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35 ATTACHMENT Proposed Guidelines for the Selection and Application of Warning Lights on Roadway Operations Equipment The proposed guidelines are the recommendations of purposes of these guidelines, the maintenance vehicle refers NCHRP Project 13-02 contractor staff at Virginia Polytechnic to any type of vehicle used on the roadway, whether it is being Institute and State University. These guidelines have not been used for new construction, inspection, or general maintenance. approved by NCHRP or any AASHTO committee or formally The design of the warning-light systems may differ based on the accepted for adoption by AASHTO. 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: Introduction Roadway operations equipment used for construction, · Will the vehicle be used primarily while moving or maintenance, utility work, and other similar activities gener- stopped? ally operates within the roadway right-of-way. These vehicles · Will the vehicle be used primarily in the daytime or night- and mobile equipment operate on all types of roadways, during time? daytime and nighttime hours, and under all weather condi- · Will the vehicle be used primarily in bad weather or good tions. To improve motorist and work-crew safety, equipment weather? must be readily seen and recognized and, therefore, warning · Will there be maintenance workers present around the lights are provided on the equipment to alert motorists of vehicle as pedestrians? potentially hazardous situations. Amber warning lights have traditionally been used, although lights of other colors are often Many vehicles are multi-purpose (i.e., they are used for many added with the intent of helping the traveling public better see different tasks on the roadway). For example, a vehicle may the equipment. Combinations of amber, blue, and white lights be used for clearing snow in the winter and in construction and other forms of warning lights (e.g., lighted bars, lighted and maintenance activities during the summer. The lighting "arrow sticks," strobes, light emitting diodes [LED], and alter- system on these vehicles needs to be designed and laid out to nating flashes) are used. There is a concern that this variety of include the considerations for all of the planned or expected lighting on roadway operations equipment has evolved with- vehicle uses. out adequate consideration of the effects on the awareness and responsiveness of motorists. Safety Issues These guidelines have been developed based on the results of a series of experiments that considered more than 40 light- Safety with respect to maintenance vehicles must consider ing configurations in both static and dynamic environments. not only the maintenance vehicle and its crew but also the The presence of maintenance personnel, the identification of safety of other drivers. the maintenance vehicle, attention-getting, glare, peripheral detection, and urgency were all metrics in the experiments. Maintenance Vehicle and Crew Differing experimental conditions such as weather, the pres- ence of other vehicles, and time of day were also considered in The safety of the maintenance-vehicle crew also has two the experiments. conditions to be considered: when the maintenance crew is in One of the primary considerations in the use of these the vehicle and when one or more crew members is outside guidelines is the purpose of the maintenance vehicle. For the of the vehicle, possibly working on the road.
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36 For the case in which the maintenance crew is in the vehicle, Other Drivers the key to safety is to make the vehicle as conspicuous as pos- Glare is the primary issue of a warning-light system for sible (i.e., the maintenance vehicle and its actions and purpose other drivers. Bright warning lights and oppressive flashing are able to be perceived by other roadway users). provide disability glare and discomfort glare for a driver of an For the case in which the maintenance crew is outside of the oncoming vehicle or a vehicle passing a maintenance vehicle vehicle, a higher effective-intensity light source was found to from behind. The warning-light system may limit the driver hinder safety by limiting the detection of a pedestrian around of another vehicle's ability to travel safely. a vehicle. This factor will limit the overall intensity of the sys- The glare is primarily a result of the intensity of the light tem. Using too many lights or lights with too high effective source. The research showed that a high-effective-intensity light intensity may impede the ability of other drivers to detect a source created a greater glare response than a low-effective- pedestrian; limiting the effective intensity of the light sources intensity light source. A high-effective-intensity light source on the vehicle will mitigate this issue. limited the ability of an approaching driver to see the pedestrian For the vehicle conspicuity, one of the requirements that was standing behind the maintenance vehicle. Glare and pedes- first identified in the research was the use of internally illu- trian detection also limit the maximum effective intensity of minated sources. Passive devices, such as retroreflective tape, the warning-light system and limit the number and type of did not draw the driver's attention or provide any