Street Lighting Design: Layout & Calculations

Street lighting design sets the amount, distribution and control of light needed for people to use a road after dark. A design does not try to reproduce daylight. It aims to provide useful visibility while controlling glare, spill light, energy use and effects on the surrounding area. Road lighting can:

• Help people detect the road, other users and hazards
• Make junctions, crossings and changes in road geometry easier to identify
• Support wayfinding and access to roadside property
• Improve perceived personal security when coordinated with the wider streetscape

Basic Features of Street Light Luminaires

A roadway luminaire is selected by its measured photometric distribution, mounting arrangement and environmental performance. Common features include:

• A fixed mounting position and tilt defined in the design calculation
• An optical distribution shaped to place light on the carriageway, footway or other target area
• Controlled forward, side and back light; the distribution may be asymmetric to suit the road geometry
• Limits on glare, uplight and spill light set by the project criteria

Main Objectives of Street Lighting Design Scheme

A street-lighting scheme should:

1. Provide the maintained visibility criteria required for the road class and users
2. Control disability glare and avoid abrupt changes in visual adaptation
3. Meet energy, maintenance, environmental and roadside-safety requirements

Which Lamps are Used in Street Lighting?

LED luminaires dominate new road-lighting work because their optics and controls can direct and dim light efficiently. Existing installations may still contain these source types:

1. High-pressure sodium lamps in legacy installations
2. Metal Halide Lamps in some older white-light systems
3. Low-pressure sodium lamps in older systems
4. Incandescent Lamp, which is obsolete for normal roadway lighting
5. LED modules in current roadway luminaires
6. Compact fluorescent lamps in limited older low-output applications

Main Factors in the Street Lighting Design Scheme

1. Select the Maintained Lighting Criterion
   For motorised traffic, road-surface luminance helps predict the contrast seen by a driver. Illuminance criteria are often used for pedestrian areas, conflict areas or where pavement reflection data are unsuitable. The designer applies the class and values adopted by the road authority, including the required treatment of road edges and adjacent areas.
2. Meet the Required Uniformity
   Uniformity describes how values vary across the calculation grid. Overall luminance uniformity is the minimum divided by the average luminance:
   
   Longitudinal uniformity is a separate minimum-to-maximum ratio evaluated along specified driving lines. The applicable standard defines the grid, observer positions and limits.
3. Limit Glare
   Disability glare reduces visual performance and is commonly evaluated by threshold increment in road-lighting calculations. Discomfort glare causes annoyance or pain. Optical control, mounting height, tilt, spacing and output all affect glare.
4. Choose Suitable Light-Source Characteristics
   Spectral power distribution, colour rendering and correlated colour temperature affect appearance and visual tasks. The project must also consider environmental policy and the response of people, wildlife and nearby residents.
5. Provide Visual Guidance
   The pattern of luminaires and the lit road surface should make the route, intersections, crossings, curves and changes in width easy to read without creating misleading cues.

Types of Road to Implement Various Street Lighting Design Schemes

The Type A to E grouping below is a simplified legacy classification. It can help describe traffic context, but it must not replace the current lighting classes and maintained criteria adopted by the road owner or jurisdiction.
Type A of Street Lighting Design

• High-volume, high-speed motor traffic
• Physically separated carriageways
• No at-grade crossings
• Controlled access
• Example: an expressway

Type B of Street Lighting Design

• High-volume, high-speed motor traffic
• Separate facilities for slower traffic or pedestrians
• Example: a trunk road

Type C of Street Lighting Design

• High-volume mixed traffic at moderate speed
• Urban or rural context
• Examples: ring roads and radial roads

Type D of Street Lighting Design

• Low-speed traffic with frequent pedestrian activity
• Urban centre or retail setting
• Example: a shopping street

Type E of Street Lighting Design

• Mixed traffic with a low speed limit
• Local connection between residential areas
• Example: a local street

Street Light Luminaire

A laboratory measures a luminaire’s luminous-intensity distribution with a goniophotometer. The resulting photometric data can be shown as polar diagrams and supplied in a standard electronic file for lighting calculations.

Roadway photometry commonly uses the C-γ coordinate system. A C-plane is a vertical plane through the luminaire’s photometric centre. The γ angle is measured within that plane from the downward vertical axis to the direction being evaluated.

