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

Chapter: Chapter 1 - Introduction and Research Approach

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Suggested Citation:"Chapter 1 - Introduction and Research Approach." National Academies of Sciences, Engineering, and Medicine. 2008. Selection and Application of Warning Lights on Roadway Operations Equipment. Washington, DC: The National Academies Press. doi: 10.17226/14190.
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Suggested Citation:"Chapter 1 - Introduction and Research Approach." National Academies of Sciences, Engineering, and Medicine. 2008. Selection and Application of Warning Lights on Roadway Operations Equipment. Washington, DC: The National Academies Press. doi: 10.17226/14190.
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Suggested Citation:"Chapter 1 - Introduction and Research Approach." National Academies of Sciences, Engineering, and Medicine. 2008. Selection and Application of Warning Lights on Roadway Operations Equipment. Washington, DC: The National Academies Press. doi: 10.17226/14190.
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Suggested Citation:"Chapter 1 - Introduction and Research Approach." National Academies of Sciences, Engineering, and Medicine. 2008. Selection and Application of Warning Lights on Roadway Operations Equipment. Washington, DC: The National Academies Press. doi: 10.17226/14190.
×
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Suggested Citation:"Chapter 1 - Introduction and Research Approach." National Academies of Sciences, Engineering, and Medicine. 2008. Selection and Application of Warning Lights on Roadway Operations Equipment. Washington, DC: The National Academies Press. doi: 10.17226/14190.
×
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Suggested Citation:"Chapter 1 - Introduction and Research Approach." National Academies of Sciences, Engineering, and Medicine. 2008. Selection and Application of Warning Lights on Roadway Operations Equipment. Washington, DC: The National Academies Press. doi: 10.17226/14190.
×
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Suggested Citation:"Chapter 1 - Introduction and Research Approach." National Academies of Sciences, Engineering, and Medicine. 2008. Selection and Application of Warning Lights on Roadway Operations Equipment. Washington, DC: The National Academies Press. doi: 10.17226/14190.
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Page 11

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5Background Roadway operations equipment used for construction, maintenance, utility work, and other similar activities gener- ally operate 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; therefore, warning lights are provided on the equipment to alert motorists of poten- tially hazardous situations. Amber warning lights have tradi- tionally 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. There are no widely accepted guidelines for selecting warn- ing lights on roadway operations equipment that consider the type of equipment, weather conditions, daytime and nighttime operation, color of vehicle, or other relevant factors. Research was needed to develop guidelines for use by transportation agency personnel in selecting and procuring lights that will sub- stantially enhance motorist awareness. NCHRP Project 13-02 was conducted to address this need. Objective The objective of this research was to develop guidelines for selection and application of warning lights to improve the con- spicuity and recognizability of roadway operations equipment (i.e., vehicles and mobile equipment) used for construction, maintenance, utility work, and other similar activities. This objective was accomplished through the following tasks: 1. Collection and review of relevant literature, specifications and guidelines, research findings, current practices, gov- ernment requirements, and other information relative to selection and application of warning lights on equipment used in roadway operations. Motorist response and other criteria used in the selection process were also identified and discussed. This information was assembled from published and unpublished reports, contacts with transportation agen- cies and industry organizations, and other domestic and foreign sources. 2. Identification and discussion of the factors related to the design and selection of warning lights. These factors in- cluded type and purpose of vehicle; daytime and night- time operations; weather conditions (e.g., snow, rain, fog, and dust); light details (e.g., color, type of light source, con- figuration and effective intensity of lights, flash patterns and parameters, location of lights on vehicles, and durability); color of the vehicle and markings on it; and distinguish- ability from emergency response vehicles. 3. Assessment of the relevance and importance of the identi- fied factors to the selection and application of an effective lighting system, development of a prioritized list of these factors, and recommendation of specific factors for further research. 4. Preparation of a detailed work plan that included exper- imental investigations for addressing the recommended factors and developing the guidelines. 5. Conduct of the recommended investigations and develop- ment and validation of the guidelines for the selection and application of warning lights on vehicles and mobile equip- ment used in roadway operations. The guidelines also in- cluded the technical information necessary for developing procurement specifications. This task was composed of the photometric evaluation of the light sources, and static and dynamic human factors tests. 6. Submittal of a final report that documented the entire research effort. C H A P T E R 1 Introduction and Research Approach