attention- light sources placed on the maintenance vehicle. getting cues to an approaching vehicle. The warning system The position of the warning-light system also impacts glare. must provide active illumination for vehicle safety. The research showed that a light positioned close to the height Flashing lights were found in the research record to be more of an opposing driver's line of sight created a greater glare conspicuous than continuous lights and provided a sense of response than a high-mounted lighting system. This response urgency. An asynchronous flashing pattern (flashing side to was particularly evident with 360° sources (lights that are seen side) also provided a higher attention-getting rating than a from all angles), as a passing driver will be able to see that source synchronous flash pattern (both sides flashing at once). Finally, even when they are very close to the maintenance vehicle. This amber light sources and white light sources also provided consideration requires locating the light system as high on the better responses than blue or red. Another issue with the color vehicle as possible. 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 Lighting Issues and Considerations services. Not only must the characteristics of maintenance-vehicle Light sources with a higher effective intensity will provide lighting systems be considered in terms of safety, but they also better attention-getting than a light source with a lower effec- must be considered in terms of vehicle design and usage. 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 Vehicle Color identify the vehicle sooner than a flash pattern that is similar Vehicle color was not evaluated in this project. Nevertheless, to other lighting systems being used. Using a double flash or principles of vision science indicate that a higher contrast be- varying the effective intensity (such as with a rotating beacon) tween the vehicle color and the light color will provide better allowed the maintenance vehicle to be identified at a longer visibility. For example, if the vehicle color is white, use of distance than other flash patterns. Also, when a vehicle is white warning lights should be avoided. A black background approached from the rear, the tail lights are primarily used for the light source may provide the best possible condition for vehicle identification; locating the warning-light system for lighting visibility. high on the vehicle away from the tail lights improved vehicle identification. Environmental Issues Another consideration for vehicle safety was the time of day. The appearance of a lighting system against the sky lim- The weather and the time of day for the vehicle usage must ited the performance of the lighting system. For operation also be considered for the lighting system. in daytime, it is important that the background behind the lighting system be controlled by having the light appear either Adverse Weather against the rear of the vehicle or against a shield that provides adequate contrast and maintains the performance of the light- The use of the vehicle in adverse weather conditions will ing system. impact its visibility; the vehicle identification distance is di-
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37 minished by adverse weather. The presence of moisture in the In visually complex environments, a high effective intensity atmosphere will cause absorption of the light from the warning may be used to provide increased visibility of the vehicle while system and will cause the light to scatter. A higher effective in- not causing too much glare for other drivers. tensity causes greater scatter and therefore a greater glare ex- perience at night. However, higher effective intensity improves Lighting Selection the visibility of the vehicle. The lighting effective intensity is limited by the glare in this condition, and additional lighting The lighting requirements are based on requirements for in adverse weather will likely cause difficulty to opposing and safety of the vehicle and other drivers. passing vehicles. Light Source Selection Ambient Light There seems to be no benefit of one light source over another in general use. Because the spectral output of the source is very The time of day during which the vehicle will primarily be pure, solid-state LED sources seem to provide a benefit with used influences the characteristics of the lighting system. For some light colors. LED sources also provide an equivalent daytime use, the lighting system must provide high conspicu- amount of light at a reduced wattage that may be a benefit to ity, while for nighttime use, the lighting system must pro- the vehicle in terms of electrical system loading. Many of vide conspicuity, while not creating excessive glare for other the visual effects of the low- and high-mounted beacons can drivers. be achieved using LED light sources. 