Calculations - Roadway - Concepts

The Calculation Points - Roadway command allows you to specify several lighting metrics associated with roadway lighting applications in a straight segment of roadway. The resulting calculations are calculated based on the user’s selection of the roadway Standard to which they will be designing. AGi32 includes default input parameters for grid placement and point spacing based on ten roadway Standards in use around the world:

This table shows the calculation metrics that are defined in each Standard (See the Roadway Standards command for more details and to setup a custom standard):

  IES RP-8-14 IES RP-8-2000 IES RP-22-2011 CIE 140-2000* CIE 140-2000-TN-007-2017 BSEN 13201-3 .2015 ABNT NBR5101-2012 ABNT NBR5101-2012(C) Australia 1158.2-2005 New Zealand 1158.2-2005
Pavement Luminance X X X X X X X   X X
Illuminance X X X X X X X X X X
Veiling Luminance X X X X X X     X  
Veiling Luminance filtering for foveal aligned sources and sources >60 degrees       X X X     X X
Visibility Level (STV) X X                
Longitudinal Uniformity       X X X X   X X
Vertical Illuminance           X        
Semi-cylindrical Illuminance       X X X X      
Hemispherical Illuminance           X        
Surround Illuminance       X X X X   X X
Veiling Luminance w/TI       X X X X   X X
Mesopic Luminance         X          

* Mesopic Luminance calculations are in CIE TN-007:2017.

Notes:

Roadway Luminance Calculations With Obstructive and/or Reflective Surfaces

Tunnels: In Full Radiosity calculations, the Roadway Luminance calculation can take reflective or obstructive entities around or within the luminance grid into account, per IES Recommended Practice for Tunnel Lighting, RP-22, recommendations. This is also true for the associated illuminance and veiling luminance calculations.

Consideration of Surfaces in Roadway Calculations:

More information is available in the Roadway Luminance Calculations With Reflective Surfaces topic.

Technical Details

The Roadway Optimizer tool and Roadway Luminance command provide assistance in computing calculations for straight segments of roadways using IES, CIE, BSEN, CIE-Australia, or CIE-New Zealand methods. Some of the criteria and recommended values are provided here.

Terminology, by Standard

IES RP-8-00 and RP-8-14

AGi32 creates the calculation grids used to calculate Pavement Luminance, Illuminance, and Veiling Luminance per the last two revisions of RP-8 (RP-8-00, displayed as RP-8-2000, and RP-8-14).

Average Pavement Luminance (Lavg) - Luminance on the pavement is based on the quantity and direction of light, observer location, and the pavement reflectance characteristics. The overall average luminance of the road surface as observed from a specific point in cd/m2. Metric units are used regardless of current settings. Observer position is 1.45 meters above the pavement surface and 83.07 meters back from each computation point along a longitudinal line parallel to the direction of travel. Observer line of sight is 1 degree below horizontal. Observer position changes relative to the curb line to align with each row of calculation points. A Roadway Luminance grid represents the calculation points for a single directional flow of traffic. To consider the entire roadway, a calculation grid should be created for each direction of traffic flow. The directional flow of traffic is determined by the specified order of the curb points defining the length of the roadway. Traffic flows in the direction of first point to second point. The third point defines the width of the roadway from curb to curb in one direction of traffic flow.

Illuminance - Illuminance is a measure of the lumens incident on the pavement divided by the area. The units are lumens per square foot (footcandles) or lumens per square meter (lux). Illuminance is not impacted by the pavement's surface type or the angle of observation.

Veiling Luminance (Lv) - A measure of disability glare, veiling luminance is a luminance superimposed over the eye's retinal image that is produced by stray light within the eye. In the IES method, Lv is computed at the same points as the pavement luminance with the observer 83.07 meters back from each computation point. Observer height is 1.45 m above the road surface, with line of sight 1 degree below horizontal. Line of Sight (LOS) filtering for foveal sources is not included. Off axis >60 degrees filtering is not included.

Veiling Luminance ratio - The IES method uses the maximum value of Veiling Luminance (Lv,max) divided by the Average Pavement Luminance (Lavg) as a measure of the disability glare produced by a lighting system.

