Section 2: Illumination Levels

Introduction

This section describes TxDOT's standards for illumination levels for various roadway lighting situations.

Illuminance and Luminance

Lighting levels for typical roadways may be designed by calculating either the illuminance levels, luminance levels, or both.
Illuminance in roadway lighting is a measurement of the amount of light that hits the pavement surface. Illuminance is measured in foot-candles (US customary units) or lux (SI units). TxDOT projects are typically calculated in foot-candles. Illuminance layouts may be designed with lighting design software or with illuminance templates. The illuminance at any point will be the sum of illuminance from one or several contributing sources.
Luminance in roadway lighting is a measurement of the reflected light from the pavement surface that is visible to the motorist's eye. Luminance is measured in candela per square meter (cd/m2).The reflectance characteristics of the pavement must be known to calculate luminance, and lighting design software is required for roadway luminance calculations.
The level and uniformity of illuminance or luminance along a highway depends on several factors, including the lumen output of the light source, luminaire distribution, mounting height, luminaire position, pavement reflectance, and pole spacing and arrangement. The same average level can be obtained by different installation arrangements, such as a few high-output light sources or a greater number of low-output sources.
The proper light loss factor (LLF) should be used in all lighting calculations. LLF is a depreciation factor applied to the calculated initial average illuminance or luminance. Each TxDOT LED luminaire has a unique LLF that is shown with that fixture on the Material Producer’s List (MPL) for Roadway Illumination and Electrical Supplies. TxDOT HPS luminaires use a LLF of 0.65 for all models.

Continuous Lighting

Continuous roadway lighting includes uniform lighting of all main lanes, direct connectors, and complete interchange lighting of all interchanges. TxDOT does not normally light frontage roads or ramps with continuous lighting, but lights the intersection of ramp and frontage road with safety lighting.

Illuminance and Luminance Design Values for Continuous Lighting

The recommended lighting levels for continuous lighting are in the table below. These values are based on Table 3-5a from the AASHTO Roadway Lighting Design Guide.
The table below contains the recommended illuminance and luminance values for continuous lighting of roadways based on roadway type and general land use. Either the illuminance method, the luminance method, or both methods may be used to specify the light levels for the roadway.
Illuminance and Luminance Design Values
Roadway and Walkway Classification (4)
Area Classifications
Illuminance Method
Luminance Method
Additional Values
(both methods)
Average Maintained Illuminance (Eavg)
Minimum Illuminance Emin
Illuminance Uniformity Ratio
Eavg/Emin
Average Maintained Luminance
Veiling Luminance Ratio
R2
Lavg
Uniformity
General Land Use
(foot-candles) (min)
(foot-candles)
avg/min (max)(3)
cd/m2 (min)
Lavg/Lmin (max)
Lmax/Lmin (max)
Lv(max)/Lavg(max)(2)
Interstate and other freeways
Commercial
0.6 to 1.1
0.2
3:1 or 4:1
0.4 to 1.0
3.5:1
6:1
0.3:1
Intermediate
0.6 to 0.9
0.2
3:1 or 4:1
0.4 to 0.8
3.5:1
6:1
0.3:1
Residential
0.6 to 0.8
0.2
3:1 or 4:1
0.4 to 0.6
3.5:1
6:1
0.3:1
Other Principal Arterials
(partial or no control of access)
Commercial
1.6
As
uniformity ratio allows
3:1
1.2
3:1
5:1
0.3:1
Intermediate
1.2
3:1
0.9
3:1
5:1
0.3:1
Residential
0.8
3:1
0.6
3.5:1
6:1
0.3:1
Minor Arterials
Commercial
1.4
4:1
1.2
3:1
5:1
0.3:1
Intermediate
1.0
4:1
0.9
3:1
5:1
0.3:1
Residential
0.7
4:1
0.6
3.5:1
6:1
0.3:1
Collectors
Commercial
1.1
4:1
0.8
3:1
5:1
0.4:1
Intermediate
0.8
4:1
0.6
3.5:1
6:1
0.4:1
Residential
0.6
4:1
0.4
4:1
8:1
0.4:1
Local
Commercial
0.8
6:1
0.6
6:1
10:1
0.4:1
Intermediate
0.7
6:1
0.5
6:1
10:1
0.4:1
Residential
0.4
6:1
0.3
6:1
10:1
0.4:1
Alleys
Commercial
0.6
6:1
0.4
6:1
10:1
0.4:1
Intermediate
0.4
6:1
0.3
6:1
10:1
0.4:1
Residential
0.3
6:1
0.2
6:1
10:1
0.4:1
Sidewalks
Commercial
1.3
3:1
Use illuminance requirements
Intermediate
0.8
4:1
Residential
0.4
6:1
Pedestrian Ways and Bicycle Ways(1)
All
2.0
3:1
Notes:
  • Meet either the Illuminance design method requirements or the Luminance design method requirements and meet veiling luminance requirements for both the Illuminance and Luminance design methods.
  • There may be situations when a higher level of illuminance or luminance is justified. The higher values for freeways may be justified when deemed advantageous by the agency to mitigate off-road sources.
  • Physical roadway conditions may require adjustment of spacing determined from base levels of illuminance or luminance indicated above.
  1. Assumes a separate facility. For Pedestrian Ways and Bicycle Ways adjacent to roadway, use roadway design values. Use R3 requirements for walkway/bikeway surface materials other than pavement types shown. Other design guidelines such as IESNA or CIE may be used for pedestrian ways and bikeways when deemed appropriate.
  2. Lv(max) refers to the maximum point along the pavement, not the maximum in lamp life. The Maintenance Factor applies to both the Lv term and the Lavg term.
  3. Higher uniformity ratios are acceptable for elevated ramps near high-mast poles.
  4. See AASHTO publication entitled “A Policy on Geometric Design of Highways and Streets” for roadway and walkway classifications.

