocation issue. The local agency shall be responsible for preparing relocation plans and submitting plans through the TDOT Utilities Office. The TDOT Regional Utilities Office will determine reimbursement eligibility. Relocation plans shall be completed by the local agency for review and approval by the TDOT Traffic Design Division.
  - Installation or relocation of roadway lighting in a state project typically occurs at the local agency’s request. The TDOT Traffic Services Manager shall approve the installation or relocation of roadway lighting projects.
  - The local governing agency may request that relocation be installed under the state project as a non-participating item when the local agency working through the TDOT Utilities Office prepares relocation plans and submits funds to cover relocation costs prior to letting.
  - All requests for roadway lighting installations on non-interstate highways will be reviewed and approved by the TDOT Traffic Services Manager.
  - Installation and maintenance of roadway lighting along state highways are the responsibilities of local governing agencies. Those agencies desiring to install street lighting must submit street lighting photometric data to TDOT and obtain utility permits. Contact the regional utility offices for more information on obtaining utility permits.

Chapter 2: Lighting Fundamentals

This chapter describes fundamental concepts and technical terminology related to roadway lighting and illumination.

2.1 Illumination

Per the Federal Highway Administration (FHWA), illumination describes the amount of light incident on a surface.
Two main aspects are the quantity and quality of illumination.
Quantity includes metrics such as luminous intensity, illuminance, and luminance.
Quality includes glare, uniformity of illuminance, and color rendition.

2.2 Lighting Metrics and Terminology

Luminous Intensity: Light-emitting power of a light source in a specific direction, measured in candela.
Illuminance: Measure of the amount of light falling onto a surface.
- Measured in lux (lumens per square meter; SI unit) or foot-candle (lumens per square foot).
- Types: horizontal (light on the ground) and vertical (light on a vertical surface, helps drivers see pedestrians).
Luminance: Amount of light emitted from a surface.
- Measured in candela per square meter.
- Brightness of a surface, a direct measure of what a driver sees on the illuminated roadway.
- Veiling luminance is the brightness of a light source compared with the roadway surface.
Glare: Sensation produced by luminances within the visual field that are sufficiently greater than the luminance to which the eyes are adapted to cause annoyance, discomfort, or loss in visual performance and visibility.
- Types: direct (from a light source) and indirect (from a bright object).
- Discomfort glare causes discomfort without visual impairment.
- Disability glare results in reduced visual performance and visibility.
Contrast: How clearly an object is visible compared with its background.
- Positive contrast: object is brighter than its background.
- Negative contrast: background is brighter than the object.
- Positive contrast is desirable.
Light Trespass: Light crossing into the surrounding neighborhood or area beyond the area it was intended to illuminate, causing light pollution.
Veiling Luminance Ratio: Ratio of maximum veiling luminance at the driver’s eye to the average pavement luminance.
- Indicates the veiling effect of glare as a percentage of average overall luminance.
- Used by the IES and AASHTO as a measure of glare.
Visual Comfort Probability: Percentage of drivers who will find the lighting system and its environment comfortable with regard to visual glare.
Uniformity Ratio: Evenness of the light distribution over a target area.
- Typically the ratio of average level of illuminance to the minimum level of illuminance.
- Also, the ratio of maximum level of illuminance to minimum level of illuminance is calculated.

2.3 Characteristics of LED Light Sources

Efficacy: Light output of the light source measured in lumens per watt. Higher is more efficient but doesn't measure light delivery efficiency.
Unit Power Density (UPD): Ratio of the rated luminaire watts to the amount of lighting delivered for a given area. A lower UPD indicates better lighting.
Correlated Color Temperature (CCT): Color appearance of a light source in degrees Kelvin (K).
- LEDs are available from 2,700 K (yellow tint) to 7,000 K (blue tint).
- HPS lighting is 2,200K, Incandescent is 2,800, Metal Halide is 4,000K, Direct Sunlight is 4,800K, and blue sky is 10,000 K.
Color Rendering Index (CRI): Color appearance of an object under a light source compared to a reference light source.
- Measure of the light source's ability to accurately reflect the true colors of the target objects, measured on a scale of 0 to 100.
- LEDs have the best CRI in the range of 70 to 95, suitable for roadway lighting.

