Testing the quality of road lighting according to the criterion of illuminance

The quality of road lighting and the way in which urban spaces and public spaces are illuminated have been widely discussed in recent years in view of the increasing use of LED lighting, which is significantly different from traditional lighting systems with discharge lamps.

New technology affects the change in the construction of the luminaire, the level of luminance (brightness) of the source, the colour temperature and the photometric shape i.e. the luminous intensity distribution (LID), which in turn allows for significant savings of electric energy. Currently, in many cities and municipalities, numerous modernisations of road lighting are being carried out. Old luminaires are replaced by luminaires made in LED technology. However, there are situations in which, as a result of such replacement, the expected improvement in the quality of road lighting does not occur, and sometimes LED lighting is criticized only because it replaced the previously used lighting, to which users were already accustomed.

So how can cities, municipalities and road managers verify the quality of lighting performance in the scope of basic lighting parameters? In the previous article we discussed the ways to verify lighting through the measurements of luminance on the road, which apply to roads of M class (Motor), that is, mainly those with high traffic intensity, and expressways. In this article we will discuss the principles of lighting design for other classes of road lighting, where the illuminance method is used during design and verification. We will also discuss the standards and regulations in force and present additional good practices and conditions, which influence the way in which road lighting is designed and evaluated but which are not required by law.

Which roads, why and how do we illuminate?

A properly designed and properly executed road lighting installation is intended to provide all traffic participants, i.e. drivers, cyclists and pedestrians, with adequate visual comfort and has an impact on the improvement of traffic safety and on the improvement of the general feeling of safety after dark. In many countries, for economic reasons, the lighting of all streets and roads is not used, and the applicable regulations specify the places and sections that must be illuminated at night [1].

Such regulations usually focus on specific locations or conditions of public roads where road lighting should be used such as:

  • when the road crosses an illuminated area and traffic participants may experience glare,
  • at a junction or crossroads with a road already illuminated, with a transition zone between the illuminated and non-illuminated section of the road with decreasing illuminance and a length of not less than 200 m – on motorways (class A) or expressways (class S) and 100 m – on roads with accelerated traffic (class GP) and on roads of lower classes,
  • at the intersection with the expressway and at the roundabout intersection,
  • on the main roads with accelerated traffic when there is a crossroads with islands in kerbstones,
  • between illuminated sections – if the length of the section does not exceed 500 m,
  • in the section adjacent to the illuminated bridge,
  • at toll booths,
  • on a single carriageway street with four or more lanes,
  • at the crossroads in the built-up area, where public buildings and public transport stops are located,
  • and in the area of pedestrian crossings and access to public transport stops in the built-up area.

Apart from the cases mentioned above, it is usually the responsibility of the municipality to plan the lighting of public places and roads in the municipality [2].

Another important normative document is the latest standard EN 13201-2: 2016 Lighting of roads. The regulations contained in this standard define the basic lighting requirements for ensuring high visual efficiency and comfort of vision for all road users, with the lowest possible investment and operating costs of the lighting system. Technical report TR 13201-1:2016 [3], the first part of this document, does not have the status of a Polish standard, but defines the way of choosing the lighting class. Individual parts of the standard define respectively: quantitative requirements for particular classes of lighting [4], method of calculation of lighting parameters [5], methods of measurement of lighting quality [6] and method of assessment of energy efficiency of road lighting [7].

The basic criterion when designing road lighting is luminance and illuminance. The appropriate level of luminance or illuminance should be adopted depending on the conditions in which the road user is located, his speed of movement, and thus the time he has to notice an obstacle, both on the road and in its immediate environment, depending on the shape of the road and depending on who is the main user moving on the road.