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 Signal Colors halogen light at a lower effective intensity may provide higher It is recommended that only amber lighting and white light- conspicuity than LED light at a higher effective intensity. There ing be used in maintenance vehicles, with amber being the pre- is no evident glare in the daytime condition and therefore no dominant color. These colors provide increased detectability maximum effective intensity limit. Another issue for the day- and are least confused with other on-road activities such as law time condition is that of the location of the light source. The enforcement and emergency response. light appearing against the sky will limit the contrast of the source and will therefore limit the conspicuity of the light Light Type Selection source. The light must appear against a controlled background for the conspicuity to remain constant. Flashing Lights The sources used to provide adequate daytime conspicuity It is recommended that the predominant light pattern be will cause significant glare for opposing and passing drivers flashing. A pattern that alternates from one side of the vehicle at night. At night, adequate visibility can be found at a much to the other is preferable to one in which lights on both sides of lower effective intensity level. The effective intensity of the the vehicle are flashing at the same time. It is also recom- lighting system must be maintained between a level that pro- mended that a slower flash frequency be used, because there vides conspicuity and one that does not cause too much glare; was better response to the longer flash durations (as compared the photometric effective intensity values are discussed below. to the short flash durations required by high flash frequencies). Research has shown that a flash rate of 1 Hz is preferable to Visually Complex Environments 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- Research has shown that for a visually complex environ- ables vehicle identification and should improve response. A ment a higher effective intensity may be required to provide rotating beacon provides the appearance of flashing, and when adequate performance as compared to a simple rural envi- two beacons are used, they rarely appear to be synchronized. ronment. Glare ratings are lower when the warning-light sys- tem is rated on a road with an overhead lighting system and Steady Lights opposing traffic as compared to a rural test track. Similarly, the high-effective-intensity light source causes vehicles to It is recommended that, if a steady (continuous burning) change lanes to pass earlier than a lower effective intensity light is used on the vehicle to meet federal vehicle lighting light source does. In situations where other vehicles are present, requirements (the most recent should be consulted), it should the glare ratings are also reduced, because the warning lights be used only as a supplement to the flashing light systems. are interspersed with other light sources. Because steady lights have many other vehicle uses such as
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38 clearance indicators, brake lights, and vehicle headlights, Table 1. Recommended photometric limits they should not be used to warn drivers of the presence of for warning light systems on each maintenance vehicles. approachable side of a vehicle. Intensity (by Form Factor Method) Lighting Layout and Positioning Light Source Daytime Nighttime In the layout of the vehicle lighting, positioning the lighting Minimum Minimum Maximum such that it appears against a portion of the vehicle and not Halogen 3500 cd 900 cd 2200 cd against the sky will provide a consistent contrast and will allow LED 4000 cd 1650 cd -- for increased daytime and nighttime conspicuity. However, Strobe 3500 cd 1200 cd 2200 cd this configuration limits the ability of the light to be seen from Note that a maximum value for the LED sources was not found. 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 maximum value for those lights that are simultaneously illu- mitigated by use of flat-mount LEDs or strobe lights mounted minated. For example, if there are two light sources of equal against a solid surface. It may thus be necessary to replicate the power flashing on the rear of a vehicle asynchronously, only lights at the front, back, and sides of the vehicle. Lighting that one of the lights is counted in the total because both are not is viewable from 360° around the vehicle (providing light to simultaneously illuminated. However, if two pairs of lights are all angles of approach) will enhance the safety of the crew. used and two are illuminated simultaneously, two of the light The lighting system should be positioned such that the light sources are included in the sum. A higher effective intensity does not cause excessive glare to approaching and passing may be required for vehicles that are primarily used in urban drivers. Similarly, the light should be placed away from the tail and visually complex environments. lights of the vehicle to allow those lights to be seen. Therefore, Because most roadway vehicles are used both in the day and the lights should be mounted high on the vehicle above the at night, it is important to note the difference between the day- typical eye height of other drivers. time and nighttime system. The capability to either dim the The lights should also be placed to outline the vehicle (i.e., lighting available or switch off some lighting for nighttime on either side of the vehicle and on any portion of the vehicle operation would be an important addition to the warning-light that extends beyond the lane such as a plow blade or a trailer system installed on the vehicle. extension). The values identified in Table 1 are specified using the Form Factor method (28) as the metric for