2.4 Lighting Sources: HID Lamps

This manual recommends and focuses on LED lighting; HID lamps are not covered in detail.

2.5 Lighting Sources: LED Roadway Lighting

LED is a solid-state semiconductor lighting source device.

2.5.1 LED Light Sources versus Other Light Sources

Directional Light: LEDs emit light directionally (up to 180 degrees) unlike other sources, which emit light 360 degrees, resulting in higher energy efficiency.
Energy Consumption: LEDs offer high lumen output with low energy consumption, leading to higher energy efficiency. White LED packages have an improved efficiency of more than 200 lumens per watt.
Temperature Effects: LEDs work well in a wide range of temperatures without significant degradation; they work more efficiently in lower ambient temperatures.
Lifespan: Outdoor LED lamps have a useful lifespan (L70, when 70% of initial light is produced) of about 50,000 to 100,000 operating hours or more.
Carbon Footprint: LEDs have low carbon emissions compared with other lighting sources.
Size: LEDs use a small and bright point source of light, requiring less space.
Dimming and Voltage: LEDs have high dimming capabilities, can operate at power less than their rated power, and can operate at low voltages.

2.5.1.1 Advantages of LED over HPS

Higher energy efficacy: LEDs have around 200 lumens per watt, whereas HPS light sources have about 140 lumens per watt.
Longer lifespan: LEDs range from 50,000 to 100,000 hours, whereas HPS is about 24,000 hours.
Less heat emissions: LEDs emit much less heat, whereas almost 15 percent of the energy is lost as heat emissions in HPS lighting.
High color-rendering properties: LEDs have a CRI value between 65 and 95, whereas HPS lighting varies between 20 and 30.
Higher color temperature: LEDs are available in a wide range of color temperature values. Higher color temperature values produce cooler and bluish light.

2.6 Luminaire

A luminaire, also known as a light fixture, may be defined as an assembly of a lighting source, light reflectors, housing, and so on that is used to direct light emitted from the lighting source.

2.6.1 Luminaire Classification System

The Illuminating Engineering Society (IES) developed the Luminaire Classification System (LCS) for defining luminaire distribution and efficiency better.
IES classifies outdoor luminaires based on the shape of the area illuminated.
Table 2-1 lists the IES classifications by intensity distribution for outdoor luminaires:

Type	Description	Application	Plan View
Type I	Narrow, symmetric illuminance pattern	Walkway Path Roadway
Type II	Slightly wider, more asymmetric illuminance pattern	Walkways Roadways Bike paths
Type III	Wide, asymmetric illuminance pattern	Roadway (highways) Parking
Type IV	Asymmetric, forward throw illuminance pattern	Wall mount or pole mount	perimeter applications
Type V	Symmetrical circular illuminance pattern	Parking Area lighting

2.6.2 Backlight, Uplight, and Glare (BUG) Rating System

The Backlight, Uplight, and Glare (BUG) rating system quantifies the amount of trespass light produced by a light fixture.
Backlight (B) is the light directed behind the fixture, uplight (U) is the light directed upward above the horizontal plane and glare (G) is the light emitted from the luminaire at high angles.
BUG ratings are classified into zones based on the Model Lighting Ordinance by IES and the International Dark-Sky Association.
*
Lighting zones are categorized into the following five groups:
- LZ0: No ambient lighting – Areas such as “wilderness areas, parks and preserves, and undeveloped rural areas”
- LZ1: Low ambient lighting – Areas such as “rural and low-density residential areas”
- LZ2: Moderate ambient lighting – Areas such as “light commercial business districts and high density or mixed-use residential districts”
- LZ3: Moderately high ambient lighting – Areas such as “large cities’ business district[s]”
- LZ4: High ambient lighting – Special case areas such as “high intensity business or industrial zone districts”
For all zones, roadway lighting should consider the following BUG ratings:
- B: The lowest back light rating possible will achieve sidewalk lighting levels and surround levels, which should take priority in the design.
- U: Up-light rating would be U0 is recommended to reduce the potential of skyglow.
- G: The design should minimize the glare rating with a maximum G of 3.

