Web-Based Training for FHWA Roadway Lighting Workshop

Module 3: Street and Roadway Lighting Design
Web-Based Training for FHWA Roadway Lighting 

Workshop 

Participant Workbook 

Module 3: Street and Roadway Lighting Design 

(Other modules include: 

Module 1: Roadway Lighting Design Overview 

Module 2: Lighting Hardware and Light Source Considerations for Roadway Lighting 

Module 4: Other Roadway Lighting Topics) 

May 2018 
NOTICE 

This document is disseminated under the sponsorship of the U.S. Department of 

Transportation in the interest of information exchange. The United States Government 

assumes no liability for its contents or the use thereof. This Report does not constitute a 

standard, specification, or regulation. 

The contents of this Report reflect the views of the contractor, who is responsible for the 

accuracy of the data presented herein. The contents do not necessarily reflect the official 

policy of the U.S. Department of Transportation. 

The United States Government does not endorse products or manufacturers named 

herein. Trade or manufacturers’ names appear herein solely because they are considered 

essential to the object of this report. 

QUALITY ASSURANCE STATEMENT 

The Federal Highway Administration (FHWA) provides high-quality information to serve 

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TECHNICAL REPORT DOCUMENTATION PAGE 

1. Report No. 2. Government Accession No. 3. Recipient's Catalog No. 

FHWA-SA-18-035 

4. Title and Subtitle 5. Report Date 

Web-Based Training for FHWA Roadway Lighting Workshop 

Module 3: Street and Roadway Lighting Design May 2018 

6. Performing Organization Code 

7. Author(s) 8. Performing Organization Report No. 

Daniel C. Frering, John D. Bullough, Kevin Chiang, Leverson Boodlal 

9. Performing Organization Name and Address 10. Work Unit No. (TRAIS) 

KLS 

Engineering, 

LLC (Prime), 

45155 Research 

Pl, Suite 200, 

Ashburn, VA 

20147 

Lighting Research Center, 

Rensselaer Polytechnic 

Institute (Subcontractor), 21 

Union Street, Troy, NY 12180 

11. Contract or Grant No. 

DTFH6116D00017, TOPR No. 16-16 

12. Sponsoring Agency Name and Address 13. Type of Report and Period Covered 

Federal Highway Administration 

Office of Safety 

1200 New Jersey Ave. SE 

Washington, DC 20590 

Report (2016-2018) 

14. Sponsoring Agency Code 

FHWA 

15. Supplementary Notes 

Joseph Cheung from FHWA served as the Project Manager. Cathy Satterfield, George Merritt, Michelle Arnold and 

Wilmari Valentin Medina from FHWA provided helpful technical comments. 

16. Abstract 

This document serves as a participant workbook for Web-Based Training for FHWA Roadway Lighting Workshop, 

Module 3: Street and Roadway Lighting Design. Module 3 covers lighting design criteria, calculations, field 

measurements, and light pollution. Other modules include Module 1: Roadway Lighting Design Overview, Module 

2: Lighting Hardware and Light Source Considerations for Roadway Lighting, and Module 4: Other Roadway 

Lighting Topics. 

17. Key Words 18. Distribution Statement 

No restrictions Roadway lighting, safety, visibility, crash avoidance

19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No. of Pages 22. Price 

Unclassified Unclassified 67 

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized
List of Acronyms and Abbreviations

AASHTO American Association of State Highway and Transportation Officials 

BUG Backlight-uplight-glare 

CCT Correlated color temperature 

cd Candela 

cd/m2 Candela per square meter 

CU Coefficient of utilization 

fc Footcandle 

HPS High pressure sodium 

HS House side 

IES Illuminating Engineering Society 

LED Light emitting diode 

LDD Luminaire dirt depreciation 

LLD Lamp lumen depreciation 

LLF Light loss factor 

LM Lighting measurement guide 

LZ Lighting zone (environmental) 

MH Mounting height 

OSP Outdoor site-lighting performance 

Qo Pavement brightness factor 

RP Recommended practice 

SS Street side 

TM Technical memorandum 
Module 3: Street and Roadway Lighting Design 

Street and Roadway Lighting Design Overview 

Module 3, Slide 1 

This section of the Participant Workbook will help you review the content for Module 3, 

titled Street and Roadway Lighting Design. The module slide title and slide number at 

the top of each workbook page will help you locate content within the workbook that 

matches content in the online modules. 
Navigating This Course 

Module 3, Slide 2 

This slide in the online module describes how to navigate the course module. It is 

similar to the instructions in this workbook. This section of the workbook also contains 

an extended example problem where you can apply the knowledge gained from this 

module. 
Course Objectives 

Module 3, Slide 3 

Following completion of this module, the user will be able to accomplish the following 

objectives: 

 Select design criteria for roadway lighting types. 

 Estimate luminaire spacing for different road types. 

 Calculate average illuminance and illuminance uniformity. 

 Analyze lighting pollution and apply mitigation techniques. 
Table of Contents 

Module 3, Slide 4

Section 1: Lighting Design Criteria – illuminance, luminance, veiling luminance, 

pavement characteristics, uniformity. 

Section 2: Calculations – light loss factors, coefficient of utilization, luminaire spacing, 

luminaire layouts, roadway lighting design software. 

Section 3: Field Measurements – luminance, illuminance, uniformity. 

