Selection and Application of Warning Lights on Roadway Operations Equipment (2008)

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35 The proposed guidelines are the recommendations of NCHRP Project 13-02 contractor staff at Virginia Polytechnic Institute and State University. These guidelines have not been approved by NCHRP or any AASHTO committee or formally accepted for adoption by AASHTO. Introduction Roadway operations equipment used for construction, maintenance, utility work, and other similar activities gener- ally operates within the roadway right-of-way. These vehicles and mobile equipment operate on all types of roadways, during daytime and nighttime hours, and under all weather condi- tions. To improve motorist and work-crew safety, equipment must be readily seen and recognized and, therefore, warning lights are provided on the equipment to alert motorists of potentially hazardous situations. Amber warning lights have traditionally been used, although lights of other colors are often added with the intent of helping the traveling public better see the equipment. Combinations of amber, blue, and white lights and other forms of warning lights (e.g., lighted bars, lighted “arrow sticks,” strobes, light emitting diodes [LED], and alter- nating flashes) are used. There is a concern that this variety of lighting on roadway operations equipment has evolved with- out adequate consideration of the effects on the awareness and responsiveness of motorists. These guidelines have been developed based on the results of a series of experiments that considered more than 40 light- ing configurations in both static and dynamic environments. The presence of maintenance personnel, the identification of the maintenance vehicle, attention-getting, glare, peripheral detection, and urgency were all metrics in the experiments. Differing experimental conditions such as weather, the pres- ence of other vehicles, and time of day were also considered in the experiments. One of the primary considerations in the use of these guidelines is the purpose of the maintenance vehicle. For the purposes of these guidelines, the maintenance vehicle refers to any type of vehicle used on the roadway, whether it is being used for new construction, inspection, or general maintenance. The design of the warning-light systems may differ based on the vehicle’s intended usage. For example, a snow plow will have different criteria than a small truck. The following are typical questions to be considered: • Will the vehicle be used primarily while moving or stopped? • Will the vehicle be used primarily in the daytime or night- time? • Will the vehicle be used primarily in bad weather or good weather? • Will there be maintenance workers present around the vehicle as pedestrians? Many vehicles are multi-purpose (i.e., they are used for many different tasks on the roadway). For example, a vehicle may be used for clearing snow in the winter and in construction and maintenance activities during the summer. The lighting system on these vehicles needs to be designed and laid out to include the considerations for all of the planned or expected vehicle uses. Safety Issues Safety with respect to maintenance vehicles must consider not only the maintenance vehicle and its crew but also the safety of other drivers. Maintenance Vehicle and Crew The safety of the maintenance-vehicle crew also has two conditions to be considered: when the maintenance crew is in the vehicle and when one or more crew members is outside of the vehicle, possibly working on the road. A T T A C H M E N T Proposed Guidelines for the Selection and Application of Warning Lights on Roadway Operations Equipment

For the case in which the maintenance crew is in the vehicle, the key to safety is to make the vehicle as conspicuous as pos- sible (i.e., the maintenance vehicle and its actions and purpose are able to be perceived by other roadway users). For the case in which the maintenance crew is outside of the vehicle, a higher effective-intensity light source was found to hinder safety by limiting the detection of a pedestrian around a vehicle. This factor will limit the overall intensity of the sys- tem. Using too many lights or lights with too high effective intensity may impede the ability of other drivers to detect a pedestrian; limiting the effective intensity of the light sources on the vehicle will mitigate this issue. For the vehicle conspicuity, one of the requirements that was first identified in the research was the use of internally illu- minated sources. Passive devices, such as retroreflective tape, did not draw the driver’s attention or provide any attention- getting cues to an approaching vehicle. The warning system must provide active illumination for vehicle safety. Flashing lights were found in the research record to be more conspicuous than continuous lights and provided a sense of urgency. An asynchronous flashing pattern (flashing side to side) also provided a higher attention-getting rating than a synchronous flash pattern (both sides flashing at once). Finally, amber light sources and white light sources also provided better responses than blue or red. Another issue with the color is the relationship of the color to the vehicle type. Amber and white were more commonly identified with maintenance ve- hicles, while blue and red were identified with police and fire services. Light sources with a higher effective intensity will provide better attention-getting than a light source with a lower effec- tive intensity. However, this