6Report Organization This report documents the work performed in this project. Chapter 1 describes the research approach and summarizes the findings of the literature review. Chapters 2, 3, and 4 describe the photometric characterization, the static screening, and the performance experiments, respectively. Chapter 5 pro- vides conclusions and suggested research. A more in-depth discussion of the experiments is included as appendixes to this report (not published herein but available on the TRB website at www.trb.org/news/blurb_detail.asp?id=9632). Trade or manufacturers’ names appear in the report solely because they are considered essential to the object of this report. Literature Review Currently, there is a wide disparity in the design of warning lights used on roadway maintenance vehicles. The Manual on Uniform Traffic Control Devices (MUTCD) provides some guidance for the use of traffic control, signage, and auxiliary safety vehicles, but these recommendations are not specific as to the nature of the lighting on the vehicles themselves. Recent studies have highlighted the lack of specific criteria for the use of warning lights. Kamyab and McDonald (1) found that there is a great disparity among states in the use of warning lights and that disparity exists even within local agencies. Several studies have also investigated different aspects of lighting systems, from color to configuration, but no single study has been undertaken to provide comprehensive guide- lines for the marking of maintenance vehicles. Because many types of maintenance vehicles are required to operate in bad weather environments and during both daytime and night- time, these environments are particularly critical aspects of the warning-light guidelines. A more detailed literature review is provided in Appendix A. Hazard Detection and Recognition Hazard detection refers to the first stage of information pro- cessing in which an object is perceived by one’s senses. Hazard recognition refers to a later stage of information processing in which drivers use their memories to relate the object to previous experiences. Recognition typically involves mental operations (or attention) and as a result takes longer than de- tection. However, certain characteristics of objects can support pre-attentive processing, or recognition without the applica- tion of attention. Contrast sensitivity is a main determinant of one’s ability to detect objects of interest in a visual scene because the human eye emphasizes regions of differences in illumination—because they possess the most information. Detection and recognition of objects in the road is context dependent. Drivers scan the roadway by looking in the direc- tion they expect to see an object of importance. Cox (2) points out that warning-light placement faces the following constraints in terms of capturing attention: • Motorists make multiple decisions on proximal events while driving. As a result, distant low-effective-intensity lights may not be detected since they have no immediate interest. • Drivers’ eyes are cast downward in the natural human pos- ture. As a result, drivers do not naturally keep a special lookout for distant objects. • A meaningful proportion of motorists is colorblind or has poor visual acuity. Conspicuity The conspicuity of an object, which refers to how well it captures one’s attention, comes in two types: attention con- spicuity and search conspicuity. Attention conspicuity is attrib- uted to an object’s characteristics, such as proximity, color, and movement. Search conspicuity refers to an object’s ability to capture one’s attention when one is actively searching for it, such as a retroreflective street sign placed in a consistent location (3). Because the presence of a maintenance vehicle is unexpected, vehicles that have poor attention conspicuity are less likely to be detected at a safe distance because motorists are not actively searching for them. Detection and recognition of unexpected events have further implications than simply attention conspicuity. The next sec- tion explains why response time to unexpected events is longer than that to expected events. Reacting to the Unexpected Whenever individuals respond to events that have been perceived, they are transmitting information. Information is defined by Shannon and Weaver (4) as the reduction of uncertainty. Information is potentially available in an event any time there is uncertainty about what the event will be. Information theory states that the uncertainty of an event is dictated by the number of possible events that can occur, the probability of each event occurring, and the context or sequential constraints that relate each event together (5). Dewar and Olson (6) explain that motorists operate with a set of expectancies, predisposing them to believe that things will happen in a certain way. There is an increase in driver perception-response time when the expectancies are violated; this increase can lead to increased driver errors and crashes. To account for these predispositions, advanced warning signs are used with the intention of establishing expectancy for upcoming hazardous conditions. However, when confronted with unexpected events, motorists require assistance in coping