Section 4: Light Pollution – environmental zones, sky glow, light trespass. 
Lighting Design Criteria 

Module 3, Slide 5 

This module section discusses lighting design criteria. 
Road Surface Illuminance 

Module 3, Slide 6 

Illustration of a typical roadway luminaire distribution.

Illuminance was discussed in module 1 of this course. It is the amount of light (lumens) 

incident on a surface, for example a roadway, per unit area, such as square foot 

(footcandle) or square meter (lux). Illuminance is used as a design criterion for 

intersections and for curved roadways. It is also used in the field to evaluate whether a 

particular roadway meets recommended lighting conditions (illuminance or luminance). 

The illuminance on a road surface is based on two main factors: 

 The light intensity of a luminaire. 

 The luminaire mounting height and geometry. 

The farther the luminaire is from the surface it is illuminating, the lower the illuminance 

will be. Illuminance is inversely proportional to the square of the distance between the 

luminaire and the surface being illuminated. Thus, in a roadway application, as 

luminaire mounting height increases, illuminance on the roadway decreases 

exponentially. 
Road Surface Illuminance (cont'd.) 

Module 3, Slide 7

Road surface illuminance. 

The calculation for illuminance is relatively simple. 

Illuminance (E) = Intensity (I) x cosine of angle from the luminaire () 

 Distance squared (D2) 

If we wanted to calculate the illuminance at a point directly under a luminaire, we simply 

need to know the intensity of the luminaire at its center beam [e.g., 1000 candelas (cd)], 

the cosine of the angle from the luminaire (the angle directly under the luminaire is zero 

and so has a cosine of one) and the distance between the luminaire and the roadway 

surface (e.g., 5 meters), as shown in the equation below: 

1000 cd x 1 = 40 lux 

 25

As shown in the illustration above, to calculate the illuminance at a point 10 meters 

away from this same luminaire, we would need to use the cosine of the angle between 

where the luminaire is mounted and the point we wish to calculate (a 63o angle, , has a 

cosine of 0.45), the total intensity of the luminaire at that angle (800 cd), and the distance 

between that point and the luminaire (the distance D is 11 meters). The calculation is: 

800 cd x 0.45 = 3 lux 

 121
 Road Surface Illuminance (cont'd.) 

Module 3, Slide 8

A typical illuminance map produced by a computer software program. The colors on the 

road indicate different illuminance levels (in this case, in footcandles) on the roadway 

surface. 

Before the era of computers, engineers used the point method to calculate illuminance, 

as described on the previous slide, at enough points along a roadway to provide a good 

idea of what the overall light levels were throughout an entire roadway lighting 

installation. However, this is rarely done today. 

Today these types of illuminance calculations are done using computer software. This 

course module will discuss some of these software types in later slides. 
Road Surface Luminance 

Module 3, Slide 9

Illustration of roadway luminance measurements. The observation angle from the point 

on the road to the observer is 1 degree. The observer distance from the point on the 

road where luminance is being measured or calculated is 83.07 meters. The observer 

height from the road surface is 1.45 meters. 

The next design criterion used for roadways is luminance. This is used for relatively 

straight sections of roadways and streets. The definition of luminance was discussed in 

Module 1 of this course. Unlike illuminance, which is light on a surface, luminance is 

what we actually see. 

The luminance of a road surface is based on: 

 The illuminance on the road surface. 

 The geometry of the installation. 

 The reflection characteristics of the pavement. 

 The location of the observer. 

The observer location used in the luminance calculation is assumed to be 83.07 meters 

from the calculation point, at an eye height of 1.45 meters. This results in an 

observation angle of 1 degree. This observer location is used for all calculation points. 

All pavement reflection characteristics are also measured at the 1 degree observation 

angle. 
Pavement Surface Characteristics 

Module 3, Slide 10

Roadway surface types. Diffuse surface reflection occurs when a rough surface 

causes reflected rays of light to travel in different directions. Specular surface reflection 

occurs when a smooth surface causes reflected rays to travel in the same direction. An 

absorbing surface prevents reflection or diffusion of light striking on the surface.

The type of pavement used for roadways will affect the luminance and the calculation of 

the visibility of small targets or objects on the roadway. Roadway surfaces are 

classified based on their reflection characteristics. 

Pavement is a mixture of aggregate stone and a binder material, usually concrete or 

asphalt. Studies show that reflection depends on the texture, type, and structure of the 

materials used. Reflection from pavement is a combination of diffuse and specular 

reflection. 
Pavement Surface Classification 

Module 3, Slide 11

Reflection properties have been measured for hundreds of different road surfaces since 

the 1960s. All of these measurements have been under a 1 degree observation angle 

for roadway lighting. The results were generalized into four road classes based on 

reflection criteria, shown on the chart above. In the chart, Qo represents the general 

brightness of the surface. 
Veiling Luminance 

Module 3, Slide 12

A glaring luminaire along a roadway at night. Lighting engineers use the veiling 

luminance metric to limit potential glare from roadway luminaires.

Another factor to consider in roadway lighting design is veiling luminance. Lighting 

engineers use veiling luminance as a way to limit glare from roadway luminaires. Veiling 

luminance is the effect produced by bright sources or objects in the visual field that 

causes decreased visibility and visual performance. 

Veiling luminance compares the luminance of the luminaires on the roadway to the 

luminance of the area surrounding them. This results in a ratio between the two 

luminances, the veiling luminance produced by the luminaire to the average luminance 

of the roadway area. 