Definitions for Illuminance and Luminance Design Values Table

Commercial.
That portion of a municipality in a business development where ordinarily there are large numbers of pedestrians and a heavy demand for parking space during periods of peak traffic or a sustained high pedestrian volume and a continuously heavy demand for off-street parking space during business hours. This definition applies to densely developed business areas outside of, as well as those that are within, the central part of a municipality.
Intermediate.
 That portion of a municipality which is outside of a downtown area but generally within the zone of influence of a business or industrial development, often characterized by moderately heavy nighttime pedestrian traffic and a somewhat lower parking turnover than is found in a commercial area. This definition includes densely developed apartment areas, hospitals, public libraries, and neighborhood recreational centers.
Residential.
 A residential development, or a mixture of residential and commercial establishments, characterized by few pedestrians and a low parking demand or turnover at night. This definition includes areas with single family homes, townhouses, and/or small apartments. Regional parks, cemeteries, and vacant lands are also included.
Principal Arterial.
 A roadway with high mobility and limited access. Principal arterials include freeways with full control of access and divided highways with partial control of access.
Minor Arterial.
 A roadway with moderate mobility and limited access. Minor arterials interconnect principal arterials and primarily serve through traffic.
Collector.
 A roadway with moderate mobility and moderate access. Collectors connect local roads to arterials.
Local.
 A roadway with high access and limited mobility. Local streets permit access to abutting land.

Illumination for Intersections

The recommended lighting levels for intersections of continuously lit roadways are in the table below. They are based on the functional classifications of the intersecting roadways and level of pedestrian use. The values are taken from Table 8 in ANSI/IES RP-8-14, Roadway Lighting.
The functional classifications of roadways are based on the Institute of Transportation Engineers (ITE) Guidelines for Residential Subdivision Street Design:
  • Major - over 3,500 average daily traffic (ADT)
  • Collector - 1,500 to 3,500 ADT
  • Local - 100 to 1,500 ADT
Illumination for Intersections
Functional Classification
Average Maintained Illumination at Pavement by Pedestrian Area Classification in fc
Uniformity
E
avg
/E
min
High
Medium
Low
Major/Major
3.4
2.6
1.8
3.0
Major/Collector
2.9
2.2
1.5
3.0
Major/Local
2.6
2.0
1.3
3.0
Collector/Collector
2.4
1.8
1.5
4.0
Collector/Local
2.1
1.6
1.0
4.0
Local/Local
1.8
1.4
0.8
6.0

Safety Lighting

Safety lighting includes lighting rural and urban interchanges, highway intersections, railroad crossings, pedestrian conflict areas, and other points of nighttime hazard.

Lighting of Isolated Intersections and Interchanges

The recommended lighting levels for isolated intersections, interchanges, and railroad crossings are shown in the table below. These values are based on Table 9 from ANSI/IES RP-8-14, Roadway Lighting. For roadway intersections and interchanges, the road classification with the highest light level should be used. Roadway classifications in this table are defined the same as in the Illuminance and Luminance Design Values table displayed earlier in this section.
Illumination of Isolated Intersections
Road Classification
Average Illuminance
fc
Uniformity Ratio
Eavg/Emin
Arterial
0.9
3.0
Collector
0.6
4.0
Local
0.4
6.0

Pedestrian Areas

Lighting for pedestrian areas should be designed using both horizontal and vertical illuminance. Horizontal illuminance levels help the pedestrian to see the walkway. Vertical illuminance levels help the pedestrian to see and recognize other pedestrians in the walkway. The table below contains the recommended values for pedestrian areas. The table is based on Tables 4, 5, 6, and 7 from ANSI/IES RP-8-14, Roadway Lighting. The area classifications for the table below are defined as follows:
  • High Pedestrian Conflict Area -
     Commercial areas in urban environments with high nighttime pedestrian activity.
  • Medium Pedestrian Conflict Area -
     Areas with moderate night pedestrian activities near community facilities such as libraries and recreation centers.
  • Low Pedestrian Conflict Area -
     Residential areas.
  • Pedestrian Portion of Pedestrian/Vehicular Underpass -
    Pedestrian areas such as sidewalks alongside a vehicle underpass, or underpasses designed for pedestrians only.
Illumination for Pedestrian Areas
Maintained Illuminance Values for Walkways
Area Classification
Description
E
avg
 (fc)
EV
min
 (fc)
E
avg
/E
min
*
High Pedestrian Conflict
Areas
Mixed Vehicle and Pedestrian
2.0
1.0
4.0
Pedestrian Only
1.0
0.5
4.0
Medium Pedestrian
Conflict Areas
Pedestrian Areas
0.5
0.2
4.0
Low Pedestrian
Conflict Areas
Rural/Semi-Rural Areas
0.2
0.06
10.0
Low Density Residential (2 or fewer dwelling units per acre)
0.3
0.08
6.0
Medium Density Residential (2.1 to 6.0 dwelling units per acre)
0.4
0.1
4.0
Pedestrian Portion of Pedestrian/Vehicular Underpasses
Day
10.0
5.0
4.0
Night
4.0
2.0
3.0