with the change in task demands. A form of guidance, termed positive guidance, has been developed to aid people in such situations. Positive Guidance Positive guidance (6) is a way of providing information to allow the driver to detect a hazard in a roadway environment that may be visually cluttered, recognize its threat potential, select an appropriate speed and path, and complete the required maneuver safely. The positive guidance concept acknowledges three levels of driver performance: control, guidance, and navigation (6). Control. The control level encompasses the interaction between the driver and the vehicle. Drivers control vehicles through the steering wheel, accelerator, and brake. Guidance. The guidance level describes the selection and maintenance of a safe speed and path. Dewar and Olson (6) state that information at the guidance level comes from the highway, traffic, and traffic control devices. Navigation. The navigation level refers to the planning and execution of a trip from origin to destination. Decisions at the navigation level are made at select points based on infor- mation extrapolated from maps, verbal directions, experience, guide signs, and landmarks. Information Placement Dewar and Olson (6) also explain that, to execute positive guidance in traffic control devices, four principles of informa- tion placement must be followed: primacy, spreading, coding, and redundancy. Primacy requires information on signs to be placed according to importance to the driver. Spreading requires information content to be spread out across multiple signs when its content is too great to place on one sign. Coding requires pieces of information to be organized into larger units (e.g., using specific colors and shapes for street signs). Redundancy requires information to be presented in more than one way at the same time (e.g., an emergency vehicle’s visual warning lights and auditory siren). Flashing Warning Lights Flashes are bursts of light which, by definition, are un- expected because they do not occur in nature (save for light- ning). This characteristic is their most important feature and why they are so good at capturing attention. Holmes (7) sug- gests that flashing lights have their own language. The flash’s characteristics (such as flash frequency, effective intensity, and duration) are elements of a language that can be learned. Holmes also suggests that people need to be educated on how to recognize flashing signals (because they are artificial) and how to interpret their meanings. The characteristics affecting warning-light conspicuity in- clude contrast brightness, flash effective intensity, flash color, flash frequency, flash duration, flash shape, flash type, flash pat- tern, flash size, number of elements, and apparent motion, and steady-burn light color. Contrast Brightness Contrast brightness refers to the direct comparison of one reflecting surface with another. Contrast brightness of a flash- ing light signal is obtained from the difference in illumination between the lamp-illuminated bulb, called a “roundel,” and the background. Flash Effective Intensity Roufs (8) defines flash threshold as the minimum effective intensity increment required for perceiving the flash. For short flashes, the threshold is driven by the product of the flash effec- tive intensity and duration. The threshold for long flashes is mainly determined by the effective intensity. Flash Color Cook et al. (9) investigated the conspicuity of warning bea- cons according to flash color and found that when effective intensity is held constant, amber has the poorest detection time under both day and night conditions. Blue light mini- mizes the effects of disability glare and daytime discomfort glare. Green light has the quickest detection time during day conditions, but is the poorest for disability glare and dis- comfort glare. Red light yields the quickest detection times and gives rise to the least discomfort glare. Flash Frequency Misinterpreting flashing lights designed to communicate a message to motorists can be as dangerous as missing the sig- nal. To avoid misinterpretation, the flashing light signal must be seen for the duration of one period. Holmes (7) states that the flashing signal should be repetitive and have a maximum interval of 5 s to continuously retain the observer’s attention. Flash Duration The flash duration is defined as the length of time during which the light is on in one flash cycle. Brown and Gibbs (10) found that, as the flash frequency duration decreased for fre- quencies in the range of 1.5 to 3 Hz, there was a corresponding 7