Roadway lighting guidelines, such as the Illuminating Engineering Society’s (IES’s) 

recommended practice for roadway lighting (RP-8) set limits on this ratio for various 

types of roadways as a means of limiting potential glare for drivers. 
Veiling Luminance (cont'd.) 

Module 3, Slide 13 

An illustration of the method used to calculate veiling luminance. 

Engineers calculate veiling luminance for a driver looking at a point on the road located 

83 meters ahead. It is determined by the vertical illuminance (E, in lux) from the 

luminaire falling on the driver's eye location, and the angle (A, in degrees) between the 

luminaire and the driver’s line of sight. It is expressed in terms of a luminance value (L, 

in candelas per square meter) that is added to the luminances of an object and its 

background according to the formula: 

 L = 10E / A2 
Lighting Uniformity 

Module 3, Slide 14

An expressway with relatively uniform lighting. 

The last design criterion we will discuss is lighting uniformity. Uniformity 

recommendations apply to both illuminance and luminance, depending on which 

measure you are using. As the name suggests, uniformity is a measure of how evenly 

light is distributed along a portion of roadway. Greater uniformity improves both visibility 

and visual comfort for drivers. 

In RP-8, the IES publishes recommended uniformity metrics for various types of 

roadways to help ensure that the lighting is evenly distributed. These metrics apply to 

both luminance and illuminance calculations. Like veiling luminance, these uniformity 

metrics are in the form of a ratio. The ratio is between the highest level on the roadway 

(max), luminance or illuminance, and the lowest (min), luminance or illuminance. For 

example, if the recommended uniformity ratio for a straight roadway were 6, then the 

maximum luminance could not exceed 6 times the minimum luminance anywhere on 

the section of roadway for which lighting is being designed. 
Recommended Lighting Design Criteria for Roadways 

Module 3, Slide 15 

The chart above is adapted from IES RP-8 and shows the recommended design criteria 

(average luminance Lavg, maximum uniformity ratio Lmax/Lmin, maximum veiling 

luminance ratio LVmax/Lavg) for some types of roadways in North America. Engineers 

and others who design lighting installations for roadways follow these guidelines to help 

ensure that the lighting provides the characteristics necessary to promote good visibility 

and visual comfort for drivers. Lighting software programs used to assist with roadway 

lighting design provide the information necessary to ensure compliance with the 

recommendations shown in this chart. 
Knowledge Check 

Module 3, Slide 16 

True or False: 

Veiling luminance is a metric used to prevent glare from roadway lighting. 

 A) True 

 B) False 
Knowledge Check Answer 

Module 3, Slide 17

Potentially glaring luminaires along a highway at night. 

The correct answer is A) True. 

Veiling luminance is the effect produced by bright sources or objects in the visual field 

that decreases visibility and visual performance. The veiling luminance metric is 

designed to prevent disability glare for drivers and improve visibility and visual comfort. 

(See Module 3, Slide 12)
Calculations for Luminaire Spacing and Layout 

Module 3, Slide 18

The next section of this module will discuss calculations for the spacing and layout of 

luminaires for roadway lighting. 
Maintained Light Levels 

Module 3, Slide 19 

A lamp being changed in a roadway luminaire. Light loss factors take into account 

frequency of lamp changes and luminaire cleanings.

All calculations for roadway lighting should result in maintained light levels, whether 

luminance or illuminance. Roadway lighting design uses the maintained 

illuminance/luminance method, which aims to provide not less than the required light 

levels at the worst point in the maintenance cycle of luminaire cleaning and lamp, light 

emitting diode (LED) module, or luminaire replacement. 

To properly calculate maintained light levels we use a series of light loss factors (LLF). 

These factors take into account such things as dirt accumulation on luminaire surfaces, 

lumen depreciation from the lamp or LED, and maintenance procedures such as how 

often crews clean luminaires and change lamps. 
Light Loss Factors (LLF) 

Module 3, Slide 20

A very dirty post-top street luminaire. It's easy to see how the dirt will decrease the light 

output, compared to a clean luminaire. 

LLFs are applied to lighting design to compensate for light depreciation over time. The 

two main factors to consider are: 

Lamp Lumen Depreciation (LLD) – LLD is a multiplier that a lighting engineer uses in 

calculations to relate the initial rated output of a light source to its anticipated minimum 

output based on a particular relamping schedule. 

Luminaire Dirt Depreciation (LDD) – LDD is a multiplier used in lighting calculations to 

reduce the initial light level provided by clean luminaires to the light level they will 

provide because of dirt collection on the luminaires at the time when it is anticipated that 

the luminaires will be cleaned. 
Lamp Lumen Depreciation (LLD) 

Module 3, Slide 21

A LLD chart for a high pressure sodium (HPS) lamp. 

To determine the LLD for a particular roadway lighting installation, you must first 

determine how often you expect to change the lamp or light source, for example, every 

five years. Once you have determined that, you estimate a number of hours that the 

lamp will operate over the time period, for example, 11 hours per night (on average) for 

1,825 nights. This equals approximately 20,000 hours. 

You then use the information provided by the light source manufacturers to determine 

the percentage of initial light output that will remain at the time when the lamp is 

expected to be changed. This is often provided in a graph similar to the one shown 

above. 
Luminaire Dirt Depreciation (LDD) 

Module 3, Slide 22

To determine the LDD factor to apply in your calculations, you first determine how 

frequently the luminaires will be cleaned, for example, every five years. Cleaning is 

typically done at the same time as relamping. 

Once the maintenance scheduled is determined, you then need to decide on the type of 

environment in which the luminaire is or will be installed from very clean, to very dirty. 

