Published: 4th Sep 2013
A comprehensive guidebook to nighttime construction safety has been published by the Transportation Research Board under the National Cooperative Highway Research Program (NCHRP). Lighting the work zone is discussed as just one of many important considerations.
What follows is an excerpt from NCHRP Report 726: A Guidebook for Nighttime Construction: Impacts on Safety, Quality, and Productivity, which gives a superb overview of technical aspects to consider, and a range of lighting options that are available.
The importance of proper lighting arrangements in nighttime work zones is obvious. Construction work zone lighting and glare specifications should require appropriate levels of lighting based on work tasks. In addition, an agency must have practical methods for inspecting nighttime work zone lighting arrangements.
- The more quantitative the contract specifications are about required work zone lighting levels the better, because the specifications then provide measurable outcomes that Standards and Testing Agencies (STAs) can easily check
- Drive-through inspections are important but are not sufficient for ensuring proper work zone lighting levels
Nighttime construction lighting arrangements have an impact on project safety, quality, cost, and productivity (El-Rayes and Hyari 2005a). One of the main reasons illumination levels have a strong impact on the other aspects of nighttime construction is the fact that light influences human performance and alertness.
The quality of work zone lighting can be evaluated using three main parameters: illuminance, lighting uniformity, and glare. A good specification requires a minimum level of average illuminance on the jobsite. This illuminance specification ensures the intensity of light incident on the surfaces of the project. Light uniformity ensures that lighting is distributed equally across the construction jobsite. Finally, the third quality parameter specified is maximum glare, which is quantified using the levels of luminance prevalent on the site (El-Rayes and Hyari 2005a).
Illuminance represents the time rate of light flow, measured in lumens, that falls upon a surface area. The unit of measurement of illuminance is lux, which is measured in lumens per square metre. Illuminance can be measured on a jobsite using a simple device called an illuminance meter.
To calculate horizontal illuminance on a jobsite, measurements need to be taken from different locations. To ensure the uniformity of the measurement, readings over a uniform grid of points that covers the entire jobsite are recommended.
The value of illuminance at any point on the jobsite grid depends on the values of lighting intensity reaching that point from each luminaire being used. The size of the grid for measuring illuminance depends on the task detail level, but grid point spacing should generally range from one to three feet.
Illuminance is the amount of light that falls on a surface. It is measured in lux (metric) or foot-candles (imperial). Most agency specifications require certain levels of illuminance for the performance of construction tasks. The measurement is performed by holding the illuminance meter horizontally at the specified height and taking the measurement.
The amount of lighting available for performing a specific task can be measured using luminance. This measure is often confused with the above-described illuminance measure; it is therefore important to distinguish between these two measures.
Illuminance is a measure of the amount of light incident on a unit surface area. Luminance, on the other hand, is the amount of light, measured in luminous flux, leaving a surface at a given time. Luminance, therefore, is measured in candelas per square metre and is more suitable for measuring visibility of objects (Ellis 2001).
Given that luminance needs a target for which measurements are taken, its measuring devices allow users to focus from a distance on specific targets.
Most agency specifications do not have a requirement for luminance, but it is an important parameter to measure because it determines the amount of glare.
Lighting uniformity is an important lighting quality parameter used to evaluate the suitability of lighting arrangements in nighttime work zones. This parameter is based on the calculation of the levels of illuminance on the jobsite.
Lighting uniformity is calculated as the ratio between the above-described average illuminance, Eavg, and the minimum illuminance at the darkest spot on the jobsite, Emin.
The following equation shows the calculation of the uniformity ratio (El-Rayes and Hyari 2005a):
Note that smaller values of this ratio are better because they indicate a smaller difference between average illuminance and the levels of lighting at the darkest spot on the jobsite (Nassar 2008).
Unlike lighting uniformity, glare calculations are dependent on luminance values prevalent on the jobsite. Luminance calculations require consideration of the reflectance characteristics of the surfaces present at the jobsite.
The largest surfaces to consider are naturally the pavement surfaces in the work area, which are classified into four main categories by the International Engineering Society of North America (IESNA), based on the ability of the pavement material to reflect light (El-Rayes and Hyari 2005a).