2.7 Luminaire Efficiency

May be defined as the percentage of lamp lumens produced that actually exit the fixture.
It is the percentage of luminous flux emitted by the luminaire compared with the luminous flux emitted by the lamp or light source.
In general, the higher the efficiency of the luminaire, the more visual discomfort to the eyes.
Thus, a trade-off is required between luminaire efficiency and visual comfort to select the best luminaire to suit the site conditions.

2.8 Luminaire Maintenance

Ensures the proper functioning and long life of a lighting system.
Components requiring maintenance include lamps, luminaires, support structures and electrical components.
Categories of lighting system maintenance include luminaire maintenance, support structure maintenance, electrical distribution and control maintenance, and external factors.
The extent and frequency of maintenance should be considered while designing the lighting system.

2.8.1 Luminaire Maintenance Factors

Luminaire Dirt Depreciation (LDD): Deterioration of optical materials, reduction in output lumens, and change in distribution characteristics caused by dirt accumulation.
Lamp Lumen Depreciation (LLD): Gradual loss and decrease in the lumen output of the lamp due to the normal in-service aging of the light source.
Equipment Factor (EF): Accounts for luminaire losses resulting from factors like manufacturing tolerances, voltage drops, ballast/driver factor, etc.
Light Loss Factor (LLF): The product of the previous three loss factors: LDD, LLD, and EF.
LLF = LDD * LLD * EF
The LLF value to be used for LED calculations generally ranges between 0.80-0.90.

Chapter 3: Lighting Applications

3.1 Overview

Lighting applications vary with the type of facility.
The section describes applications and their warrants.
Warrants establish a basis but don't obligate the agency to provide lighting.
Warrants may be considered with safety, funding, and engineering judgment.
Lighting facilitates the safe flow of traffic by illuminating unusual conditions that require extra caution.
Applications and warrants adopted from Table 3-5a of the 2018 AASHTO Roadway Lighting Design Guide.

3.2 Lighting on Freeways

Freeways are divided highways with complete control of access.
Lighting types:
- Continuous freeway lighting
- Complete interchange lighting
- Partial interchange lighting

3.2.1 Continuous Freeway Lighting

Provides uniform lighting on all main lanes, direct connections, and interchanges.
Frontage roads are not normally continuously lighted.
Warranting conditions (Table 3-1):
- CFL-1: Sections in/near cities where ADT \ge 30,000.
- CFL-2: Three or more successive interchanges are located with an average spacing of 1.5 miles or less, and adjacent areas outside the right-of-way are substantially urban in character.
- CFL-3: Sections of two miles or more passing through a substantially developed suburban or urban area where:
  - Local traffic operates on a complete street grid having some form of street lighting, parts of which are visible from the freeway.
  - The freeway passes through a series of developments that include lighted roads, streets, parking areas, yards, etc.—that are lighted.
  - Separate cross streets, both with and without connecting ramps, occur with an average spacing of 0.5 miles or less, some of which are lighted as part of the local street system.
  - The freeway cross section elements, such as median and borders, are substantially reduced in width below desirable sections used in relatively open country.
- CFL-4: Sections where the ratio of nighttime to daytime crash rate is at least 2.0 times the statewide average for all unlighted similar sections, and a study indicates that lighting may be expected to result in a significant reduction in the night crash rate.
- CFL-5: Local agency finds sufficient benefit and agrees to finance the installation, maintenance, and operation of the lighting facilities.