You then use a graph like the one above provided by the luminaire manufacturer to 

determine the LDD to use in your calculations. 
Light Loss Factor Calculation 

Module 3, Slide 23

Below are three sample calculations for total light loss factor using the graphs provided 

on the previous slides. Each calculation is for a different maintenance period. As would 

be expected, the longer the maintenance period, the higher the light loss factor. 

Note that if maintenance of a roadway lighting installation is done by spot relamping 

(relamping of luminaires only when a lamp is reported to be burned out) you should use 

a five-year maintenance schedule. 

Three-Year – Maintenance Cycle LLF Calculation: 

0.81 (LLF) = 0.9 (LLD) x 0.9 (LDD) 

Four-Year – Maintenance Cycle LLF Calculation: 

0.75 (LLF) = 0.84 (LLD) x 0.89 (LDD) 

Five-Year – Maintenance Cycle Calculation, or Maintenance by Spot Relamping 

LLF Calculation: 

0.69 (LLF) = 0.78 (LLD) x 0.88 (LDD) 
Light Loss Factor for LEDs 

Module 3, Slide 24

An LED roadway luminaire. 

Unlike all other light sources, the life of LEDs is not based on light source failure, but 

rather on lumen depreciation. The life of an LED is an estimate based on when the 

LEDs within a luminaire are expected to lose 30 percent of their light output. Therefore 

the LLF calculation for an LED luminaire is usually done using this LLD number (0.7) 

and the expected cleaning schedule for the luminaires (usually every 5 to 10 years). 

So, a LLF calculation for an LED luminaire typically looks like this: 

0.63 (LLF) = 0.70 (LLD) x 0.9 (LDD) 

This is a conservative estimate based on the belief that LED luminaires will likely be left 

in place as long as they are providing light. Using an LLF of 0.63 for LED luminaires will 

help to ensure that the light levels calculated will still be available at the end of an LED 

luminaire’s life. However, this will also result in a lighting installation that is significantly 

over-lighted when it is first installed. Some LED products can compensate for this by 

allowing the user to dim the lighting when first installed and then to slowly raise the 

power provided to the LEDs as their lumen output depreciates. 
Coefficient of Utilization 

Module 3, Slide 25

In this chart, about 43 percent (CU=0.43) of the luminaire's lumens would fall on the 

street, if the street were two mounting heights wide.

Before beginning a discussion of luminaire spacing, we need to understand the 

coefficient of utilization of a roadway luminaire. 

The coefficient of utilization (CU) is a value between zero and one that indicates the 

proportion of lumens emitted by a roadway luminaire that will fall on the roadway. The 

CU value is used in preliminary estimates of the roadway illuminance produced by a 

lighting system. The width of the roadway is given in terms of multiples of the 

luminaire’s mounting height. 

The chart above shows one CU curve labeled SS (street side), where the horizontal 

axis represents the width of the roadway, in multiples of the luminaire’s mounting height. 

The values along the vertical axis indicate the proportion of the luminaire’s lumens that 

will fall on the roadway surface. 

The other CU curve, labeled HS (house side), can be used to calculate the proportion of 

lumens from a luminaire that would fall outside the street area, behind the luminaire 

(and toward any houses located on the same side of the street as the luminaire). 
Luminaire Pole Spacing 

Module 3, Slide 26

The next few slides will explain how to estimate pole layout spacing based on average 

illuminance. Average luminance can be calculated from the illuminance value. 

The example shown on the next slide assumes a collector road as defined in the 

AASHTO Roadway Lighting Design Guide. The road has a medium classification for 

pedestrian conflict and asphalt pavement. Using the chart contained in the AASHTO 

design guide, the average maintained horizontal illuminance should be 0.9 fc (9.0 lux). 
Luminaire Pole Spacing (cont'd.) 

Module 3, Slide 27

The calculation for roadway luminaire pole spacing is: 

Spacing = (LL x CU x LLF) / (Eh x W) 

LL – Lamp lumens (or luminaire lumens if LED luminaire). These are initial lumens 

from the lamp or LED luminaire, when it is first installed. 

CU – Coefficient of utilization for roadway width (use the CU table provided by the 

luminaire manufacturer in the photometric report of the luminaire you have selected). 

LLF – Light loss factor accounts for lamp lumen depreciation and luminaire dirt 

depreciation 

Eh – Average maintained design horizontal illuminance in footcandles (fc). In a 

roadway lighting design, this is the amount of light you want to provide on the 

pavement. 

W – Width of lighted roadway in feet (ft). This is the width of the roadway this luminaire 

is intended to light. For example, if it is a 6-lane roadway, and the luminaire you are 

using is only expected to light three lanes, you only use the width of those 3 lanes. 
Determine Coefficient of Utilization 

Module 3, Slide 28

To obtain the CU, divide the width of the roadway you are lighting by the mounting 

height of the luminaires. 

This is the ratio shown on the X axis of the CU table. 

Follow this line up until you reach the CU line for street side (SS) lumens. 

The intersection of this line with the CU line indicates the CU value for the roadway. 
Luminaire Pole Spacing 

Module 3, Slide 29

A diagram showing spacing of luminaire poles, width of lighted roadway, and mounting 

height of luminaires.