decrease in reaction times. However, for signals with frequen- cies of 1 Hz and 0.33 Hz, Gerathewohl (11) found that longer flash durations yielded shorter reaction times. Flash Shape The flash shape refers to the temporal distribution of light in the flash cycle. Howard and Finch (12) state that for flashes that last longer than the critical duration of 50 ms, the square wave pattern is more effective than a triangle shape wave of equal flash energy. Flash Type Cook et al. (9) investigated the conspicuity of warning beacons according to flash type. They found that strobe warn- ing beacons were subjectively considered to convey greater urgency, while rotating warning beacons were considered to be less annoying and minimized the effects of disability glare. Flash Pattern Cook et al. (9) also investigated the conspicuity of warning beacons according to flash pattern. They found that when more than one warning beacon was present on a vehicle, bea- cons that flashed simultaneously were detected significantly faster than beacons that flashed alternately. Simultaneously flashing beacons were also subjectively rated as more con- spicuous, while those that flashed alternately had the lowest discomfort glare. Signal Size Many investigations on steady lights at threshold levels have concluded that lamp size does not play a significant role in determining its conspicuity. However, the perception of light under road conditions is quite different than under labora- tory conditions. Cole and Brown (13) concluded that effective intensity is independent of signal size for light signals with a high probability of being seen (called optimum signal lumi- nance) and that, if the lamp is of optimum luminance, its size does not matter. Number of Lights Cook et al. (9) investigated the conspicuity of warning beacons according to the number of elements utilized. From subjective ratings, they found that the greater the number of warning beacons, the greater the perceived conspicuity. In 1990, Hanscom and Pain (14) developed guidelines for warning-light systems on service vehicles engaged in short- term or moving maintenance operations. Based on the results of both a closed-field and field experiment, the following conclusions were made: • If only one type of light is used, four-way flashers pro- vide the most accurate information about closure rate and service vehicle speed. • Adding more of the same type of lights does not increase the amount of information provided to the driver or en- hance the driver’s ability to extract information from the lights. • Changing the location of the light(s) does not increase the information or ability to extract information; it is important that the light can be seen from all directions. • Lighting parameters had little effect on driver response. • Adding a four-way flasher to any other warning light in- creases the amount of information provided to the driver, and combining a roof-mounted flasher light and rotating light increases the information to the driver. Apparent Motion Under certain conditions, it is possible to create a sense of motion between two stationary sources of light by flashing the two lights on and off with one source temporally trailing the other. Foster (15) showed that a model developed to de- scribe certain real-motion effects also translated to describe the existence of an apparent-motion effect. Steady-Burn Light Color Color is an established coding dimension for inter-vehicle signaling. Projector et al. (16), however, reject the use of color-coding owing to variation in observer vision, desatura- tion of colors in haze and fog, and variation in filter efficien- cies, but note that color is useful as a redundant perceptual dimension. Hazard Analysis In investigating the effective conspicuity of new warning lights, factors that present potential drawbacks must also be considered. Disability glare, discomfort glare, distraction, and eleptogenic response are such factors. Disability Glare Disability glare occurs when a bright light source impairs an individual’s ability to see objects. The effect of disability glare caused by warning beacons, as stated by Cook et al. (9), was assessed by subjects’ ability to detect a pedestrian in their vicinity. They found that disability glare was worsened by amber beacons, strobe beacons, and maximum intensities. 8