3.2.1.1 Design Criteria

Lighting levels meet criteria if the three values (minimum, average, and uniformity) are taken collectively and not individually.
Luminance values for freeways (Table 3-2):
- Freeway Class A: Average Luminance L{avg} (cd/m^2) = 0.6, Uniformity Ratio L{avg} / L{{min}} = 3.5, Uniformity Ratio L{{max}} / L{{min}} = 6.0, Veiling Luminance Ratio L{{Vmax}} / L_{avg} = 0.3
- Freeway Class B: Average Luminance L{avg} (cd/m^2) = 0.4, Uniformity Ratio L{avg} / L{{min}} = 3.5, Uniformity Ratio L{{max}} / L{{min}} = 6.0, Veiling Luminance Ratio L{{Vmax}} / L_{avg} = 0.3
Illuminance values for freeways (Table 3-3):
- Freeway Class A:
  - R1: Average Maintained Horizontal Illuminance (Eh) = 0.6 (6 lux), Uniformity Ratio (Avg/Min.) = 3:1 to 4:1, Veiling Luminance Ratio LVmax/Lavg = 0.3
  - R2 and R3: Average Maintained Horizontal Illuminance (Eh) = 0.9 (9 lux), Uniformity Ratio (Avg/Min.) = 3:1 to 4:1, Veiling Luminance Ratio LVmax/Lavg = 0.3
  - R4: Average Maintained Horizontal Illuminance (Eh) = 0.8 (8 lux), Uniformity Ratio (Avg/Min.) = 3:1 to 4:1, Veiling Luminance Ratio LVmax/Lavg = 0.3
- Freeway Class B:
  - R1: Average Maintained Horizontal Illuminance (Eh) = 0.4 (4 lux), Uniformity Ratio (Avg/Min.) = 3:1 to 4:1, Veiling Luminance Ratio LVmax/Lavg = 0.3
  - R2 and R3: Average Maintained Horizontal Illuminance (Eh) = 0.6 (6 lux), Uniformity Ratio (Avg/Min.) = 3:1 to 4:1, Veiling Luminance Ratio LVmax/Lavg = 0.3
  - R4: Average Maintained Horizontal Illuminance (Eh) = 0.5 (5 lux), Uniformity Ratio (Avg/Min.) = 3:1 to 4:1, Veiling Luminance Ratio LVmax/Lavg = 0.3
Mounting height: 30 to 70 feet. Mid-mast light standards (40 to 70 feet) on the median.
Start with Type II or Type III light distribution without uplight (U0).
Spacing-to-mounting-height ratio is 6:1.
Use illuminance calculations for curves and hills.
Curves require closer spacing.
Viaducts and flyovers may require alternative lighting solutions.

3.2.2 Complete Interchange Lighting

Provides relative uniform lighting within the limits of interchange including main lanes, direct connections, ramp terminals, underpasses, and frontage roads or crossroad intersections.
Warranting conditions (Table 3-4):
- CIL-1: Total ADT ramp traffic entering and leaving the freeway within the interchange areas exceeds 10,000 (urban), 8,000 (suburban), or 5,000 (rural).
- CIL-2: Current ADT on the crossroad exceeds 10,000 (urban), 8,000 (suburban), or 5,000 (rural).
- CIL-3: Existing substantial commercial or industrial development is lighted during hours of darkness in the immediate vicinity of the interchange, or crossroad approach legs are lighted for 0.5 miles or more on each side of the interchange.
- CIL-4: Ratio of nighttime to daytime crash rate within the interchange area is at least 1.5 times the statewide average for all the unlighted similar sections, and a study indicates that lighting may be expected to result in a significant reduction in the night crash rate.
- CIL-5: Local agency finds sufficient benefit and agrees to finance the installation, maintenance, and operation of the lighting facilities.
- CIL-6: Complete interchange lighting should be considered where continuous freeway lighting is provided.
Design Criteria
- Lighting levels will be considered as meeting the criteria if the three values (minimum, average, and uniformity) are taken collectively and not individually.
- Luminance values for complete interchange lightings (Table 3-5):
- Freeway A: Average Luminance (cd/m^2) = 0.6, Uniformity Ratio = 3.5:1, Maximum Luminance Ratio LVmax/Lavg = 0.3
- Freeway B: Average Luminance (cd/m^2) = 0.4, Uniformity Ratio = 3.5:1, Maximum Luminance Ratio LVmax/Lavg = 0.3
Mounting height is typically 30 to 70 feet. Mid-mast light standards (40 feet to 70 feet) are typically included in large roadway projects and are typically installed on the median.
It is recommended to begin light distribution design with Type III without any uplight (U0).
It is recommended to design the spacing with the spacing-to-mounting-height ratio 5:1.
- Light standards should follow the roadway through the interchange.
- When possible, a light standard should be placed adjacent to the gore area at off-ramps. Placing light poles within the clear zone at a gore area is not desirable.
- Lights should be located on the insides of curves to minimize potential strikes from vehicles.
- Light standards present near residential neighborhoods have greater light trespass potential.
- Low backlight ratings (B0 through B2) and low glare ratings (G0 through G2) should be considered to minimize light trespass.
TDOT does not design, use, recommend, or approve partial interchange lighting.