Luminaire Spacing Worksheet 

Example

LL 16000 lm

CU [Street Side (SS)] 0.35

LLF 0.75

Eh (average) 0.9 fc

W 40 ft

MH 27 ft

W/MH (40’/27’) 1.5

Spacing 117 ft

Spacing = (LL x CU x LLF) / (Eh x W)

 = (16,000 x 0.35 x 0.75) / (0.9 x 40) 

 Luminaire Spacing = 117 ft 
Roadway Luminaire Layout 

Module 3, Slide 30

A lighting engineer has several choices of how to configure the layout of luminaires 

along the roadway. The pole arrangement configuration will depend, in part, upon the 

availability of clear space along the median or sides of the road, but mainly on the width 

of the road. A layout with poles along a single side of the road is best for narrower 

roads. Median layouts can be advantageous if there is a well-defined median and few 

intersections, such as along freeways. A staggered layout is appropriate for roads of 

medium width (about 1.5 to 2 times the pole height). Wider roads may require a twosided layout with pole locations opposite each other along the road, in order to provide 

sufficient coverage. 

The diagram above shows the four possible layouts of pole-mounted luminaires along a 

roadway. 
Roadway Lighting Design Software 

Module 3, Slide 31

An example of a rendering produced by a lighting design software program. The colors 

represent different light levels from red (highest) to dark blue (lowest).

The previous two sections of this course module discussed several roadway lighting 

calculations. While each of these calculations can be done by hand, most often a 

lighting engineer will use a lighting design software program to perform these 

calculations and compare the overall performance of different luminaires and lighting 

design options. 

Lighting design software is used to simulate the lighted environment in terms of spatial 

geometry, luminaire types, and relevant metrics, such as illuminance and luminance 

levels, as well as lighting uniformity. These programs produce photometrically accurate 

lighting calculations, and, in some cases, renderings. 

Simple software packages are often free but have some design constraints. 

Sophisticated software programs allow more realistic designs, like curved roadways and 

intersections as well as accounting for obstructions and reflections, but they can have a 

steep learning curve. Some programs are free; others have a moderate purchase price 

($100 – $1000). 
Simple Software Tools 

Module 3, Slide 32

Left: Portion of a calculation grid output display from the Visual Roadway Tool. 

Right: Part of the roadway descriptor input screen from the Street Lighting Tool. 

One simple application software program is the web-based Visual Roadway Tool 

courtesy of Acuity Brands lighting. It models straight segments of roadway for 

illuminance or luminance calculations. Another simple tool is the Street Lighting Tool 

from the Super-Efficient Equipment and Appliance Deployment (SEAD) Initiative. 

These tools use manufacturers' photometric files. They cannot model curved roadways 

or intersections, and do not account for reflected light or obstructions. 

They are available for free. 
Sophisticated Software Tool 

Module 3, Slide 33

Examples of renderings produced by AGi32.

AGi32, developed and marketed by Lighting Analysts, is an example of a sophisticated 

software tool. 

It can use any photometry and is able to model almost any outdoor environment for 

calculation purposes and rendering. Background images, such as satellite views, can be 

imported and scaled for layout purposes. The tool can also import 2D and 3D CAD files 

and Sketch-up files. 

It has a steep learning curve, but in-person training and YouTube tutorials are available. 

The current cost is $900 for a single-user license; annual support and upgrade 

subscriptions are available. 
Knowledge Check 

Module 3, Slide 34

Select the correct answer. 

The coefficient of utilization (CU) is a measure of the percentage of light emitted by a 

luminaire that ________. 

 A) is emitted in the forward direction 

 B) is emitted below the luminaire 

 C) reaches the roadway surface 

 D) does not cause unwanted glare 
Knowledge Check Answer 

Module 3, Slide 35

An illustration of the useful light falling on the roadway surface. 

The correct answer is C. 

The CU is a value between zero and one that indicates the proportion of lumens emitted 

by a roadway luminaire that will fall on the roadway. (See Module 3, Slide 25)
Field Measurements of Lighting 

Module 3, Slide 36

This section discusses why and how to take field measurements of a roadway lighting 

installation. 
Why Take Field Measurements? 

Module 3, Slide 37

A busy freeway at night. Lanes would need to be closed down to take field 

measurements of the lighting. 

There are several reasons why it is important to take field measurements of a lighting 

project once it has been installed, as well as over time as the system ages. 

 Validate luminaire performance – ensure that the luminaires and any lighting 

controls are operating properly. 

 Validate luminaire photometrics – ensure that the luminaires are providing the 

expected light output and distribution. 

 Review color temperature – ensure that the color temperature (CCT) of the 

installation is as specified. 

 Confirm that all lighting conditions (light levels, uniformity) comply with 

recommended practice, and design specifications. 
Taking Field Measurements 

Module 3, Slide 38

 

Left: IES LM-50. Right: Illuminance meter.

IES publication Lighting Measurement Guide number 50 (IES-LM-50) provides detailed 

instructions on how lighting measurements should be conducted in the field. 

When taking field measurements, you must first mark off measurement grid points on 

the roadway, as IES-LM50 requires. Because luminance is difficult to measure, use an 

illuminance meter for all sections of roadway, even those designed using luminance as 

the design metric. 

Place your illuminance meter on each of the grid points you have laid out and take a 

reading at each point without blocking any of the light from the luminaires. A hand-held 

illuminance meter with a separate sensor (as shown above) that can be placed on a 

movable (wheeled) platform will likely work best in this application. 

Safety is very important. If you are working in traffic when taking lighting measurements, 

traffic control and lane closure will likely be required.
Taking Field Measurements (cont'd.) 