was offset by the flash characteris- tics. A flash that provides a different flash pattern than the other lighting systems in the road environment allowed the driver to identify the vehicle sooner than a flash pattern that is similar to other lighting systems being used. Using a double flash or varying the effective intensity (such as with a rotating beacon) allowed the maintenance vehicle to be identified at a longer distance than other flash patterns. Also, when a vehicle is approached from the rear, the tail lights are primarily used for vehicle identification; locating the warning-light system high on the vehicle away from the tail lights improved vehicle identification. Another consideration for vehicle safety was the time of day. The appearance of a lighting system against the sky lim- ited the performance of the lighting system. For operation in daytime, it is important that the background behind the lighting system be controlled by having the light appear either against the rear of the vehicle or against a shield that provides adequate contrast and maintains the performance of the light- ing system. Other Drivers Glare is the primary issue of a warning-light system for other drivers. Bright warning lights and oppressive flashing provide disability glare and discomfort glare for a driver of an oncoming vehicle or a vehicle passing a maintenance vehicle from behind. The warning-light system may limit the driver of another vehicle’s ability to travel safely. The glare is primarily a result of the intensity of the light source. The research showed that a high-effective-intensity light source created a greater glare response than a low-effective- intensity light source. A high-effective-intensity light source limited the ability of an approaching driver to see the pedestrian standing behind the maintenance vehicle. Glare and pedes- trian detection also limit the maximum effective intensity of the warning-light system and limit the number and type of light sources placed on the maintenance vehicle. The position of the warning-light system also impacts glare. The research showed that a light positioned close to the height of an opposing driver’s line of sight created a greater glare response than a high-mounted lighting system. This response was particularly evident with 360° sources (lights that are seen from all angles), as a passing driver will be able to see that source even when they are very close to the maintenance vehicle. This consideration requires locating the light system as high on the vehicle as possible. Lighting Issues and Considerations Not only must the characteristics of maintenance-vehicle lighting systems be considered in terms of safety, but they also must be considered in terms of vehicle design and usage. Vehicle Color Vehicle color was not evaluated in this project. Nevertheless, principles of vision science indicate that a higher contrast be- tween the vehicle color and the light color will provide better visibility. For example, if the vehicle color is white, use of white warning lights should be avoided. A black background for the light source may provide the best possible condition for lighting visibility. Environmental Issues The weather and the time of day for the vehicle usage must also be considered for the lighting system. Adverse Weather The use of the vehicle in adverse weather conditions will impact its visibility; the vehicle identification distance is di- 36

minished by adverse weather. The presence of moisture in the atmosphere will cause absorption of the light from the warning system and will cause the light to scatter. A higher effective in- tensity causes greater scatter and therefore a greater glare ex- perience at night. However, higher effective intensity improves the visibility of the vehicle. The lighting effective intensity is limited by the glare in this condition, and additional lighting in adverse weather will likely cause difficulty to opposing and passing vehicles. Ambient Light The time of day during which the vehicle will primarily be used influences the characteristics of the lighting system. For daytime use, the lighting system must provide high conspicu- ity, while for nighttime use, the lighting system must pro- vide conspicuity, while not creating excessive glare for other drivers. A higher effective intensity of the light source must be used to provide adequate daytime conspicuity. This value may vary by the type of light source used. The research showed that halogen light at a lower effective intensity may provide higher conspicuity than LED light at a higher effective intensity. There is no evident glare in the daytime condition and therefore no maximum effective intensity limit. Another issue for the day- time condition is that of the location of the light source. The light appearing against the sky will limit the contrast of the source and will therefore limit the conspicuity of the light source. The light must appear against a controlled background for the conspicuity to remain constant. The sources used to provide adequate daytime conspicuity will cause significant glare for opposing and passing drivers at night. At night, adequate visibility can be found at a much lower effective intensity level. The effective intensity of the lighting system must be maintained between a level that pro- vides conspicuity and one that does not cause too much glare; the photometric effective intensity values are discussed below. Visually Complex Environments Research has shown that for a visually complex environ- ment