3.3 Lighting on Streets and Highways

Urban streets and highways have high traffic volumes, at-grade intersections, turning movements, signalization, and varying geometry.
Installation is recommended for safety, efficiency, and comfort for motorists, pedestrians, and other modal transportation users.
TDOT recommends lighting on state routes next to interstates if these state routes are being widened.
Factors to consider: urban/rural, traffic volumes, intersections, turning movements, signalization, channelization, varying geometry.

3.3.1 TDOT Requirements

TDOT provides lighting for interstate highways and bridges.
New lighting installations will be reviewed by TDOT using breakaway, nonbreakaway, and utility distribution poles (joint usage).
Consider installing lighting to provide adequate levels of illumination; minimize the amount of glare; and reduce the number of poles required.
Street lighting plans submitted to TDOT must provide photometric calculations using AGI32 software only and the type of lighting equipment to be installed.
Poles used for streetlights must be shown on the lighting design.
Contact TDOT to discuss and resolve problems or concerns prior to lighting plan submittal.
The TDOT Traffic Services Manager will make the final administrative and engineering determinations.

3.3.2 General Design Considerations on Street Lighting

Recommended mounting height for existing utility poles is 45 feet. Mounting heights less than 45 feet may be approved in order to use existing utility poles to the fullest extent; however, the effectiveness of a satisfactory lighting job should not be jeopardized just to use existing utility poles.
In the relocation of utility poles on state highway right-of-way, every effort shall be taken to relocate these poles to provide for their use for roadway lighting.
Pole setback from the edge of the pavement shall be 20 feet minimum, or at the right-of-way line if located less than 20 feet from the edge of pavement.
In urban areas, poles shall be located as near to the right-of-way line as possible, but in no case shall they be less than 2 feet from the face of the curb.
Poles shall not be set in the median of the roadway, except where a 20-foot minimum setback can be obtained on both sides of the divided roadway, or where protected by an existing guardrail for other safety considerations.
Street lighting installed in depressed medians may be considered on a case-by-case basis, because this type of installation is a variance to TDOT’s street lighting policies.
Light standards may be installed in depressed medians that have a minimum width of 48 feet provided minimum clear zone requirements are met.
Mast arm length shall be no greater than 6 feet, except as approved by the TDOT Traffic Services Manager.
Concrete pole bases should be flushed but shall not extend over 4 inches above ground level. In parking lots, concrete bases shall be 3 feet above ground level for protection.
Lighting standard mountings shall be of an approved AASHTO breakaway type. Consider non-breakaway mountings in highly developed areas with high pedestrian activity.
All poles must be installed a minimum of 4 feet behind the face of the guardrail.
Poles to be located behind existing guardrails, rock bluffs, embankments, or ditches are not required to be of the breakaway type.
The breakaway poles that are used for street lighting installation must meet AASHTO’s breakaway requirements.
Non-breakaway poles recommended specifically for street lighting installations must be located outside the clear zone.
If the right-of-way is limited and sidewalk, curb, and gutter are not provided along highways, then poles equipped with AASHTO-approved breakaway bases must be installed.
Non-breakaway poles may be used where joint use of utility poles for roadway lighting and electrical distribution is practical and the effectiveness of a satisfactory lighting job would not be jeopardized.
Joint use of utility poles is an economical system, which reduces the number of fixed objects along the roadway.
Mounting heights less than 45 feet may be approved in order to use existing poles to the fullest extent; however, the effectiveness of a satisfactory lighting job should not be jeopardized just to use existing poles.
All installation must meet the minimum requirements set by the National Electrical Safety Code.
The design engineer should coordinate with the local utility company to determine the mounting height.
Offset lighting may be used in a lighting system required to be located 20 feet or greater from the edge of the highway.
Offset lighting may be considered if the design parameters cannot be met due to geometric constraints.
Rapid changes in levels of illumination may be compensated for by using transition lighting or adaptation lighting.
When transition lighting is provided, the roadway sector requiring transition lighting should be illuminated so as to allow the motorist’s eyes to adjust to a different level of illumination.
Equation 1 is a practical formula for calculating the required roadway length for transition lighting. Roadway Length for Transition Lighting
L = S \times C \times T
- L = length of transition lighting
- S = speed along roadway section in miles per hour (mph) (design speed)
- C = 1.47 (converts mph to feet per second)
- T = 15 seconds (recommended exposure time to allow motorist’s eyes to adjust to a different level of illumination)

