Road Lighting Design

Road lighting design scheme must be robust such that viewers can feel easy to continue their movement on the road and through the road in a safe level. Road lighting scheme never brings the day light appearance. Only moving objects must be caught in perfect visual sensation by the viewers due to a perfect design scheme.

What are the Basic Features of Road Light Luminaires?

The basic features of road way lighting luminaires are given below.

  • Road way luminaires are mounted horizontally and thus have fixed vertical aiming.
  • Road way lighting luminaires have particular intensity distributions which are desired to light long narrow horizontal stripes on one side of the luminaire, while minimizing the intensities on the other side of the luminaire.
  • The intensity distributions up and down the narrow strip are generally the same.
  • Any fixed aimed luminaire which does not have this type of intensity distribution is called an area luminaire.

What are Main Objectives of Road Lighting Design Scheme?

The main objectives of road lighting design scheme are given below:

  1. Perfect visual sensation for safety
  2. Illuminated environment for quick movement of the vehicles
  3. Clear view of objects for comfortable movement of the road users.

Which Lamps are Used in Road Lighting?

Various types of lamps are used in road lighting luminaires. They are

  1. High pressure sodium lamp
  2. Metal Halide Lamps
  3. Low pressure sodium lamps
  4. Incandescent Lamp (not recommended)
  5. LED
  6. CFL (used in Lanes or streets only not widely)

What are the Main Factors to be Considered in the Road Lighting Design Scheme?

  1. Luminance Level Should be Proper
    Luminance always influences the contrast sensitivity of the obstructions with respect to the back ground. If the road is brighter, then darker surroundings makes the car driver adapted, unless the driver will be unable to perceive the objects in the surroundings. As per CIE, 5m away from the road on both sides will be lit by Illuminance level at least 50% of that on the road.
  2. Luminance Uniformity must be Achieved
    To provide visual comfort to the viewer’s eyes, enough luminous uniformity is needed. Luminous uniformity means the ratio between minimum luminance level to average luminance level, i.e.

    It is termed as longitudinal uniformity ratio as it is measured along the line passing through the viewers position in the middle of the traffic facing the traffic flow.
  3. Degree of Glare Llimitation is always taken into Design Scheme
    Glare means visual discomfort due to high luminance. There are two types of glare created by the road light luminaires, first type is disability glare and second type is discomfort glare. Disability glare is not a strong factor, rather discomfort glare is a common factor due to unplanned road lighting scheme.
  4. Lamp Spectra for Visual Sharpness depends on the Proper Luminaries
    It is very much essential to make an object as per its size and dimension.
  5. Effectiveness of Visual Guidance is also Important Factor
    It helps a viewer to guess how far another object is from his position.

What are the Types of Road to Implement Various Road Lighting Design Schemes?

As per CIE 12 roads are broadly classified into five types.
Type A of Road Lighting Design

  • Heavy and high speed traffic.
  • The roads are separated with the separators.
  • No crossing is allowed.
  • Controlled access
  • As the example: express ways.

Type B of Road Lighting Design

  • Heavy and high speed traffic.
  • Separate road for slow traffic movement or pedestrians.
  • As the example: Trunk road.

Type C of Road Lighting Design

  • Heavy mixed traffic with moderate speed.
  • Rural and urban roads.
  • As the examples: Ring Road or Radial Road.

Type D of Road Lighting Design

  • Slow traffic and pedestrians’ purpose.
  • Road in the city or shopping center.
  • As the example: Shopping streets.

Type E of Road Lighting Design

  • Mixed Traffic with limited speed.
  • Connector road between residential areas.
  • As the example: Local street.

Road Light Luminaire

Intensity distribution of the road light luminaire is measured with mirror Goniophotometer. And it is graphically represented by polar intensity diagram.
But Intensity distribution of the road light is measured following C-ɣ photometric convention. In C-ɣ photometry, C is the angle on the road surface plane and ɣ is the angle created between vertical axis of the luminaire and lumen throwing direction, or in other word, ɣ is the angle of incidence.
Initially on the surface of the road, point specific Illuminance values are collected.
Then intensity I is calculated from the equation of the Illuminance,

Where, EP is the Illuminance at point P on the road and h is the vertical height from the point P to the luminaire. After calculation of the intensity, we put all the intensity values making a C-ɣ table as per their angular position.
intensity distribution chart
The format of C-ɣ table is shown above. In this above chart C’ is the position of maximum intensity on the table.
road light luminaire
Three basic planes of intensity are considered on the road surface with respect to one luminaire.

  1. Plane 1: C-0o to C-180o along the road.
  2. Plane 2: C-90o to C-270o across the road.
  3. Plane 3: Principle Plane, through the point of maximum intensity of the light, i.e. C’ to C’ + 180o

To obtain C’ we have to prepare intensity distribution chart of the road light luminaire on the road. Where intensity will meet at maximum value this is the degree value of C’. To draw the principle plane axis we have to add 180o with C’.

