Where the light is BETTER

June 16, 2008

Most roadwork today involves reconstruction, rehabilitation and maintenance occurring near lanes of traffic on existing roadways. More and more agencies are doing roadwork on high-volume facilities at night to reduce traffic delays and complaints by the public that would typically be caused by doing the work during the day. In addition to several obvious advantages of nighttime work, such as fewer traffic delays and cooler temperatures for equipment and materials, there also are certain disadvantages.

Most roadwork today involves reconstruction, rehabilitation and maintenance occurring near lanes of traffic on existing roadways. More and more agencies are doing roadwork on high-volume facilities at night to reduce traffic delays and complaints by the public that would typically be caused by doing the work during the day. In addition to several obvious advantages of nighttime work, such as fewer traffic delays and cooler temperatures for equipment and materials, there also are certain disadvantages.

Night work comprises many complex issues and a variety of challenges, which include lighting conditions, safety, quality, manpower availability and administrative considerations.

Different ways of seeing it

Lighting is one factor that must be properly addressed for nighttime construction and maintenance activities, as it affects driver and worker safety, work quality, worker productivity and worker morale. Illumination guidelines for nighttime highway work have recently been developed based on past practices, existing illumination standards and regulations for construction, correlation to illumination standards for nonhighway construction tasks and subjective evaluations. The Manual on Uniform Traffic Control Devices (MUTCD) has generally adopted these lighting levels.

Generally speaking, standard highway lighting, light from nearby businesses or residences or light from conventional vehicle headlights are generally inadequate to properly light the areas where the work is performed. Thus, additional lighting is needed.

Typically, work-zone illumination is provided by three types of lighting systems: temporary systems, portable systems or equipment-mounted systems. Temporary systems use existing or temporary poles to mount lights such that the entire work zone is lit. Portable systems combine the lights, power supply and pole into one device that can be easily moved from one location to another. Trailer-mounted light towers are the most common type of portable lighting system. Finally, equipment-mounted systems offer better mobility and are useful to increase the level of lighting in front of or behind equipment.

The type of work-zone illumination used is highly dependent upon the duration of the work activity and geometric constraints. For mobile, short-duration and some short-term stationary operations, portable lighting may not be applicable since mobile operations can cover long distances during a single work period, and for shorter-duration activities the setup and removal of portable lighting can take longer to perform than the actual work or could considerably increase the amount of time it takes to complete the work activity.

Thus, equipment-mounted lighting becomes more critical. For longer-duration activities, both portable and equipment-mounted lighting systems may be used. Also, in some cases fixed temporary lighting may be used.

Geometric constraints such as limited or no shoulders, bridges, working adjacent to open lanes of traffic, horizontal and vertical curvature and intersections also affect the quality of work-zone lighting being provided. Nighttime highway work on roadways with limited or no shoulders or on bridges may hinder or completely remove the ability to use portable light towers to illuminate the work area. In such instances, equipment-mounted lighting becomes critical.

No way to measure

So in the field, how do we ensure adequate illumination is provided? Although specific criteria are available upon which to judge lighting adequacy, only a small portion of field personnel actually use any type of light-measuring equipment to evaluate or verify lighting adequacy, glare or light trespass. One of the most critical weaknesses in the current state of the art in work-zone lighting is a lack of easy processes and tools to allow field personnel to quickly measure and evaluate the adequacy of a lighting system. Presently, the only way that field personnel can determine if the lighting provided meets such criteria is through an onerous data collection process, setting up a grid system and taking multiple measurements from illumination devices that typically require some degree of calibration each time they are used. Even if such a validation is conducted at one point in time and space (at the beginning of the work shift, for example) there is no guarantee that the system will continue to meet the requirements as the equipment and work activity moves along the roadway. This challenge is especially true if portable light towers are being used.

It is clear that methods need to be developed to facilitate the field validation process for night work lighting systems. For example, tools that allow quick and standardized measurement of illumination around the work operation (possibly vehicle-mounted sensors) could be developed. In addition, it may be possible to use computer machine vision technology such as is used for video vehicle detection systems to quickly and objectively measure a work operation from multiple perspectives and provide easy-to-interpret outputs of illumination and other visual features such as shadows or glare.

Lighted equipment

Related to the challenges in measuring and validating the illumination provided within a nighttime highway work zone, the interactions between work vehicles and the light sources present can create shadows that fall onto key work areas. The severity and location of these shadows will depend on the following:

  • Size and shape of the work vehicles;
  • The position, intensity and aim of the light sources; and
  • The movement of work equipment along the roadway.

Shadows can be especially problematic for fixed lighting systems because of the movement of the work zone. Of course, improperly positioned equipment-based lighting can likewise create shadow problems that actually move with the operation. Presumably, many of the potential work areas around vehicles and equipment where shadows could occur can be identified by equipment manufacturers, and equipment-mounted lights could be properly integrated into the design of the equipment.