Weighted Average VL (STV) – A measure of the visibility of small targets in the roadway considering the target luminance, veiling luminance and the background pavement luminance against which they are seen. The weighted average of all Visibility Levels (STV) is given. An Visibility Level grid may be included in the calculated results. It is recommended that this grid be examined, as it provides information about contrast bands . When considering Visibility Level and STV, designs are created that produce negative and positive contrast, all positive contrast, and all negative contrast. The weighted average of the calculated Visibility Levels is displayed as the STV.


IES RP-22-11

AGi32 creates the calculation grids used to calculate Pavement Illuminance, Luminance and Veiling Luminance per the latest revision of RP-22 (RP-22-11, displayed as RP-22-2011).

Average Pavement Luminance (Lavg) - Luminance on the pavement is based on the quantity and direction of light, observer location, and the pavement reflectance characteristics. The overall average luminance of the road surface as observed from a specific point in cd/m2. Metric units are used regardless of current settings. Observer position is 1.45 meters above the pavement surface and 83.07 meters back from each computation point along a longitudinal line parallel to the direction of travel. Observer line of sight is 1 degree below horizontal. Observer position changes relative to the curb line to align with each row of calculation points. A Roadway Luminance grid represents the calculation points for a single directional flow of traffic. To consider the entire roadway, a calculation grid should be created for each direction of traffic flow. The directional flow of traffic is determined by the specified order of the curb points defining the length of the roadway. Traffic flows in the direction of first point to second point. The third point defines the width of the roadway from curb to curb in one direction of traffic flow.

Illuminance - Illuminance is a measure of the lumens incident on the pavement divided by the area. The units are lumens per square foot (footcandles) or lumens per square meter (lux). Illuminance is not impacted by the pavement's surface type or the angle of observation.

Veiling Luminance (Lv) - A measure of disability glare, veiling luminance is a luminance superimposed over the eye's retinal image that is produced by stray light within the eye. In the IES method, Lv is computed at the same points as the pavement luminance with the observer 83.07 meters back from each computation point. Observer height is 1.45 m above the road surface, with line of sight 1 degree below horizontal. Line of Sight (LOS) filtering for foveal sources is not included. Off axis >60 degrees filtering is not included.

Veiling Luminance ratio - The IES method uses the maximum value of Veiling Luminance (Lv,max) divided by the Average Pavement Luminance (Lavg) as a measure of the disability glare produced by a lighting system.


CIE 140-2000

Note:CIE 115-2010 uses the same calculation methods, including observer location, point spacing, etc.

Average Pavement Luminance - The overall average luminance of the road surface as observed from a specific point in cd/m2. Metric units are used, regardless of current settings. By default, observer position is 1/4 roadway width inset from the curb, 1.5 m above the pavement surface and 60m back from the first lateral row of computation points in the grid. Line of sight is 0.5 - 1.5 degrees below horizontal, depending on the distance to the points. Observer position is static relative to the curb line.

Illuminance - Illuminance is a measure of the lumens incident on the pavement divided by the area. The units are lumens per square meter (lux). Illuminance is not impacted by the pavement's surface type or the angle of observation.

Longitudinal Uniformity - The ratio of minimum to maximum pavement luminance, illuminance or veiling luminance is taken along each longitudinal line of computation points. The worst-case ratio is displayed as the Longitudinal Uniformity Ratio. For the CIE method, an additional grid of pavement luminance points is computed, with observer position centered in the middle of each lane, 60m back from the first lateral row of computation points in the grid, and viewing along the line of points in that lane.

Surround Ratio - This is a ratio of the average horizontal illuminance on the two longitudinal strips each adjacent to the two edges of the carriageway (the surround illuminance), divided by the average horizontal illuminance on two longitudinal strips each adjacent to the two edges of the carriageway, but lying on the carriageway. The width of all four strips shall be the same, and equal to 5m, or half the width of the carriageway, or the width of the unobstructed strip lying off the carriageway, whichever is the least. For dual carriageways, both carriageways together are treated as a single carriageway unless they are separated by more than 10m.

Veiling Luminance (Lv) - A measure of disability glare, Veiling Luminance is a luminance superimposed over the eye's retinal image produced by stray light within the eye. In the CIE method, Lv is computed along a single row of calculation points directly in front of the observer position. By default, the distance of the observer to the first lateral row of computation points in the grid is 2.75 x (mounting ht - 1.5) meters and 1/4 road width from the curb line. 0.5 - 1.5 degrees below horizontal, depending on the distance to the points. The maximum value of Lv is used to compute the relative Threshold Increment (TI). NOTE: The CIE 140-2000 method of calculating veiling luminance is per 1000 lamp lumens. This generates rather small values. Therefore, it is recommended that if you are calculating veiling luminance with the CIE standard, you first open the Units dialog (bottom toolbar in Model Mode or top toolbar in Roadway Optimizer) and set the Number of Decimals value to 2 or 3. This will help to avoid generating all zeros.