For a known intensity in the direction of a calculation point, direct horizontal illuminance follows the inverse-square and cosine relationship shown below:
The geometric relation is:

Here E_P is the illuminance at point P and h is the mounting height above the calculation plane. Modern design software reads intensities from the luminaire’s C-γ photometric table rather than deriving the table from road measurements.

The table maps intensity to each C-plane and γ angle. C’ marks the C-plane containing the maximum intensity in this legacy illustration.

Three reference planes are shown:

1. Plane 1: C-0^o to C-180^o, normally aligned along the road
2. Plane 2: C-90^o to C-270^o, normally aligned across the road
3. Plane 3: the principal plane through the maximum intensity, from C’ to C’ + 180^o

C’ is read from the measured photometric data at the direction of maximum intensity. The opposite half of that principal plane is C’ + 180^o. A design should use the manufacturer’s tested photometric file with the same luminaire, optics, output and tilt as the proposed installation.

Spread and Throw Angle of Street Light Luminaire

The two main terms related to the street light luminaire are:

1. Spread angle: a legacy descriptor for how far the main distribution extends across the road
2. Throw angle: a legacy descriptor for how far the main distribution extends along the road

The older graphical definitions are shown below. Current design work normally uses the complete photometric file rather than reducing the distribution to two angles:

Pole Arrangement Schemes in Street Lighting Design

Single Sided

A single-sided layout can suit a relatively narrow road. W = H is only a preliminary rule of thumb, not a design requirement.
Photometric calculation determines the mounting height, outreach and pole spacing that meet the maintained criteria.

Double Sided

An opposite double-sided layout places paired poles on both sides and can serve a wider carriageway. W = 2H is an initial layout guide only.
The calculation must check luminance or illuminance, uniformity and glare for the actual optics and pavement.

Staggered Sided or Zigzag Pattern

A staggered layout alternates poles between the two sides of the road. W = 1.5 H is a preliminary guide rather than a fixed limit.
Spacing is adjusted through calculation to avoid dark bands, excessive glare and poor longitudinal uniformity.

Central Verge Position

A central-verge layout can use single or twin outreach luminaires to serve divided carriageways. It may reduce roadside poles but adds maintenance-access and median-safety constraints.
The road width alone does not set the spacing; the final layout comes from photometric and roadside-safety analysis.

What are the Street Light Design Parameters?

Road-lighting criteria are calculated over a defined grid, normally covering one representative pole cycle. The designer then verifies the completed installation by measurement where the specification requires it.

1. Average maintained road-surface luminance in cd/m², or average maintained illuminance in lux when the illuminance method applies
2. Overall uniformity (U₀) over the specified calculation area for the selected luminance or illuminance method
3. Longitudinal uniformity along each required driving line
4. Transverse uniformity where the adopted method requires this legacy measure
5. Disability glare, commonly expressed as threshold increment
6. Discomfort glare or luminaire glare limits specified by the road authority
7. Installed power, energy use and controls, which may be normalised by lit area or road length.
   The legacy expression is:
   In this simplified layout count, the number of luminaires (n) = 1 for a single-sided pole position
   = 2 for an opposite double-sided pole position
   = 2 across a pair of staggered pole positions.

The best Arduino starter kits and a photoresistor can demonstrate dusk sensing, but they are not a road-lighting controller. A field system needs rated equipment, safe failure behaviour, manual override, surge protection and a control strategy that continues to meet the maintained lighting class.

How to Compute Average Illuminance on the Road Surface?

A preliminary average-illuminance estimate can use the lumen method with a maintenance factor (MF) and coefficient of utilisation (COU). Final roadway design should use point-by-point photometric calculations.

In the expression,
Φ_L = initial luminous flux of one lamp or luminaire,
A_eff = effective lit road area for one pole cycle = span × width = S × W

N = number of luminaires contributing to that pole cycle.
For the simplified layouts shown,
N = 1 for a single-sided arrangement and
N = 2 for opposite or staggered arrangements,
n = number of lamps or light engines per luminaire, if the legacy formula separates that quantity.
COU is the fraction of luminaire flux reaching the defined road area. It depends on the luminaire distribution and geometry, and should come from valid photometric data rather than a generic curve.