For equipment already owned by an agency or contractor for which shadow lighting has not been taken into consideration, the challenge is much greater. In the field, some work crews make d0 by pointing other vehicles with headlights on the work area to create adequate lighting for the workers. Unfortunately, vehicle headlights are not mounted a sufficient distance above the roadway to avoid glare issues with either workers or with drivers traveling in lanes adjacent to the work area. In addition, the use of vehicle headlights pointed toward the work area and oncoming traffic can create significant anxiety with approaching drivers who can perceive themselves to be to the left of oncoming traffic and not where they should be. Consequently, this practice should be avoided at all costs. Rather, the equipment should be retrofitted with equipment lighting that allows it to be operated as intended.

Each piece of equipment and each task have unique task-lighting requirements. Issues can exist with respect to where and how the lights are mounted on the equipment, ensuring that the power supply for the lights is adequate and in limiting the amount of vibration of the lights themselves. Obviously, there are significant constraints on where any equipment-mounted lighting is positioned on the equipment. The lights must not affect the key functions of the equipment itself or of any other nearby equipment, nor must it interfere with equipment-operator interactions with the work crew on foot. Minimum mounting heights are needed to control glare to drivers and workers, whereas maximum mounting heights may be needed to allow the lights to clear vertical obstructions such as overpasses. Lateral clearance also must be considered, especially when the work is adjacent to open lanes of traffic.

Typically, alternators already installed on construction equipment can be used to power equipment-mounted lighting. However, the alternator capacity for a particular piece of equipment controls the wattage of the lights and the voltage of the lighting system that can be powered. Furthermore, the other electrical demands of the equipment draw from this alternator capacity. Therefore, what is important is not total alternator capacity, but rather the spare capacity of the alternator available for retrofitted lights. Some contractors have mounted separate portable generators as power sources on construction equipment to operate 120-volt lighting systems.

Vibration is a major concern for some types of lights when mounted on construction equipment. Vibrations caused by construction equipment will vary by equipment type, mounting location on the equipment and use of poles or mast arms to elevate or extend the light. No single set of vibration parameters (amplitude or frequency) exist against which light performance characteristics can be measured.

If equipment-mounted lighting is not used to mitigate shadows, the number and position of portable light towers becomes especially influential on shadow effects. Shadows of fixed objects, such as an overpass or sign structure, also can be created during nighttime work operations. Presumably, such shadows are fairly easily identifiable and remedied through repositioning of portable light towers in most cases. However, this issue could be problematic under special geometric situations, for example limited vertical clearance within an overpass to allow portable light towers to be fully erected.

Glaring problem

Field personnel must remain vigilant against creating glare for either the driver or worker when implementing lighting for night work operations. Unfortunately, an easily implemented quantitative procedure for measuring and verifying that glare is below a maximum threshold has yet to be identified. Glare to drivers is an issue especially when the work activity is adjacent to open lanes of traffic and at intersections, where drivers are approaching the work area from multiple directions. However, glare also is an important consideration when the work area is located adjacent to the roadway. Horizontal and vertical curvature also may increase glare to drivers, as well as decrease the illumination provided to the work area by portable light towers. It is important to remember that glare issues can develop in real-time, especially if equipment moves along the roadway.

Portable light towers can provide illumination levels seven to 12 times the lowest adequate illumination level. This abundance of light can exacerbate glare problems for drivers if the light sources are improperly positioned. Such light abundance also may adversely affect drivers’ night vision and may decrease drivers’ ability to distinguish low-contrast objects such as workers or idle equipment. The vision of workers also may be impaired by bright and direct lighting sources. Low mounting heights, in other words close proximity to drivers’ and workers’ line of sight, used for some equipment-mounted lighting may result in glare to drivers and workers.

In most instances, glare can be reduced by physically moving the lights, altering the mounting height, re-aiming the lights or some combination thereof. However, aiming portable light towers is typically a trial-and-error process that is not easily accomplished with existing equipment. Wireless controls could be used to alter the aim of individual lights in portable light towers in order to reduce the time and effort needed to re-aim these devices. Other glare-control measures, such as cutoff lights, visors, louvers, shields, screens and barriers, also can be used.

Overall, it is imperative that lighting be provided for nighttime highway construction and maintenance activities. However, highway agencies and contractors must be cognizant of the many challenges that exist in providing work-zone lighting. Making agencies and contractors aware of possible mitigation strategies to address these challenges is an important first step in minimizing the adverse safety consequences of improper lighting systems and implementations.

About The Author: Finley is an associate research engineer at the Texas Transportation Institute. Ullman is a senior research engineer at the Texas Transportation Institute.

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