These pavement reflectance characteristics are used to determine the luminance of the pavement. The importance of pavement luminance stems from the fact that the sensation of glare depends on the amount of veiling luminance experienced relative to the level of luminance the eyes adapt to when exposed to this veiling luminance – in other words, the pavement luminance.
To calculate the glare produced by lighting arrangements, the following three parameters need consideration:
- Observer positions
- Pavement luminance at possible observer positions
- Veiling luminance at each observer position
These positions and lines of sight are used to calculate glare and thereby determine the maximum glare for all positions. This maximum computed glare is the measure used to determine the suitability of work zone lighting arrangements (El-Rayes and Hyari 2005a).
The calculation of the amount of glare experienced by drivers because of work zone lighting arrangements starts by determining the likely positions of the drivers and their lines of sight. These positions are determined based on traffic lanes and the expected height from the ground to the drivers’ eyes. This height is expected to vary based on the type of vehicle. The determination of possible driver positions and lines of sight is followed by measuring the pavement luminance experienced by drivers at these positions or locations using the luminance meter as explained in the previous sections.
Glare can be decreased by increasing pavement luminance, by increasing background lighting. This decreased glare is achieved by increasing the lighting ahead of the work zone.
Note: Increasing the mounting height of the lighting equipment in a work zone decreases glare. This means extending the mast of light towers higher or having higher poles for fixing lights on construction equipment. The trade-off in doing so to decrease glare is that the increased height decreases horizontal illuminance.
Recommended lighting levels
A major problem with nighttime work zone lighting arrangements is the insufficiency of the lighting provided to perform the construction or maintenance task. The level of lighting needed for specific construction activities depends on factors related to the humans performing the activities, as well as factors relating to the task at hand and the environment in which it takes place.
For example, visual ability is a human factor that is known to vary among workers. Lighting standards therefore assume reasonable visual abilities that are considered to be normal, while giving some allowance for variations among individuals. This assumption allows for visual standards to be based mainly on the visual requirements of the specific tasks construction activities entail.
- Recommended illumination levels vary by task
- Tasks requiring higher levels of precision need higher levels of lighting
- A minimum illuminance of 54 lux needs to be maintained in general working areas for the safety of workers and the driving public
One indication of the adequacy of the levels of illuminance is what could be termed the ‘no flashlight rule’. If the illuminance provided by the lighting equipment on the job is sufficient, field personnel should not need to use a flashlight for additional lighting in a work zone.
While clearly the example that follows centres on a road and bridge construction project in Florida, in the United States, it offers readers a valuable rationale for particular works’ choices.
A number of STAs have developed good temporary lighting specifications for nighttime highway construction and maintenance activities.
The Florida Department of Transportation (FDOT) has a detailed nighttime construction lighting specification which contains definitive illumination levels that should be achieved, and specifies possible lighting equipment that could be used to achieve appropriate lighting levels.
Road and bridge construction
What follows are the sample lighting specifications for road and bridge construction from the FDOT in 2010.
During active nighttime operations, furnish, place and maintain lighting sufficient to permit proper workmanship and inspection. Use lighting with 5 ft-cd minimum intensity. Arrange the lighting to prevent interference with traffic or produce undue glare to property owners. Operate such lighting only during active nighttime construction activities. Provide a light meter to demonstrate that the minimum light intensity is being maintained.
Lighting may be accomplished by the use of portable floodlights, standard equipment lights, existing street lights, temporary street lights, or other lighting methods approved by the Engineer.
Submit a lighting plan at the Preconstruction Conference for review and acceptance by the Engineer. Submit the plan on standard size plan sheets (not larger than 24 by 36 inches), and on a scale of either 100 or 50 feet to 1 inch. Do not start night work prior to the Engineer’s acceptance of the lighting plan.
During active nighttime operations, furnish, place and maintain variable message signs to alert approaching motorists of lighted construction zones ahead. Operate the variable message signs only during active construction activities.
Include compensation for lighting for night work in the contract prices for the various items of the contract. Take ownership of all lighting equipment for night work.