Module 3, Slide 39

The illustration above shows the recommended measurement points for one luminaire 

cycle.

At least 10 measurements must be taken per luminaire cycle. A luminaire cycle is the 

distance between two luminaires on one side of the roadway. These measurement 

points cannot be spaced more than 5 meters apart. Two lines of measurement points 

are needed per driving lane, one-half lane width apart. 
Converting Illuminance to Luminance 

Module 3, Slide 40

Once you have taken all of your illuminance measurements, you can calculate an 

average illuminance for the section of roadway you have measured. If it is a straight 

section of roadway, you will then need to convert your average illuminance measure to 

an average luminance value in order to check if the design meets the specifications and 

recommendations for the particular roadway classification. 

Use the following equations for the conversion: 

For asphalt roadways (R3): 

luminance (cd/m2) = illuminance (lux) 

 15

For concrete roadways (R1): 

luminance (cd/m2) = illuminance (lux) 

 10

If the average illuminance that you have measured on the roadway is 30 lux on a R3 

type pavement, that would equate to a luminance of 2 cd/m2. 
Assess Lighting Uniformity 

Module 3, Slide 41

A relatively uniform lighted stretch of roadway.

The final task you will need to perform is to check if the installation meets uniformity 

requirements. 

You first need to determine the highest and the lowest illuminance point you measured. 

Compare these two points to see if the installation meets the max to min (maximum to 

minimum) uniformity requirements. 

For example, if the maximum allowed uniformity ratio is 3 to 1, and your highest 

measured point was 50 lux and your lowest was 6 lux, that ratio (8.3 to 1) does not meet 

the requirement. 
Knowledge Check 

Module 3, Slide 42

Select the correct answer:

When taking illuminance measurements along a roadway the measurement points 

cannot be spaced farther than _____ meters apart. 

 A) fifteen 

 B) five 

 C) ten 

 D) two 
Knowledge Check Answer 

Module 3, Slide 43

A worker laying out a grid to take lighting measurements. 

The correct answer is B, the measurement points cannot be spaced farther than five 

meters apart. 

At least 10 measurements must be taken per luminaire cycle. A luminaire cycle is the 

distance between two luminaires on one side of the roadway. These measurement 

points cannot be spaced more than 5 meters apart. (See Module 3, Slide 39)
Light Pollution 

Module 3, Slide 44

This section of the module will discuss various aspects of light pollution and its control. 
What is Light Pollution? 

Module 3, Slide 45

Light pollution is often caused by the way light is emitted from lighting equipment. 

Choosing proper equipment and carefully mounting and aiming it can make a significant 

difference.

Light pollution was briefly discussed in Module 2 of this course in the section on 

luminaire “BUG” (backlight, uplight, glare) ratings. Light pollution is an unwanted 

consequence of outdoor lighting and includes such effects as sky glow, light trespass, 

and glare. The illustration above shows both useful light and the components of light 

pollution. Sky glow is a brightening of the sky caused by both natural and human-made 

factors. The key factor of sky glow that contributes to light pollution is outdoor lighting. 
Environmental Lighting Zones 

Module 3, Slide 46

A brightly lighted area around a city. This is an example of zone four lighting.

One thing the lighting community has done to help limit light pollution is to define 

various environmental lighting zones (LZ). These zones help to guide lighting engineers 

in deciding where, when, and at what quantity to use outdoor lighting, including for 

roadways, streets, and parking areas. The IES publication Technical Memorandum 11

(TM-11) defines these zones. 

IES TM-11 defines five environmental lighting zones from zone zero where it 

recommends no outdoor lighting at night to zone four where there is relatively little 

restriction placed on lighting. 

Local municipalities may adopt these lighting zone recommendations as part of local 

lighting ordinances. 
Environmental Lighting Zones (cont'd.) 

Module 3, Slide 47

LZ0: No ambient lighting – Areas where the natural environment will be seriously and 

adversely affected by lighting. An example of this zone might be near or within a 

national park or wildlife reserve. Roads are unlighted. 

LZ1: Low ambient lighting – Areas where lighting might adversely affect flora and 

fauna or disturb the character of the area. This is typically a very rural area or park. 

LZ2: Moderate ambient lighting – 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. This is typically a rural area, with 

some limited use at night. 

LZ3: Moderately high ambient lighting – 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 and/or convenience and it is often uniform and/or 

continuous. After curfew, lighting may be reduced as activity levels decline. This is 

typically a suburban area. 

LZ4: High ambient lighting – 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 and/or convenience and it is mostly uniform and/or 

continuous. After curfew, lighting may be reduced in some areas as activity levels 

decline. This is typically an urban area. 

 
Environmental Lighting Zones (cont'd.) 

Module 3, Slide 48

The recommended maximum illuminance level for each of the lighting zones is shown 

above (from IES TM-11). These are shown as “pre-curfew” and “post-curfew.” Precurfew is from dusk until 11:00 p.m. local time, when the area being illuminated is more 

likely to be in use. Post-curfew is from 11:00 p.m. to 7:00 a.m. local time. 

Recommended lighting levels are higher during pre-curfew time. 
Luminaire "BUG" Rating 

Module 3, Slide 49

Left: Backlight, uplight and glare zones. Right: Backlight subzones.

As mentioned in Module 2 of this course, one way to help limit light pollution is to select 

a luminaire that does not send light into areas where it is not needed, for example 

behind the luminaire off of the roadway (backlight), directly up into the sky (uplight), and 

light at high angles (likely to cause glare for drivers and pedestrians). 