CIE-140-2000 provides the ability to filter the luminaires considered in the calculation of Lv to eliminate those directly in the foveal area, defined as 0.1 - 1.5 degrees form the observer line of sight (LOS), and for those luminaires outside of 60 degrees from the LOS. Both of these filters are engaged by default. Lv is computed per standard method of CIE-140-2000.

Relative Threshold Increment (TI) - A measure of the effect of disability glare produced by the lighting system. Threshold Increment is intended to yield the percentage increase in the luminance of the road's surface required to render an object just visible (threshold of visibility) under the proposed lighting system (glare present) as compared to the luminance required to render the object just visible in the absence of glare. Important note: Threshold Increment is calculated differently here than in AS4282-1997 or CIE 150:2003.

CIE 140-2000-TN 007-2017

Same as CIE-140-2000 with the exception that Mesopic Luminance calculations are included.

Mesopic Luminance - CIE TN 007:2017 provides an interim recommendation for the application of Mesopic Photometry in outdoor lighting, including roadways. This Technical Note from the CIE defines the Adaptation Coefficient to be used in conjunction with the Mesopic equations from CIE 191:2010. AGi32, in correlation with this document uses the average luminance of the design area (area covered by computation points) as the adaptation luminance for the purposes of calculating mesopic luminance.

BSEN 13201-3.2003

Note:BSEN 5489-1:2013 uses the same calculation methods, including observer location, point spacing, etc.

Average Pavement Luminance - The overall average luminance of the road surface as observed from a specific point in cd/m2. Metric units are used, regardless of current settings. By default, observer position is 1/4 roadway width inset from the curb, 1.5 m above the pavement surface and 60m back from the first lateral row of computation points in the grid. Line of sight is 0.5 - 1.5 degrees below horizontal, depending on the distance to the points. Observer position is static relative to the curb line.

Horizontal Illuminance - Horizontal illuminance at a point is a measure of the lumens incident on the pavement divided by the area. The units are lumens per square meter (lux). Illuminance is not impacted by the pavement's surface type or the angle of observation.

Longitudinal Uniformity - The ratio of minimum to maximum pavement luminance, illuminance or veiling luminance is taken along each longitudinal line of computation points. The worst-case ratio is displayed as the Longitudinal Uniformity Ratio. For the CIE method, an additional grid of pavement luminance points is computed, with observer position centered in the middle of each lane, 60m back from the first lateral row of computation points in the grid, and viewing along the line of points in that lane.

Vertical Illuminance - Measured in lux, vertical illuminance at a point is calculated on a plane 1.5 m above the surface of the road. The grid is located on the same points as the roadway luminance grid.

Semi-cylindrical Illuminance - Measured in lux, semi-cylindrical illuminance at a point could be considered a type of vertical illuminance, but its calculated value varies with the direction of interest. It is calculated at 1.5 meters above the road surface and on a half cylinder whose front is lined up parallel to the main directions of pedestrian movement, which for a road are usually longitudinal. The grid is located on the same points as the roadway luminance grid.

Hemispherical Illuminance - Measured in lux, hemispherical illuminance at a point is calculated at ground level on the road surface. Its formula is similar to the formula for horizontal illuminance, but differs from it in the way that the angle of incidence is considered.

Surround Ratio - This is a ratio of the average horizontal illuminance on the two longitudinal strips each adjacent to the two edges of the carriageway (the surround illuminance), divided by the average horizontal illuminance on two longitudinal strips each adjacent to the two edges of the carriageway, but lying on the carriageway. The width of all four strips shall be the same, and equal to 5m, or half the width of the carriageway, or the width of the unobstructed strip lying off the carriageway, whichever is the least. For dual carriageways, both carriageways together are treated as a single carriageway unless they are separated by more than 10m.