Types of lighting equipment
Lighting equipment is in constant development as technologies advance at a very rapid pace.
These are the four main types of lighting equipment used traditionally for nighttime work zones:
- Light towers
- Balloon lighting
- Night Lite
- High-mast lighting
Light towers are one of the most commonly used types of portable lighting equipment. Typically, they have a power generator, a retractable mast, and two to six lighting fixtures.
All of these components are mounted on a trailer that can be towed by a construction vehicle.
The luminaires installed in light towers are usually fitted with 1,000 to 1,500 watt metal halide, high-pressure sodium, or tungsten halogen bulbs. Different bulbs vary in the colour of their light and in the time it takes for them to warm up (Gambatese 2005).
Light towers have a number of features that increase their versatility and flexibility in lighting outdoor space. The retractable masts can be raised to 30 feet and rotate 360 degrees around their vertical axis. These towers can light areas ranging from five to seven acres using only four luminaires.
Balloon lighting Balloon lights are intended for use as equipment-mounted lights or in stationary locations such as flagger stations. These lights have fabric covers that are inflated with air or helium and they can be mounted on stands.
The source of light enclosed in the fabric cover is either a halogen bulb or a Hydrargyrum Medium-arc Iodide (HMI) system. Balloon lights can illuminate spaces ranging from 108,000 to 432,000 square feet.
The main advantages of balloon lighting are that it distributes light over 360 degrees, offers glare-free lighting, and can be mounted as high as 164 feet (Gambatese 2005).
Nite Lite This is a new equipment type that was utilised in an IDOT (the Illinois Department of Transportation) test.
It was a portable light composed of a 400-watt metal halide lamp in a dome-shaped casing coated with a light-diffusing material. The luminaire had a weight of 11.8 kg, and the dome had a diameter of 0.635 m.
The luminaire was powered by a 120-volt AC source that could produce up to 42,000 lumens, which could provide lighting for an area of 1,395 square metres (El-Rayes et al, 2007).
High-mast lighting The fourth type of construction lighting is a semi-permanent lighting system for the illumination of work zones. High-mast lighting systems are composed of luminaires mounted on high-mast poles. These were first used by the NYSDOT (New York State Department of Transportation) in 2005 for a three-mile stretch of I-90 at Albany, New York (Freyssinier et al, 2008).
The system was proposed as an alternative for portable light towers. The main characteristics of high-mast lighting systems include the horizontal illuminance produced. The measurements were taken in conditions that had no ambient background lighting and followed IESNA (Illuminating Engineering Society of North America) measurement guidelines. The average illuminance the system produces is more than 100 lux, which is sufficient for the movement of construction workers and some construction activities such as pavement resurfacing (Freyssinier et al, 2008).
The results of the evaluation of the system’s performance showed that it met lighting quality specifications for nighttime work zones. The comparison of the performance of high-mast lighting to portable light towers showed it provided sufficient illumination with fewer shadows and relatively low glare.
Key points to be noted with regard to different kinds of lighting are included in the bulleted summary below:
- Light towers are the most common type of construction lighting equipment. Light towers are suitable for lighting large stationary areas
- Balloon lights are used for lighting small to medium stationary areas or can be equipment-mounted
- High-mast lighting is a new type of lighting that is semi-permanent in nature
Agencies do not usually prescribe the lighting equipment used by contractors to provide appropriate lighting levels. Most contractors therefore select lighting equipment based on availability and price. These criteria may not result in equipment that results in optimum lighting arrangements. Attention should therefore be given to contractor-submitted lighting plans and proposed equipment.
IESNA pavement reflectance categories can be found in the American National Standards Institute (ANSI)/IESNA RP-8-00 report, which is available online at http://www.scribd.com/doc/38492536/Ansi-Iesna-Rp-8-00.
There are obviously many more elements to weigh up when it comes to ensuring worker safety during nighttime roadworks’ operations. Thanks to the extensive research compiled by TRB, guidance is available on a huge range of elements, from risk, cost and communications, through to lighting.
To view the full 2012 TRB report, log on to http://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_rpt_726.pdf
Published: 04th Sep 2013 in Health and Safety International