For most roadway luminaires, there should be little or no light emitted in the zones “G” 

(glare) or zone “U” (uplight), shown in the illustration on the top at left. This will limit both 

glare and sky glow. 

For the luminaire shown in the illustration above at right, if you wanted to light a 

sidewalk along the roadway, you might select a luminaire that distributes some light in 

zone “BL” (back lower). However if you wanted to limit light going beyond that point, you 

would select a luminaire with little or no light in zones “BM” (back middle), or “BH” (back 

high). This would limit light trespass off the roadway or street. 

Local outdoor lighting ordinances will typically place limits on the amount of light allowed 

in each zone, not directly lighting the roadway. 
Outdoor Site-Lighting Performance 

Module 3, Slide 50

A computer rendering showing useful light on the horizontal roadway surface, light 

leaving the vertical surface (light trespass) and light leaving the horizontal surface 

above the road, which contributes to sky glow. Each color represents a light (lux) level 

(see legend at the left of the image). 

A luminaire “BUG” rating is a good tool to limit direct light from a luminaire going where 

it is unwanted. However, a major contributor to light pollution is light that is reflected off 

the roadway and adjacent areas (e.g. walkways along the road). 

The metric called Outdoor Site-Lighting Performance (OSP) was developed to capture 

the contribution that both direct and reflected light make to light pollution. 

OSP is not a luminaire-based metric, it is a design metric. It allows a lighting engineer to 

compare various designs using commercially available lighting software. With OSP a 

“box” is constructed in the lighting software program along each side and over the 

roadway. This box allows the software program to measure all of the light leaving the 

roadway, both directly and reflected. 

For more information on using the OSP metric, follow the link at the end of this course 

module. 
Outdoor Site-Lighting Performance (cont'd.) 

Module 3, Slide 51

Left: A computer rendering (plan view) showing light leaving the roadway up into the 

sky. Right: A computer rendering (perspective view) showing light leaving the sides of 

the roadway. Colors correspond to light (lux) level.

These images show a comparison of three lighting design options for a roadway, LED 

luminaires, cobra-head luminaires with high pressure sodium (HPS) lamps, and post-top 

luminaires. The images above at right show the amount of light trespass; the image 

above at left shows the installations contribution to sky glow. 

In all cases, the LED luminaire contributes the least to light pollution (sky glow and light 

trespass), while the post top contributes the most. 
Knowledge Check 

Module 3, Slide 52

True or false: 

A roadway in environmental lighting zone one, should be lighted to a higher level than a 

roadway in zone four. 

 A) True 

 B) False 
Knowledge Check Answer 

Module 3, Slide 53

An aerial view of nighttime lighting across the globe. 

The correct answer is “false.” 

Environmental lighting zone 1 should have the lowest amount of electric lighting on the 

roadway, while zone 4 allows for the highest electric lighting levels. 
Self-Assessment Quiz 

Module 3, Slides 54 to 63

Please answer each question. 

1. Illuminance is __________ proportional to the square of the distance between 

the luminaire and the surface being illuminated. 

a.  Directly 

b.  Inversely 

2. The luminance of a road surface is based on: 

a.  The illuminance of the road surface 

b.  The geometry of the installation 

c.  The reflection characteristics of the pavement 

d.  The location of the observer 

e.  All of the above 

3. True or false? Diffuse reflection occurs when a rough roadway surface causes 

reflected rays of light to travel in different directions. 

a.  True 

b.  False 

4. Veiling luminance is used as a measure of the _______ produced by roadway 

luminaires. 

a.  Amount of light 

b.  Sky glow 

c.  Glare 

d.  Overall roadway illuminance 

5. If the required illuminance uniformity ratio for a stretch of roadway is 4 to 1, and 

the highest illuminance you measure on the roadway surface is 12 lux, and the 

lowest is 4 lux, does this roadway meet this uniformity requirement? 

a.  Yes 

b.  No

6. For a stretch of freeway 150 meters long, with luminaire mounting heights of 5 

meters, you have installed luminaires with a lamp lumen depreciation factor of 

0.84 and a luminaire dirt depreciation factor of 0.89. What would the total light 

loss factor be for the luminaires used in this installation? 

a.  1.73 

b.  0.75 

c.  526.5 

d.  3.75 

7. What is the most important factor in selecting between a single sided luminaire 

layout and an opposite side layout? 

a.  The mounting height of the luminaires 

b.  Traffic volume along the roadway 

c.  The width of the roadway 

d.  The amount of pedestrian conflict along the roadway 
Self-Assessment Quiz (cont'd.) 

Module 3, Slides 54 to 63 

8. True or false? Only light directly emitted by luminaires, and not reflected light, 

contributes to light pollution. 

a.  True 

b.  False 

See the following page for the answer key to this self-assessment. 
Self-Assessment Quiz Answer Key 

Module 3, Slides 54 to 63

The correct answers for the self-assessment are provided below. 