Discomfort Glare Discomfort glare is defined as glare that is annoying or painful, but that does not cause impairment in the visual field. Discomfort glare could potentially have safety implications because it may cause drivers to avert their gazes. Cook et al. (9) found that discomfort glare was worsened by amber and green beacons, strobe beacons, maximum flash frequencies, and simultaneous flash frequencies. Distraction A balance needs to be made between warning-beacon con- spicuity and warning-beacon distraction. Cook et al. (9) found that the presence of a warning beacon is significantly more distracting than no warning beacon at all, but the extent of the distraction was not related to flash type, frequency, or effective intensity. Eleptogenic Response (Epileptic Seizure) Some features of flashing lights, such as flashing light fre- quency, luminance, field of view, and flash type, are relevant to eleptogenic response. Frequencies above 5 Hz should be avoided. Luminance as low as 20 cd/m2 can trigger eleptogenic response; however, this exceeds the luminance required to make a warning beacon conspicuous. Lights flashing in the cen- ter of the visual field are more likely to cause an eleptogenic re- sponse. Also, drivers of emergency vehicles reported that strobe beacons cause more visual discomfort than rotating beacons. Environment Complexity Hargroves (17) states that the background has a significant effect on the conspicuity of flashing lights. Day, night, glare, and irrelevant lights can affect the conspicuity of the flashing-lights signals. Number of Irrelevant Lights Crawford (18) found that response times to light increase from 0.8 s to almost 2 s when 21 lights are added to an other- wise clear background in a dark soundproof room. He also showed that steady signals are always more effective than flash- ing ones if the proportion of flashing background lights exceeds 1 in 10, and therefore, overuse of flashing lights would defeat their purpose. Time of Day For a fixed luminance, a warning light will have a further detection distance during the night than it will during the day because the contrast of the signal is great at night. Weather The presence of snow, rain, or fog interferes with the per- centage of light reaching the driver’s eyes from the warning light. When the brightness is decreased, the signal is harder to detect. Road Geometry Because the human eye’s sensitivity to light is dependent on location of the light within the retina, it is possible that the placement of the warning light in the visual scene as a result of the road geometry will have an effect on its conspicuity. The geometry of the road, combined with the obstruction of lights from trees, rocks, and buildings, may affect the conspicuity of the warning lights. State Practices for Roadway Warning Lights The application of warning lights to maintenance vehicles differs among highway agencies in the Untied States. Warning- light specifications for some state departments of transporta- tion are presented in this section. These differences highlight the need for developing guidelines that will have nationwide applicability. Virginia The Virginia Work Area Protection Manual (19) specifies the design and application of temporary traffic control devices. The manual states that warning lights should be either a ro- tating amber light or high-effective-intensity amber strobe light, and that rotating lights shall be mounted to be view- able for 360° among other specifications of intensity, flash frequency, etc. Ohio The Ohio Department of Transportation (ODOT) (20) established a vehicle warning-light policy to assure the districts and Central Office maintain uniform lighting array, equipment light, marking, and conspicuity. This policy states that all safety lighting will be flashing lights; amber in color; composed of photo strobes, LEDs, or a combination of both; and viewable from 360°. New York The New York State Department of Transportation (NYSDOT) follows a vehicle marking and lighting standard that was developed in the mid-1980s. Few recommendations have been made to improve upon the standard; one change that has been implemented is the use of more LED lights for tail 9