Veiling Luminance (Lv) - A measure of disability glare, Veiling Luminance is a luminance superimposed over the eye's retinal image produced by stray light within the eye. In the BSEN method, Lv is computed along a single row of calculation points at the center of each lane. The observer for lane #1 looks across all lanes of traffic including those in the opposite direction. By default, the distance of the observer to the first lateral row of computation points in the grid is 2.75 x (mounting ht - 1.5) meters and 1/4 road width from the curb line. 0.5 - 1.5 degrees below horizontal, depending on the distance to the points. The maximum value of Lv is used to compute the relative Threshold Increment (TI).

BSEN 13201-3_2015 provides the ability to filter the luminaires considered in the calculation of Lv to eliminate those outside of 60 degrees from the LOS. This is selected by default. Sources within the direct line of sight (LOS), defined as 0.1 - 1.5 degrees from the observer LOS are handled by method described in CIE-146-2002. The switch to filter LOS luminaires is not selected.

Relative Threshold Increment (TI) - A measure of the effect of disability glare produced by the lighting system. Threshold Increment is intended to yield the percentage increase in the luminance of the road's surface required to render an object just visible (threshold of visibility) under the proposed lighting system (glare present) as compared to the luminance required to render the object just visible in the absence of glare. Important note: Threshold Increment is calculated differently here than in AS4282-1997 or CIE 150:2003.


ABNT NBR5101-2012

Average Pavement Luminance - The overall average luminance of the road surface as observed from a specific point in cd/m2. Metric units are used, regardless of current settings. By default, observer position is 1/4 roadway width inset from the curb, 1.5 m above the pavement surface and 60m back from the first lateral row of computation points in the grid. Line of sight is 0.5 - 1.5 degrees below horizontal, depending on the distance to the points. Observer position is static relative to the curb line.

Illuminance - Illuminance is a measure of the lumens incident on the pavement divided by the area. The units are lumens per square meter (lux). Illuminance is not impacted by the pavement's surface type or the angle of observation.

Longitudinal Uniformity - The ratio of minimum to maximum pavement luminance, illuminance or veiling luminance is taken along each longitudinal line of computation points. The worst-case ratio is displayed as the Longitudinal Uniformity Ratio. For the CIE method, an additional grid of pavement luminance points is computed, with observer position centered in the middle of each lane, 60m back from the first lateral row of computation points in the grid, and viewing along the line of points in that lane.

Semi-cylindrical Illuminance - Measured in lux, semi-cylindrical illuminance at a point could be considered a type of vertical illuminance, but its calculated value varies with the direction of interest. It is calculated at 1.5 meters above the road surface and on a half cylinder whose front is lined up parallel to the main directions of pedestrian movement, which for a road are usually longitudinal. The grid is located on the same points as the roadway luminance grid.

Surround Ratio - This is a ratio of the average horizontal illuminance on the two longitudinal strips each adjacent to the two edges of the carriageway (the surround illuminance), divided by the average horizontal illuminance on two longitudinal strips each adjacent to the two edges of the carriageway, but lying on the carriageway. The width of all four strips shall be the same, and equal to 5m, or half the width of the carriageway, or the width of the unobstructed strip lying off the carriageway, whichever is the least. For dual carriageways, both carriageways together are treated as a single carriageway unless they are separated by more than 10m.

Veiling Luminance (Lv) - A measure of disability glare, Veiling Luminance is a luminance superimposed over the eye's retinal image produced by stray light within the eye. In the CIE method, Lv is computed along a single row of calculation points directly in front of the observer position. By default, the distance of the observer to the first lateral row of computation points in the grid is 2.75 x (mounting ht - 1.5) meters and 1/4 road width from the curb line. 0.5 - 1.5 degrees below horizontal, depending on the distance to the points. The maximum value of Lv is used to compute the relative Threshold Increment (TI). NOTE: The NBR5101-2012 method of calculating veiling luminance is per 1000 lamp lumens. This generates rather small values. Therefore, it is recommended that if you are calculating veiling luminance with the CIE standard, you first open the Units dialog (bottom toolbar in Model Mode or top toolbar in Roadway Optimizer) and set the Number of Decimals value to 2 or 3. This will help to avoid generating all zeros.

ABNT NBR5101-2012 provides the ability to filter the luminaires considered in the calculation of Lv to eliminate those directly in the foveal area, defined as 0.1 - 1.5 degrees form the observer line of sight (LOS), and for those luminaires outside of 60 degrees from the LOS. Both of these filters are engaged by default. Lv is computed per standard method of CIE-140-2000.