The methods governing the choice of the class of illumination are defined in the standard [3]. The standard also determines which roads are to be designed according to the criterion of illuminance and which are to be designed according to the criterion of luminance. According to the standard, there are three basic lighting classes:

  • Class M (motor vehicles) – lighting classes on roads with motor traffic, where medium and high speeds are allowed – the design criterion is the luminance criterion;
  • Class C (conflict areas) – should be used primarily in cases where M classes cannot be used due to short viewing distances (less than 160 m), or where the position of the observer is particularly important. This situation takes place in conflict zones such as road crossings, roundabouts, shopping streets, parking lots, and areas where queues of vehicles are formed. In addition, class C also applies to pedestrian and cyclist traffic when class P is inappropriate – the design criterion is the illuminance criterion;
  • Class P (pedestrian) – refers to pedestrians and cyclists riding on pavements, bicycle lanes, parking lanes and housing estate roads, parking lots, school courtyards, etc. – the design criterion is the illuminance criterion.

How should the quality of lighting be verified?

Taking into account the criteria and principles of design by means of appropriate measurements of illuminance, we can verify whether the lighting installation has been made in accordance with the applicable regulations. Table 1 presents average values of illuminance [lx] and uniformity for conflict zones, i.e. class C.

Table 2 below shows the required illuminance levels for pedestrian traffic, i.e. class P.

In order to ensure adequate uniformity, the actual average illuminance should not exceed 1.5 times the normative value for the class.

The detailed provisions of the standard provide for additional classes: SC, HS and EV, in addition to classes C and P. These classes are designed for places where lighting is primarily used to identify people, objects, and areas with a high crime risk. If there is a special need to identify vertical surfaces, the road takes the EV class.

In addition, the standard provides a number of more detailed criteria that need to be into account during the selection of lighting classes during the design of special lighting installations. M. Zalesińska writes about this in more detail in the [Designer’s Guide] (No. 3/2019), published by the Publishing House of the Polish Chamber of Civil Engineers [8].

How should measurements be performed?

Illuminance measurements should be performed with appropriate measuring equipment, i.e. an illuminance meter, a luxmeter or a spectroradiometer, equipped with a properly constructed and calibrated head for illuminance measurements.

The rules of measurement are directly related to the design rules, because the assessment of the quality of lighting is based on the reconstruction of a situation in real conditions, which enables verification of the fulfilment of criteria and assumptions made by the designer at the stage of project implementation. In this particular case, the meter should be placed in clearly defined places on the road and in an appropriate way to measure the distribution of illuminance. Part 4 of the standard specifies the methods of measurement of illuminance. In order to apply them properly, it is important to understand the design principles and criteria contained in the standard.

It should be noted that the illuminance levels specified in Table 1 refer to measurements taken horizontally on the road surface. The arrangement of the measuring points depends on the distance between the luminaires and the width of the lane. The measurement of illuminance should be performed on the area in question that includes in longitudinal direction two consecutive luminaires in the same row and in transverse direction the width of the area with the same illumination class, i.e. if the road and adjacent pavement or bicycle path have the same illumination class, they may be considered as one area during the measurements. The measuring points should be distributed evenly within the measuring field.

The distance between the measuring points (D in [m]) in the longitudinal direction should be calculated using the formula:

where:
S – the distance between the luminaires in [m],
N – the number of measurement points in the longitudinal direction,
for S ≤ 30 m, it is N = 10,
for S > 30 m, the smallest integer giving D ≤ 3 m.

The distance between measurement points (d in [m] in the transverse direction should be calculated with the formula:

where:
Wr – the width of the road or the area under consideration in [m],
n – the number of measurement points in the transverse direction equal to 3 or more and being an integer giving d ≤ 1.5 m.

The distance between the points and the edges of the surface under consideration should be D/2 in the longitudinal direction and d/2 in the transverse direction. The location of the measurement points in the measuring field is shown in Figure 1.