Point Specific Luminance (L) on the Road Surface

Point luminance depends on the light incident on the pavement and the light reflected toward the observer.
For one incident direction it can be written L = q × E,
where q is the pavement luminance coefficient in and depends on the observation geometry and two angles, β and γ.
β describes the azimuth between the incident-light plane and observation plane.
γ is the angle of incidence within the incident-light plane.

The geometry gives:


Because r and q vary with β and γ, a luminance calculation uses the applicable pavement reflection table and observer position:

How to Compute Point Specific Illuminance from Iso-Lux Diagram of the Street Light?

An iso-lux diagram joins points of equal horizontal illuminance on a reference plane. Some diagrams express each contour as a percentage of a stated maximum E_max. If E_max is 100 lux, a 73% contour represents 73% of E_max, or 73 lux. Points with the same percentage of E_max form one contour, and the full diagram contains several such maximum-relative contours. The maximum does not have to occur at the nadir directly below an asymmetric roadway luminaire, so the E_max location and diagram scale must be checked.

The example uses an iso-lux diagram for one luminaire.

The axes are normalised by mounting height h, so the proposed point must first be converted to the same coordinate system.

To find illuminance at point P, locate P relative to every luminaire that contributes useful light.
In this example, P is one mounting height from luminaire 1, two mounting heights from luminaire 2 and 0.8 mounting height from their roadside reference line.
Read or interpolate the contribution from each matching iso-lux diagram.
Let the contribution from luminaire 1 be E_P,1 = x₁%,
the contribution from luminaire 2 be E_P,2 = x₂%,
and the contribution from luminaire 3 be E_P,3 = x₃%.
The direct contributions add at P:

The legacy normalisation estimates E_max from:

Use the tested luminaire flux and photometric data to find E_max, then apply the required maintenance factor to E_P. Modern software performs the same point-by-point superposition across the full calculation grid.

Glare in Street Lighting

Glare occurs when light in the field of view causes discomfort or reduces the ability to see. Road-lighting work distinguishes two effects:

1. Disability Glare
2. Discomfort Glare

Disability Glare

Disability glare scatters light within the eye and creates a veiling luminance over the retinal image. This reduces the contrast of road users and objects without necessarily causing discomfort. It is different from the temporary afterimage caused by staring at a bright source.

For luminance-based road classes, disability glare is commonly limited by threshold increment. A calculation uses luminaire intensities in the observer’s direction to find equivalent veiling luminance.
Older nomograms graph the same relationship. T_I is the percentage increase in road luminance needed to restore the contrast threshold when glare is present.
The legacy expression shown is:

Here L_V is equivalent veiling luminance and L_avg is average road-surface luminance. The ratio of L_V to L_avg therefore represents the veiling effect relative to the road’s adaptation luminance. CIE 140:2019 corrected and updated the TI calculation, so current software should implement the adopted edition.

Discomfort Glare

Discomfort glare causes annoyance, irritation or pain and can lead to visual fatigue. It does not necessarily reduce visual performance by the same mechanism as disability glare. Source luminance, apparent source size, position, duration and the observer’s adaptation level all affect the response. The specific luminaire index (SLI) and glare control mark below are legacy descriptors rather than universal current criteria.

The older method denotes discomfort-glare control by G:

In that equation,
SLI = specific luminaire index,
L_avg = average road-surface luminance (cd/m²),
h’ = reduced mounting height (m), and
p = number of luminaires per kilometre.
SLI is a logarithmic index derived from the luminaire distribution.

In the legacy SLI equation,
I₈₀ and I₈₈ are luminous intensities in cd at γ angles of 80 and 88 degrees in the vertical plane parallel to the road axis.
F is the apparent light-emitting area (m²) of the luminaire viewed at γ = 76 degrees from the downward vertical.
C is the colour factor assigned to the spectral distribution of the electric lamp. The historical values are C = 0.4 for low-pressure sodium and C = 0 for the listed white lamps.
In that scale, SLI < 2 indicates limited glare control, 2 ≤ SLI ≤ 4 indicates moderate control, and SLI > 4 indicates high control.
A higher SLI therefore represents stronger discomfort-glare control within this older method. Use the glare metric required by the current project standard instead of mixing methods.