Relative Threshold Increment (TI) - A measure of the effect of disability glare produced by the lighting system. Threshold Increment is intended to yield the percentage increase in the luminance of the road's surface required to render an object just visible (threshold of visibility) under the proposed lighting system (glare present) as compared to the luminance required to render the object just visible in the absence of glare.


Australia Standard 1158.2- 2005

Average Pavement Luminance - The overall average luminance of the road surface as observed from a specific point in cd/m2. Metric units are used, regardless of current settings. By default, observer position is 1/4 roadway width inset from the curb, 1.5 m above the pavement surface and 60m back from the first lateral row of computation points in the grid. Line of sight is 0.5 - 1.5 degrees below horizontal, depending on the distance to the points. Observer position is static relative to the curb line.

Illuminance - Illuminance is a measure of the lumens incident on the pavement divided by the area. The units are lumens per square meter (lux). Illuminance is not impacted by the pavement's surface type or the angle of observation.

Longitudinal Uniformity - The ratio of minimum to maximum pavement luminance, illuminance or veiling luminance is taken along each longitudinal line of computation points. The worst-case ratio is displayed as the Longitudinal Uniformity Ratio. For the CIE method, an additional grid of pavement luminance points is computed, with observer position centered in the middle of each lane, 60m back from the first lateral row of computation points in the grid, and viewing along the line of points in that lane.

Surround Ratio - This is a ratio of the average horizontal illuminance on the two longitudinal strips each adjacent to the two edges of the carriageway (the surround illuminance), divided by the average horizontal illuminance on two longitudinal strips each adjacent to the two edges of the carriageway, but lying on the carriageway. The width of all four strips shall be the same, and equal to 3 meters. The grid shall be placed at the same distance as the luminance calculation field.

Veiling Luminance (Lv) - A measure of disability glare, Veiling Luminance is a luminance superimposed over the eye's retinal image produced by stray light within the eye. In the BSEN method, Lv is computed along a single row of calculation points directly in front of the observer position. By default, the distance of the observer to the first lateral row of computation points in the grid is 2.75 x (mounting ht - 1.5) meters and 1/4 road width from the curb line. 0.5 - 1.5 degrees below horizontal, depending on the distance to the points. The maximum value of Lv is used to compute the relative Threshold Increment (TI).

AS 1158.2-2005 provides the ability to filter the luminaires considered in the calculation of Lv to eliminate those directly in the foveal area, defined as 0.1 - 1.5 degrees form the observer line of sight (LOS), and for those luminaires outside of 60 degrees from the LOS. Both of these filters are engaged by default. Lv is computed per the deprecated standard method of CIE-30-2 (1990).

Relative Threshold Increment (TI) - A measure of the effect of disability glare produced by the lighting system. Threshold Increment is intended to yield the percentage increase in the luminance of the road's surface required to render an object just visible (threshold of visibility) under the proposed lighting system (glare present) as compared to the luminance required to render the object just visible in the absence of glare. Important note: Threshold Increment is calculated differently here than in AS4282-1997 or CIE 150:2003.


New Zealand Standard 1158.2- 2005

Average Pavement Luminance - The overall average luminance of the road surface as observed from a specific point in cd/m2. Metric units are used, regardless of current settings. By default, observer position is 1/4 roadway width inset from the curb, 1.5 m above the pavement surface and 60m back from the first lateral row of computation points in the grid. Line of sight is 0.5 - 1.5 degrees below horizontal, depending on the distance to the points. Observer position is static relative to the curb line.

Illuminance - Illuminance is a measure of the lumens incident on the pavement divided by the area. The units are lumens per square meter (lux). Illuminance is not impacted by the pavement's surface type or the angle of observation.

Longitudinal Uniformity - The ratio of minimum to maximum pavement luminance, illuminance or veiling luminance is taken along each longitudinal line of computation points. The worst-case ratio is displayed as the Longitudinal Uniformity Ratio. For the CIE method, an additional grid of pavement luminance points is computed, with observer position centered in the middle of each lane, 60m back from the first lateral row of computation points in the grid, and viewing along the line of points in that lane.

Surround Ratio - This is a ratio of the average horizontal illuminance on the two longitudinal strips each adjacent to the two edges of the carriageway (the surround illuminance), divided by the average horizontal illuminance on two longitudinal strips each adjacent to the two edges of the carriageway, but lying on the carriageway. The width of all four strips shall be the same, and equal to 3 meters.The grid shall be placed at the same distance as the luminance calculation field.