a higher effective intensity may be required to provide adequate performance as compared to a simple rural envi- ronment. Glare ratings are lower when the warning-light sys- tem is rated on a road with an overhead lighting system and opposing traffic as compared to a rural test track. Similarly, the high-effective-intensity light source causes vehicles to change lanes to pass earlier than a lower effective intensity light source does. In situations where other vehicles are present, the glare ratings are also reduced, because the warning lights are interspersed with other light sources. In visually complex environments, a high effective intensity may be used to provide increased visibility of the vehicle while not causing too much glare for other drivers. Lighting Selection The lighting requirements are based on requirements for safety of the vehicle and other drivers. Light Source Selection There seems to be no benefit of one light source over another in general use. Because the spectral output of the source is very pure, solid-state LED sources seem to provide a benefit with some light colors. LED sources also provide an equivalent amount of light at a reduced wattage that may be a benefit to the vehicle in terms of electrical system loading. Many of the visual effects of the low- and high-mounted beacons can be achieved using LED light sources. Signal Colors It is recommended that only amber lighting and white light- ing be used in maintenance vehicles, with amber being the pre- dominant color. These colors provide increased detectability and are least confused with other on-road activities such as law enforcement and emergency response. Light Type Selection Flashing Lights It is recommended that the predominant light pattern be flashing. A pattern that alternates from one side of the vehicle to the other is preferable to one in which lights on both sides of the vehicle are flashing at the same time. It is also recom- mended that a slower flash frequency be used, because there was better response to the longer flash durations (as compared to the short flash durations required by high flash frequencies). Research has shown that a flash rate of 1 Hz is preferable to 4 Hz. A flash pattern such as a double flash or a pattern similar to that of a rotating beacon provides an appearance that en- ables vehicle identification and should improve response. A rotating beacon provides the appearance of flashing, and when two beacons are used, they rarely appear to be synchronized. Steady Lights It is recommended that, if a steady (continuous burning) light is used on the vehicle to meet federal vehicle lighting requirements (the most recent should be consulted), it should be used only as a supplement to the flashing light systems. Because steady lights have many other vehicle uses such as 37

clearance indicators, brake lights, and vehicle headlights, they should not be used to warn drivers of the presence of maintenance vehicles. Lighting Layout and Positioning In the layout of the vehicle lighting, positioning the lighting such that it appears against a portion of the vehicle and not against the sky will provide a consistent contrast and will allow for increased daytime and nighttime conspicuity. However, this configuration limits the ability of the light to be seen from all directions. For example, a rotating beacon placed on top of a vehicle will lose some of its conspicuity when viewed against a daytime sky, especially with the sun behind it. This effect can be mitigated by use of flat-mount LEDs or strobe lights mounted against a solid surface. It may thus be necessary to replicate the lights at the front, back, and sides of the vehicle. Lighting that is viewable from 360° around the vehicle (providing light to all angles of approach) will enhance the safety of the crew. The lighting system should be positioned such that the light does not cause excessive glare to approaching and passing drivers. Similarly, the light should be placed away from the tail lights of the vehicle to allow those lights to be seen. Therefore, the lights should be mounted high on the vehicle above the typical eye height of other drivers. The lights should also be placed to outline the vehicle (i.e., on either side of the vehicle and on any portion of the vehicle that extends beyond the lane such as a plow blade or a trailer extension). Retroreflective Tape It is recommended that retroreflective tape should be used as a supplement to a flashing warning-light system. Such tape can be used to identify vehicle shape, but should not be used as the only warning system on the vehicle. Effective Intensity Requirements As discussed, the effective intensity of the warning-light system is limited at a minimum in terms of the conspicuity of the maintenance vehicle and at a maximum by the glare appar- ent to other drivers. Nighttime and daytime requirements are different and may require two alternative warning-light sys- tems or a means to attenuate the light at night. The photo- metric limits for daytime and nighttime (listed in Table 1) were developed in a screening experiment based on the Form Factor method and then verified by the performance experi- ment. These values represent the total light output limits for the warning-light system on each of the approach sides of the vehicle (i.e., these limits apply to the sum of the output from the lighting on each of the rear, sides, and front of the vehicle). For lights that flash asynchronously, the sum represents the maximum value for those lights that are