 

Figure 1: Measuring grid used to measure illumination on the road

Where additional lighting is required to improve the visibility of information, vertical signs and facial recognition, vertical illuminance and semi-cylindrical illuminance should be determined (see requirements in Table 2 and requirements for EV and SC classes). Both vertical and cylindrical illuminances should be measured at a height of 1.5 m above the surface to be considered. Vertical illuminance should be measured on a vertical plane. It must be remembered that vertical illuminance varies with the direction of observation; therefore the plane of vertical illuminance should be directed perpendicularly to the main traffic directions, which are usually two directions parallel to the road.

Figure 2: Semi-cylindrical illuminance measured vertically at a height of 1.5 m above the road surface

The semi-cylindrical illuminance also varies according to the direction of observation; therefore the rear flat surface of the semi-cylinder should be directed perpendicularly to the main pedestrian directions, which on the road are usually in the longitudinal direction. Figure 2 shows a schematic diagram for measuring semi-cylindrical illumination with a meter equipped with a head designed to measure illuminance on flat surfaces (horizontal or vertical illumination).

Figure 3: The light emitted by luminaires with too large inclination angle in relation to the road surface:
α – the angle of the transverse direction in which the road surface is illuminated,
ß – the angle of the transverse direction in which the road surface and its immediate surroundings are illuminated.

Additional criteria for assessing the quality of lighting

  • Appropriate arrangement of luminaires

The selection and arrangement of suitable luminaires should ensure that illuminance values and uniformity of lighting are achieved that meet the requirements of the standard. Designers usually look for luminaires that can be positioned as far away as possible to achieve a high energy efficiency of the lighting installation. However, if long distances between luminaires are used the uniformity of illumination may not reach the required value. The use of LED luminaires offers a great potential for improving energy efficiency. Such luminaires are equipped with light sources of high luminance, and modern optical systems with lenses allow for appropriate shaping of photometric solids, e.g. so that they can be placed at considerable distances. Optimally designed photometric body also helps to direct as much of the luminous flux as possible in the direction of the illuminated surface. This increases the efficiency of the lighting and reduces the problem of light pollution.

  • Colour temperature CCT

High-pressure sodium lamps, commonly used in the last few decades, had a low colour rendering index and a warm light colour. The introduction of LED sources into road lighting was associated with an improvement in the colour rendering index and a significant increase in the colour temperature (cooler light). The cool colour of LED sources is conducive to achieving greater light efficiency, but in recent years, attention has been paid to designing outdoor lighting, especially within cities, with lamps with warmer colour light. Such recommendations are justified by the fact that the spectral distribution of LED sources contains a lot of blue radiation. The high proportion of blue radiation significantly influences the regulation of the circadian rhythm by suppressing the level of melanin, which is a sleep hormone. Damping the melatonin level at night disturbs the circadian rhythm, may cause difficulties in falling asleep and lead to other health problems.

A spectroradiometer with a light measuring head (GL Optic Spectis 1.0 Touch + Flicker) is used to measure the illuminance with simultaneous measurement of the spectral distribution, colour temperature and indicators related to the assessment of the influence of the lamp spectrum on the regulation of the circadian rhythm.

Literature
1. Regulation of the Minister of Transport and Maritime Economy of 23 December 2015 on the technical conditions to be met by public roads and their location, Journal of Laws of 2016, item 124.
2. Act of 10 April 1997. – Energy Law, Journal of Laws of 2006, item 625.
3. CEN/TR 13201-1:2016 – Road lighting. Part 1: Guidelines for the selection of lighting classes.
4. EN 13201-2: 2016 – Road lighting. Part 2: Operational requirements.
5. EN 13201-3: 2016 – Road lighting. Part 3: Calculation of lighting parameters.
6. EN 13201-4: 2016 – Road lighting. Part 4: Methods of measuring lighting efficiency.
7. EN 13201-5: 2016 – Road lighting. Part 5: Energy Efficiency Index.
8. M. Zalesińska, Design of road lighting, “Przewodnik projektanta” No. 3/2019 (July-September), WPIIB, 2019.

Article by:

M. Zalesińska, K. Wandachowicz, Poznań University of Technology,

M. Przybyła – GL Optic

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