lights and marker lights to reduce power draw and increase visibility. The DOT believes that the halogen rotating yellow beacon provides the best overall light for visibility and safety for the traveling public. Maine The Maine DOT (MDOT) does not have a traffic engineer- ing handbook, but amber lights are used on all of their con- struction vehicles; state law precludes the use of red or blue. Iowa The Iowa DOT conducted an investigation on the types of crashes involving snowplows and concluded that the rear end of the snowplow needs to be more visible to give approaching vehicles more time to respond. The snowplows currently use two amber rotating beacons and two amber rear-directional alternate flashing strobes (21). Retroreflective tape, warning flags, and auxiliary headlamps are also used as warning devices. Texas Texas DOT adopted a warning-light policy for use on spec- ified vehicles and equipment based on research conducted at the Texas Transportation Institute (TTI) in 1998 (22). Amber warning lights are used to identify highway maintenance and service equipment. Flashing Light Measurement Issues In the 19th century, it was recognized that intermittent light, or flashing lights, produced higher visibility than a steady light of the same intensity. Thus, efforts began to quantify the visibility as effective intensity. Effective intensity is defined as the luminous intensity of a fixed (steady) light, of the same relative spectral distribution as the flashing light, that would have the same luminous range as the flashing light under iden- tical conditions of observation (23). A singular equation has not been developed, but several options for calculation exist, in- cluding the Allard (24), modified Allard (25), Blondel-Rey (26), Blondel-Rey-Douglas (27), and Form Factor (28) methods. Retroreflective Tape During the winter months, detection and recognition of snowplows can be deterred by the snow cloud produced by these vehicles. Because the cloud of snow covers the tail light and makes detection of such vehicles even harder at night, the use of retroreflective strips has been considered. A study conducted by TTI found that the 8-inch-wide orange and fluorescent-orange magnetic strips had an insignificant impact on daytime driving, but could improve the visibility of vehi- cles during nighttime or low-visibility winter weather (21). Morgan (29) found that retroreflective tape reduced side and rear impacts into trailers in dark conditions. SAE Standards The Society of Automotive Engineers (SAE) released a stan- dard for the lighting and marking of industrial equipment on highways (J99) in March 1999 (30). The standard states that there shall be at least two amber flashing warning lamps spaced as laterally wide as practicable and mounted at the same level at least 42 in. high as measured from the lamp’s axis. SAE also developed SAE J2040 (31) in 2002 to specify the requirements for tail lights placed on vehicles of widths of 2032 mm or wider. The standard states that the color of the tail light shall be red and should have an effective projected luminous lighted lens area of at least 75 cm2. Survey of Current Equipment Available There are many products on the market with similar photo- metric characteristics, and the information provided to con- sumers is often confusing. Three technologies of flashing equipment are available. The difference among these tech- nologies is the source of the light: incandescent filament bulb, xenon or high intensity discharge (HID) flash tube (commonly referred to as strobe lights), and LED. However, because no classification system currently exists, the lighting must be judged on the source technology only. Incandescent Filament Bulb There are two types of flashing lights that use incandescent filament bulbs: rotating beacons and 360° flashing lights. The pulse width and shape of a rotating beacon are determined by the reflective optic because the bulb is on continuously. The wattage of the bulbs describes the quantity of light available to the system, but the shape and efficiency of the reflector is what controls the pulse intensity and width. The 360° flashing light ramps the current up and down quickly to create the time dependence. A dome with a Fresnel lens encases the lamp and focuses the light in the plane of the observer. Xenon or HID Flashtubes The xenon or HID flashtube lamps have a similar structure to that of the 360° flashing lights with a Fresnel dome encas- ing the flashtube light source. A significant difference between the flashtube and incandescent filament sources is the peak instantaneous intensity and pulse width. The peak instanta- neous intensity can be 1000 times more, but the pulse width is usually 1000 times less in width. 10

LED-Based Lamps Many systems that use yellow LEDs as a light source are now available. A few are based on the 360° flashing-light assembly, but many are strictly for directional purposes. It is likely that more of these assemblies will replace the 360° or rotating assembly in the future because of the electric efficiency. Literature Review Conclusions Because the presence of maintenance vehicles on the road is an uncommon event with low probability, motorists consider their encounters with maintenance vehicles to be unexpected events. Any measure that increases the distance at which mo- torists are informed about the presence of maintenance vehi- cles will increase the time available to react. Because signs are stationary countermeasures and the operation of shadow vehicles is expensive, the use of more conspicuous warning lights is desired. The literature review provided guidance on which factors (e.g., light color, types of lights, ambient lighting, lighting in- tensity, driver factors, the use of flashing, etc.) should be con- sidered in developing guidelines for the use of warning lights and therefore should be considered in this research. However, it did not provide final answers on the relevance of these factors to particular applications. Identification of Relevant Factors Based on the results of the literature review, a prioritiza- tion of the relevant factors was conducted by surveying knowl- edgeable practitioners. As an initial evaluation, the factors were characterized as lighting factors, vehicle factors, environ- mental factors, and driver factors. Of these factors, only light- ing and the vehicle factors are controllable by the responsible agency. Through the survey, the lighting factors were found to be most critical, followed by the environmental and driver fac- tors. The vehicle factors were not regarded as important as the others. Based on these findings, an experimental program was de- veloped that included an initial screening experiment to reduce the number of lighting factors to be considered followed by a dynamic experiment in which the weather conditions could be investigated. A complete description of the relevant factors and the knowl- edgeable practitioner survey is provided in Appendix B. 11

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TRB's National Cooperative Highway Research Program (NCHRP) Report 624: Selection and Application of Warning Lights on Roadway Operations Equipment explores recommended guidelines for the selection and application of warning lights on roadway operations equipment.

Appendixes A through E to NCHRP Report 624 are available online. The appendixes contain detailed information on relevant literature, the experiments performed, and data analysis associated with NCHRP Report 624.

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