What is the Spread and Throw Angle of Road Light Luminaire?

Here the two terms are related to the road light luminaire.

  1. Spread angle: it is the angle of the luminaire to direct the luminous flux across the road.
  2. Throw angle: it is the angle of the luminaire to direct the luminous flux along the road.

It is denoted by

spread and throw angle luminaire

How are Pole Arrangement Schemes in Road Lighting Design?

Single Sided

When the width (W) of the road is nearly equal to the pole height (H), i.e. W = H then the poles are arranged in one side only. Generally pole height is available of 10 meter.
The span between two poles is equal to the road width.
single sided

Double Sided

When the width (W) of the road is nearly double the pole height (H), i.e. W = 2H then the poles are arranged along both sides in opposite to each other manner.
The span between two poles may not be equal to the road width.
double sided

Staggered Sided or Zigzag Pattern

When the width (W) of the road is nearly 1.5 times of the pole height (H), i.e. W = 1.5 H then the poles are arranged in both sides in zigzag manner.
The span between two poles may not be equal to the road width.
staggered  sided or zigzag pattern

Central Verge Position

When the width (W) of the road is much greater than the pole height (H), i.e. W>>H then the poles are arranged in the central verge of the road. The luminaires are made to face towards both the road surfaces from the central verge.
The span between two poles may not equal to the road width.
central verge position

What are the Road Light Design Parameters?

Road light design parameter is measured or evaluated or simulated over the span of the road.

  1. Average maintained luminance level in Lux
  2. Over all uniformity (U0)of Illuminance for entire area (span × width) of the road
  3. Longitudinal Uniformity is measured along the length of the road (centre length by default)
  4. Transverse uniformity is measured across the road along a line passing through the nadir point.
  5. Disability glare is expressed in threshold increment.
  6. Discomfort Glare is expressed in glare control mark.
  7. Unit Power Density is measured for unit length.
    where no. of the luminaire (n) = 1 for single sided pole arrangement
    = 2 for double sided pole arrangement
    = 2 for staggered sides pole arrangement.

How to Compute Average Illuminance on the Road Surface ?

Average Illuminance is calculated by Lumen method, where maintenance factor (MF) and coefficient of utilization (COU) are taken into account.

ΦL = Lumen of the Luminaire,
Aeff = effective road surface area under Illumination = Span × Width = S × W
average illuminance computation on the road surface
N = Number of luminaire
N = 1 for single sided road lighting design and
N = 2 for double and staggered sided road lighting design,
n = number of lamps used in single luminaire = 1 for road lighting.
Coefficient of Utilization (COU) is the ratio of utilized lumen to the installed lumen. And it is obtained from the COU graph recommended by CIE.

coefficient of utilization

Point Specific Luminance (L) on the Road Surface

It is related to the point specific Illuminance (E).
It is expressed as, L = q × E,
Where, q is the luminance coefficient in and it is dependent on two angles β and ɣ.
β is the angle between plane of light incidence (plane 1) and plane observation (plane 2).
Ɣ is the angle of incidence in plane 1.
point specific luminance
So now,

As r and q both are the function of two angles β and ɣ, we should write the equation as

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

Iso-Lux diagram is the Illuminance distribution of the street light luminaire on the street or road surface. The point of maximum Illuminance is called Nadir Point. The Illuminance level of other points are given in percentage value with respect to the Emax of Nadir point. Suppose, Emax at nadir is 100 Lux, and at other point Illuminance is 73 Lux, then this point is marked as 73% of Emax. Thus all points with 73% of Emax are joint together to get Iso-Lux diagram for 73% of Emax. All Iso-Lux curves are drawn in this way. Making the nadir point center, two axes along and across the road is drawn.
Suppose, we have the Iso-Lux diagram of the street light.
computation of point specific illuminance
As per the above sample diagram of Iso-Lux, we have to divide dimension of two axes to term with respect to the luminaire height (h).
plan view of the road
Suppose at point P, we have to calculate Illuminance, and we have already the luminaire Iso-Lux diagram.
Now we find out the co-ordinate of this point P with respect to the luminaire position. Suppose this point P is at h distance from luminaire 1 and at 2h distance from luminaire 2 and at 0.8h distance from the road side of luminaires 1 and 2.
Now we have to calculate Illuminance at point P for each luminaire one by one from the Iso-Lux diagram.
Let, Illuminance contribution of luminaire 1 at point P is EP,1 = x1%,
Illuminance contribution of luminaire 2 is at point P is EP,2 = x2%,
Illuminance contribution of luminaire 3 is at point P is EP,3 = x3%,
So, ultimate Illuminance at the point P is

Again Emax is calculated from CIE recommended equation, i.e.

The value of Φ is already provided by the luminaire manufacturer. So we can get the value of Emax and hence EP at the point P.

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