1. Illuminance is __________ proportional to the square of the distance between 

the luminaire and the surface being illuminated. 

a.  Directly 

b.  Inversely (See Module 3, Slide 6)

2. The luminance of a road surface is based on: 

a.  The illuminance of the road surface 

b.  The geometry of the installation 

c.  The reflection characteristics of the pavement 

d.  The location of the observer 

e.  All of the above (See Module 3, Slide 9)

3. True or false? Diffuse reflection occurs when a rough roadway surface causes 

reflected rays of light to travel in different directions. 

a.  True (See Module 3, Slide 10)

b.  False 

4. Veiling luminance is used as a measure of the _______ produced by roadway 

luminaires. 

a.  Amount of light 

b.  Sky glow 

c.  Glare (See Module 3, Slide 12)

d.  Overall roadway illuminance 

5. If the required illuminance uniformity ratio for a stretch of roadway is 4 to 1, and 

the highest illuminance you measure on the roadway surface is 12 lux, and the 

lowest is 4 lux, does this roadway meet this uniformity requirement? 

a.  Yes (See Module 3, Slide 14)

b.  No

6. For a stretch of freeway 150 meters long, with luminaire mounting heights of 5 

meters, you have installed luminaires with a lamp lumen depreciation factor of 

0.84 and a luminaire dirt depreciation factor of 0.89. What would the total light 

loss factor be for the luminaires used in this installation? 

a.  1.73 

b.  0.75 (See Module 3, Slide 23)

c.  526.5 

d.  3.75 

7. What is the most important factor in selecting between a single sided luminaire 

layout and an opposite side layout? 

a.  The mounting height of the luminaires 

b.  Traffic volume along the roadway 

c.  The width of the roadway (See Module 3, Slide 30)

d.  The amount of pedestrian conflict along the roadway 
Self-Assessment Quiz Answer (cont'd.) 

Module 3, Slides 54 to 63 

8. True or false? Only light directly emitted by luminaires, and not reflected light, 

contributes to light pollution. 

a.  True 

b.  False (See Module 3, Slide 50)
Extended Example Problem

Suppose a luminaire to illuminate an asphalt 60-foot-wide collector road with high 

pedestrian conflict (average luminance criterion 0.8 cd/m2, with an equivalent average 

horizontal illuminance of 12 lux or 1.2 footcandles) produces 50,000 lumens. It has a 

mounting height of 40 feet. The lamp lumen depreciation for the luminaire you are using 

is 0.8 and the luminaire dirt depreciation for the environment is 0.9. 

Step 1: Enter the lamp lumens (LL), average horizontal illuminance (Eh), roadway width 

(W) and mounting height (MH) into the table on the following page. 

Step 2: Determine the width/mounting height (W/MH) ratio and enter it into the table on 

the following page. 

Step 3: Use the chart below to determine the coefficient of utilization (CU) and enter it 

into the table on the following page. 

Step 4: Determine the light loss factor (LLF) and enter it into the table on the following 

page.

Step 5: Calculate the spacing and enter it into the table on the following page.
Extended Example Problem (Continued) 

Extended Problem 

LL lm

CU [Street Side (SS)] 

LLF

Eh (average) fc

W ft

MH ft

W/MH 

Spacing ft

See the following page for the answer to this extended example problem.
Extended Example Problem (Answer Key)

Extended Problem 

LL 50,000 lm

CU [Street Side (SS)] 0.48

LLF 0.72 

Eh (average) 1.2 fc

W 60 ft

MH 40 ft

W/MH 1.5 

Spacing 240 ft

Step 1: LL = 50,000 lm; Eh = 1.2 fc, W = 60 ft, MH = 40 ft. 

Step 2: W/MH = (60 ft) / (40 ft) = 1.5. 

Step 3: According to the CU chart, the CU for a W/MH value of 1.5 is 0.48. 

Step 4: LLF = LLD x LDD = 0.8 x 0.9 = 0.72. 

Step 5: Spacing = (LL x CU x LLF) / (Eh x W) 

 = (50,000 x 0.48 x 0.72) / (1.2 x 60)

 = 240 ft. 
Links to Other Resources 

 Module 3, Slide 64 

FHWA Lighting Handbook 

https://safety.fhwa.dot.gov/roadway dept/night visib/lighting handbook

IES RP-8 Roadway Lighting 

https://www.ies.org/store/recommended-practices-and-ansi-standards/roadway-lighting

IES LM-50 Measurement of Roadway and Street Lighting Installations 

https://www.ies.org/store/measurement-testing/photometric-measurement-of-roadwayand-street-lighting-installations/

Acuity Visual Roadway Lighting Design Tool 

https://www.visual-3d.com/software/roadwaytool.aspx

SEAD Street Lighting Tool 

http://www.superefficient.org/Tools/Street-Lighting-Tool 

AGi32 Lighting Design Software 

http://www.agi32.com/ 

IES TM-10 Addressing Obtrusive Light in Conjunction with Roadway Lighting 

https://www.ies.org/store/technical-memoranda/addressing-obtrusive-light-urban-skyglow-and-light-trespass-in-conjunction-with-roadway-lighting/

Outdoor Site-Lighting Performance Metric

http://www.lrc.rpi.edu/researchAreas/pdf/instructions for building OSP box.pdf 

AASHTO Roadway Lighting Design Guide

https://bookstore.transportation.org/item details.aspx?ID=1412
Credits 

Module 3, Slide 66

Slide 10: Image courtesy of Virginia Tech Transportation Institute, WSP Global Inc., and 

DMD & Associates Ltd. 

Slide 11: Table courtesy of Virginia Tech Transportation Institute, WSP Global Inc., and 

DMD & Associates Ltd. 

Slide 46: Photograph courtesy of Virginia Tech Transportation Institute, WSP Global 

Inc., and DMD & Associates Ltd. 

All other images taken from U.S. Government publications or are released to the public 

domain by the module authors.