3.3.3 Methodologies

Luminance is the design criterion for street lighting.
IES 8-21 recommends that illuminance calculations also be performed for the resultant design in order to provide the value that can be used for field validation of the performance.
Table 3-6. Luminance Design Values for Streets
Street Classification should follow:
- Pedestrian Activity Classification, Average Luminance L{avg} cd/m^2, Average Uniformity Ratio L{avg} / L{min} and Maximum Uniformity Ratio L{max} / L_{min}
- The Average Uniformity Ratio for high and meduim conditions should be: 3.0, for the Low condition average, the ratio should be 3.5.
- The Maximum Uniformity Ratio for high and meduim conditions should be: 5.0, for the Low condition average, the ratio should be 6.0.
- Maximum Veiling Ratio L{Vmax} / L{avg} for major and minor arterial conditions is 0.3, for collector and local conditions is 0.4
Table 3-7. Illuminance Design Values for Streets
Street Classification should follow:
- Street Classification, Pedestrian Activity Classification, Average Maintained Horizontal Illuminance (Eh), and Uniformity Ratio (Avg/Min.)
- Pavement conditions R1, R2, R3 and R4 should follow.
- The Veiling Luminance Ratio for the following roads:
  - Major Arterial is, for High, Medium and Low levels conditions for the Uniformity Ratio Average with the average of 3:1 and the Veiling Ratio Average of 0.3
  - Minor Arterial is, for High, Medium and Low levels conditions for the Uniformity Ratio Average with the average of 4:1 and the Veiling Ratio Average of 0.4
  - Collector is, for High, Medium and Low levels conditions for the Uniformity Ratio Average with the average of 4:1 and the Veiling Ratio Average of 0.4
  - Local is, for High, Medium and Low levels conditions for the Uniformity Ratio Average with the average of 6:1 and the Veiling Ratio Average of 0.4
  - Alleys is, for High, Medium and Low levels conditions for the Uniformity Ratio Average with the average of 6:1 and the Veiling Ratio Average of 0.4

3.3.4 Intersection Lighting

Alerts the driver to an approaching intersection and enhances visibility and safety.
Consider conflict points at an intersection and placing luminaires on or near these conflict points.
Lighting is recommended at signalized intersections if requested by municipality and approved by TDOT.
Reduce the number of poles at an intersection; a signal pole shaft with a luminaire mast arm or offset luminaire shall be used at these locations if a light pole—existing or proposed—is within a distance of 30 feet or less from the signal poles.
The level of illumination of a signalized intersection should be dictated by the area classification.
Crash data and an engineering study shall determine the additional light poles.
Warranting conditions for general intersection lighting (Table 3-8):
- IL-1: Where facilities have raised medians.
- IL-2: Where major arterials are located in urban areas.
- IL-3: Within the fringe of corporate limits which are suburban or rural in character provided they meet the above criteria, and the Local Agency assumes all ownership responsibility, installation, operational and maintenance costs.
- IL-4: Where there are high vehicle-to-vehicle interactions (e.g., sections with numerous driveways, significant commercial or residential development, high percentage of trucks).
- IL-5: Where the driver is required to pass through a roadway section with complex geometry.
- IL-6: Where the night-to-day ratio of crash rates is higher than the statewide average for similar locations, and a study indicates that lighting may be expected to significantly reduce the night crash rate.
- IL-7: Where the Local Agency finds sufficient benefit and agrees to finance the installation, maintenance, and operation of the lighting facilities.
Warranting conditions for rural intersection lighting (Table 3-9):
- RIL-1: There are 2.4 or more crashes per million vehicles in each of three consecutive years.
- RIL-2: There are 2.0 or more crashes per million vehicles per year and 4.0 or more crashes per year in each of three consecutive years.
- RIL-3: There are 3.0 or more crashes per million vehicles per year and 7.0 or more crashes per year in each of two consecutive years.
- RIL-4: The intersection is signalized and there have been, in the past year, 5.0 or more reported nighttime crashes and a day to-night crash ratio of less than 2