simultaneously illu- minated. For example, if there are two light sources of equal power flashing on the rear of a vehicle asynchronously, only one of the lights is counted in the total because both are not simultaneously illuminated. However, if two pairs of lights are used and two are illuminated simultaneously, two of the light sources are included in the sum. A higher effective intensity may be required for vehicles that are primarily used in urban and visually complex environments. Because most roadway vehicles are used both in the day and at night, it is important to note the difference between the day- time and nighttime system. The capability to either dim the lighting available or switch off some lighting for nighttime operation would be an important addition to the warning-light system installed on the vehicle. The values identified in Table 1 are specified using the Form Factor method (28) as the metric for effective intensity. The Form Factor method evaluates the light output from the flash- ing source in terms of the maximum intensity and the energy output of the source. The effective intensity Ieff of a flash pulse I(t) is given by: where F is the Form Factor, a is a visual time constant (0.25), and Imax is the maximum of the instantaneous luminous in- tensity I(t). It is recommended that this method be used for evaluating the light source as part of the selection method for the light sources. Sample Specifications A sample specification for the warning-light system is pro- vided here. Each DOT is encouraged to develop its own spec- ification based on its needs and the practitioner’s experience; however, this document can be used as a reference to define the physical, functional, and performance requirements for the warning-light systems on the vehicles. I I a F T F I t t I T T eff d = + = • • ∫max max ; ( ) , ( ) 1 10 38 Intensity (by Form Factor Method) Daytime NighttimeLight Source Minimum Minimum Maximum Halogen 3500 cd 900 cd 2200 cd LED 4000 cd 1650 cd — Strobe 3500 cd 1200 cd 2200 cd Note that a maximum value for the LED sources was not found. Table 1. Recommended photometric limits for warning light systems on each approachable side of a vehicle.

It is the policy of the DOT to maintain warning-lighting systems on all roadway operations vehicles. This system is pro- vided in order to maintain the safety of the vehicle operator, pedestrians or personnel located adjacent to the vehicle, and the operators of the vehicles approaching or passing the roadway operations vehicles. Physical Requirements 1. The warning-light system should be visible from all angles of approach of the vehicle: specifically, the front, rear, and both sides of the vehicle. Three hundred and sixty degrees of visibility of the lighting system must be provided. 2. Multiple light sources should be provided such that the outline of the vehicle is visible, including any obstacle attached to the vehicle such as a blade or a trailer. 3. The lighting system should be located as high on the vehi- cle as possible to both provide the outline of the vehicle and reduce the direct light into an approaching driver’s line of vision. This location will also allow approaching drivers to more clearly see the vehicle’s standard lights (such as brake lights). 4. For any portion of the lighting system that is visible against the sky (such as a beacon on the roof of the vehicle), a back- ground should be provided to control the light appearance. This background may be a shield or a part of the vehicle. The shield may be mounted over or around the light source to maintain the 360° of visibility. The background should extend 100% of the width or height of the light source to each side and above the lighting unit. 5. The use of retroreflective tape should be used and should be compliant to federal regulations. However, this tape should be used as a supplement to the lights described previously. 6. The lighting system must be durable and weatherproof. Functional Requirements 1. The lighting system must provide 360° of visibility around the vehicle. 2. The warning-light system should be predominantly flash- ing. Steady burning lights can be used to supplement the flashing-light system, but should not be considered the primary lighting system. 3. The warning-light system on the vehicle should be com- posed of amber or white lighting, with amber being the predominant color. 4. On the rear of the vehicle, the lighting system must pro- vide at least two lights, one on each side of the vehicle, that flash in an asynchronous manner. The flash should have a frequency between 1 and 4 Hz. 5. It is desirable that the asynchronous lighting system on the rear of the vehicle be combined with a rotating or a simu- lated rotating beacon to provide 360° of visibility. 6. The light source used can be a halogen, strobe, or LED type. Note that the performance limitations of these may affect choice. The LED type may provide equivalent performance at a lower power requirement. Lighting Performance Requirements 1. The total of effective intensities provided from all of the lights provided to each viewing angle of the vehicle should be limited to the values provided in Table 1 of the proposed guidelines. These values are effective intensity measured by the Form Factor method. 2. Two lighting levels must be provided (a daytime and a night- time system) as specified in Table 1. The lighting levels can be achieved by either adding lighting for daytime or dim- ming lighting for nighttime. An auto-switching function should be considered. 39