Veiling Luminance (Lv) - A measure of disability glare, Veiling Luminance is a luminance superimposed over the eye's retinal image produced by stray light within the eye. In the BSEN method, Lv is computed along a single row of calculation points directly in front of the observer position. By default, the distance of the observer to the first lateral row of computation points in the grid is 2.75 x (mounting ht - 1.5) meters and 1/4 road width from the curb line. 0.5 - 1.5 degrees below horizontal, depending on the distance to the points. The maximum value of Lv is used to compute the relative Threshold Increment (TI).

AS 1158.2-2005 provides the ability to filter the luminaires considered in the calculation of Lv to eliminate those directly in the foveal area, defined as 0.1 - 1.5 degrees form the observer line of sight (LOS), and for those luminaires outside of 60 degrees from the LOS. Both of these filters are engaged by default. Lv is computed per the deprecated standard method of CIE-30-2 (1990).

Relative Threshold Increment (TI) - A measure of the effect of disability glare produced by the lighting system. Threshold Increment is intended to yield the percentage increase in the luminance of the road's surface required to render an object just visible (threshold of visibility) under the proposed lighting system (glare present) as compared to the luminance required to render the object just visible in the absence of glare. Important note: Threshold Increment is calculated differently here than in AS4282-1997 or CIE 150:2003.


R-Tables

A roadway's reflective characteristics are defined by its physical surface properties. Sufficient data has been collected on the reflectance characteristics of different pavement types to allow them to be described by reflectance tables or "R" tables.

Several typical roadway pavements are provided for your use, as defined in the R-Table pull-down menu.

R-Table

Q0  

Description

C1 0.10 CIE C1 - Concrete
C2 0.07 CIE C2 - Asphalt
N1 0.10 CIE Class = 1, Very Diffuse
N2 0.07 CIE Class = 1, Concrete
N3 0.07 CIE Class = 3, Asphalt
N4 0.08 CIE Class = 4, Glossy Asphalt
NZN2 0.09 New Zealand - Glossy Polished Asphaltic Surface
NZN4 0.09 New Zealand - Diffuse Chip Seal Surface

R1

0.10

IES RP-8 - Mostly diffuse reflectance properties characteristic of Portland cement or asphalt surface with a minimum of 15% of the aggregates composed of artificial brightener aggregates.

R2

0.07

IES RP-8 - A combination of diffuse and specular reflectances characteristic of asphalt surfaces with aggregate composed of a minimum of 60% gravel of size greater than 10 mm. Also asphalt surfaces composed of 10% - 15% artificial brightener in aggregate mix.

R3

0.07

IES RP-8 - Slightly specular reflectance typical of asphalt surfaces with dark aggregates, rough texture and some months of use. This surface is common in the United States.

R4

0.08

IES RP-8 - Mostly specular surface typical of very smooth asphalt texture.

UKPA

0.05

UK - Porous Asphalt
W1 0.11 CIE W1 - Wet Road Surface
W2 0.15 CIE W2 - Wet Road Surface
W3 0.21 CIE W3 - Wet Road Surface
W4 0.25 CIE W4 - Wet Road Surface

ZOAB

0.10

CIE Class = 2, Dutch Porous Asphalt

Q0 is a value that is related to the overall reflectance of the pavement by a factor of π: Q0 x π = overall reflectance.

Additional R-tables can be added to AGi32 easily. Please contact Lighting Analysts for details.

Note: If a user creates an R-Table and then uses it in an AGi32 calculation, that R-Table is not included with the saved AGI file.  Therefore, if that AGI file is transferred to another computer, AGi32 will not find the custom R-Table and will use the default (R3). The custom R-Table would have to be added to the new computer in the ProgramData folder.


Illuminance vs Luminance

RP-8-14's criteria are luminance based, except for curvy or hilly roads or streets. In those cases, the criteria are illuminance based.

The average pavement luminance is influenced by three factors: the quantity and direction of light, the observer location, and the reflective characteristics of the pavement. Illuminance, on the other hand, measures the light incident on the pavement surface; observer direction and pavement reflectance are irrelevant.

While there is certainly a strong relationship between luminance and illuminance, it is possible to meet the Illuminance criteria, yet be substantially far away from the recommended Luminance guidelines, and vice versa.