effective intensity. The Retroreflective Tape Form Factor method evaluates the light output from the flash- ing source in terms of the maximum intensity and the energy It is recommended that retroreflective tape should be used output of the source. The effective intensity Ieff of a flash pulse as a supplement to a flashing warning-light system. Such tape I(t) is given by: can be used to identify vehicle shape, but should not be used T as the only warning system on the vehicle. I eff = I max I (t )dt , ; F= 0 (1) a I max · T 1+ Effective Intensity Requirements F T · As discussed, the effective intensity of the warning-light where F is the Form Factor, a is a visual time constant (0.25), system is limited at a minimum in terms of the conspicuity of and Imax is the maximum of the instantaneous luminous in- the maintenance vehicle and at a maximum by the glare appar- tensity I(t). It is recommended that this method be used for ent to other drivers. Nighttime and daytime requirements are evaluating the light source as part of the selection method for different and may require two alternative warning-light sys- the light sources. tems or a means to attenuate the light at night. The photo- metric limits for daytime and nighttime (listed in Table 1) Sample Specifications were developed in a screening experiment based on the Form Factor method and then verified by the performance experi- A sample specification for the warning-light system is pro- ment. These values represent the total light output limits for vided here. Each DOT is encouraged to develop its own spec- the warning-light system on each of the approach sides of the ification based on its needs and the practitioner's experience; vehicle (i.e., these limits apply to the sum of the output from however, this document can be used as a reference to define the lighting on each of the rear, sides, and front of the vehicle). the physical, functional, and performance requirements for For lights that flash asynchronously, the sum represents the the warning-light systems on the vehicles.
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39 It is the policy of the DOT to maintain warning-lighting Functional Requirements systems on all roadway operations vehicles. This system is pro- 1. The lighting system must provide 360° of visibility around vided in order to maintain the safety of the vehicle operator, the vehicle. pedestrians or personnel located adjacent to the vehicle, and the 2. The warning-light system should be predominantly flash- operators of the vehicles approaching or passing the roadway ing. Steady burning lights can be used to supplement the operations vehicles. flashing-light system, but should not be considered the primary lighting system. Physical Requirements 3. The warning-light system on the vehicle should be com- posed of amber or white lighting, with amber being the 1. The warning-light system should be visible from all angles predominant color. of approach of the vehicle: specifically, the front, rear, and 4. On the rear of the vehicle, the lighting system must pro- both sides of the vehicle. Three hundred and sixty degrees vide at least two lights, one on each side of the vehicle, that of visibility of the lighting system must be provided. flash in an asynchronous manner. The flash should have a 2. Multiple light sources should be provided such that the frequency between 1 and 4 Hz. outline of the vehicle is visible, including any obstacle 5. It is desirable that the asynchronous lighting system on the attached to the vehicle such as a blade or a trailer. rear of the vehicle be combined with a rotating or a simu- 3. The lighting system should be located as high on the vehi- lated rotating beacon to provide 360° of visibility. cle as possible to both provide the outline of the vehicle 6. The light source used can be a halogen, strobe, or LED type. and reduce the direct light into an approaching driver's line Note that the performance limitations of these may affect of vision. This location will also allow approaching drivers choice. The LED type may provide equivalent performance to more clearly see the vehicle's standard lights (such as at a lower power requirement. 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- Lighting Performance Requirements ground should be provided to control the light appearance. 1. The total of effective intensities provided from all of the This background may be a shield or a part of the vehicle. The lights provided to each viewing angle of the vehicle should shield may be mounted over or around the light source to be limited to the values provided in Table 1 of the proposed maintain the 360° of visibility. The background should guidelines. These values are effective intensity measured by extend 100% of the width or height of the light source to the Form Factor method. each side and above the lighting unit. 2. Two lighting levels must be provided (a daytime and a night- 5. The use of retroreflective tape should be used and should be time system) as specified in Table 1. The lighting levels can compliant to federal regulations. However, this tape should be achieved by either adding lighting for daytime or dim- be used as a supplement to the lights described previously. ming lighting for nighttime. An auto-switching function 6. The lighting system must be durable and weatherproof. should be considered.