Click for next page ( 2


The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 1
1 SUMMARY Selection and Application of Warning Lights on Roadway Operations Equipment Roadway operations equipment used for construction, maintenance, utility work, and other similar activities generally operate within roadway right-of-way. These vehicles and mobile equipment operate on all types of roadways, during day and night hours, and under all weather conditions. 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 warn- ing lights (e.g., lighted bars, lighted "arrow sticks," strobe, light emitting diodes [LED], and alternating flashing) are used. There is a concern that this variety of lighting on roadway- operations equipment has evolved without adequate consideration of the effects on the awareness and responsiveness of motorists. This research project was undertaken to evaluate the effectiveness of warning lights on roadway maintenance vehicles with the goal of establishing guidelines for the application of the lighting system on the vehicle. The research included an evaluation of the lighting parameters that define the performance in terms of glare and vehicle detectability as well as an evaluation of the lighting systems in adverse weather and in a dynamic setting. Summary of Findings This research identified several aspects of the warning-light system as critical for vehicle safety; these aspects are reiterated in the proposed guidelines provided as an attachment to the report. The research also showed that a balance must be maintained between the conspicuity and safety of the maintenance vehicle and its crew and the glare imposed on the other drivers. The safety of the maintenance-vehicle crew involves both situations when the crew is in the vehicle and when the crew is outside of the vehicle, possibly working on the road. The research first identified the requirement for the use of internally illuminated sources and for the warning system to provide active illumination for the safety of the maintenance crew and the driving public. Passive reflectors such as Department of Transportation (DOT) tape did not draw the driver's attention or provide any attention-getting cues to an approaching vehicle. Flashing lights were found to be more conspicuous than continuous lights and provide a sense of urgency. An asynchronous flashing pattern (flashing side to side) provided a higher attention-getting rating than a synchronous flash pattern (both sides flashing at once). Amber light sources and white light sources provided higher responses than blue or red. With regard to the relationship of the light color to the vehicle type, amber and white are more related

OCR for page 1
2 to maintenance vehicles than the other possible colors that are closely tied to police and fire services. The research also showed that light sources with a higher effective intensity will provide higher attention-getting than a light source with a lower effective intensity, although this effect is offset by the flash characteristics. A warning-light system that provides a different flash pattern than the other lighting systems in the road environment improves the ability of the driver to identify the vehicle sooner. Using a double flash or varying the effective intensity (such as with a rotating beacon) allows the maintenance vehicle to be identified at a longer distance than other flash patterns. Also, when approaching a vehicle from the rear, drivers primarily use the vehicle's tail lights for vehicle identification; locating the warning- light system high on the vehicle away from the tail lights improves vehicle identification distance. The research showed that a warning-light system must have a higher effective intensity dur- ing the daytime in order to provide adequate daytime conspicuity. This intensity may vary by the type of light source used. The research further showed that a warning-light system with halogen lamps may provide a higher conspicuity at a lower effective intensity than warning lights using LEDs. Because there is no evident glare in the daytime environment, no maxi- mum effective-intensity limit is suggested. Another issue relevant to daytime conspicuity is the location of the light source. A warning light seen against the sky will have reduced contrast and conspicuity compared to a warning light seen against a dark background. Therefore, the light must appear against a controlled background in order for the conspicuity to remain constant. The research showed that the sources used to provide adequate daytime conspicuity will cause significant glare for opposing and passing drivers at night. Adequate conspicuity can be provided at night with a much lower effective-intensity level. Therefore, effective inten- sity of the lighting system must be maintained between the level that provides conspicuity and the level that does not cause too much glare. The photometric effective-intensity values describing these limits are discussed below. A higher effective-intensity light source was found to limit the detection of a pedestrian around a vehicle, thus, negatively affecting the safety of the maintenance crew when they are outside of the vehicles. This factor limits the overall effective intensity that should be used for the warning-light system. Using too many lights or lights with too high an effective intensity may impede the ability of other drivers to detect a pedestrian. The potential for glare is the primary consideration when specifying requirements for warning lights that will be used during nighttime. Bright warning lights and oppressive flash- ing will provide disability glare and discomfort for a driver of an oncoming vehicle or a vehicle passing a maintenance vehicle from behind. Glare from the warning-light system may limit the ability of the driving public to travel safely. Glare is primarily a result of the effective intensity of the light source. The research showed that a light source with a higher effective intensity creates a greater glare response than a light source with a lower effective intensity. Glare and pedestrian detection distance also provide a limit to the maximum effective intensity of the warning-light system. This factor will further limit the number and type of light sources placed on the maintenance vehicle. The position of the warning lights also affects glare. The research showed that a light positioned close to the height of an opposing driver's line of sight creates a greater glare response than a high-mounted lighting system. This response is particularly evident with 360 sources (lights that are seen from all angles), as a passing driver will be able to see that source even if the driver is very close to the maintenance vehicle. This consideration supports a requirement to locate the lighting system as high as possible on the vehicle.

OCR for page 1
3 Adverse weather conditions impact visibility of maintenance vehicles. The research showed that the vehicle identification distance is diminished in adverse weather conditions. The pres- ence of moisture in the atmosphere will cause absorption of the light from the warning sys- tem and will cause the light to scatter. In this research, the low-visibility condition was tested using fog as a surrogate for snow. In the experiments, a higher effective intensity caused both a greater amount of scatter and, therefore, a greater glare experience at night. However, the higher effective intensity improved the visibility of the vehicle. The effective intensity is lim- ited by the glare in this condition and additional lighting in adverse weather will likely cause difficulties to opposing and passing vehicles. The research showed that in a visually complex environment, a higher effective intensity may be required for adequate performance as compared to simple rural environments. Also, glare ratings were lower on a road with an overhead lighting system and opposing traffic than on a rural test track. Similarly, the high-effective-intensity light source caused passing vehi- cles to change lanes earlier than they did in the presence of a low-effective-intensity light source. When other vehicles were present, the glare ratings were also reduced. In visually com- plex environments, it may be possible to use a higher effective intensity to provide increased visibility of the vehicle while not causing too much glare for other drivers. Lighting Requirements and Considerations The recommended lighting requirements are based on the results of the experimental investigation and 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 colors. LED sources also provide an equivalent amount of light at a reduced wattage that may be beneficial 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 lighting be used in maintenance vehicles, with amber being the predominant color. These colors provide increased detectabil- ity 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 alter- nates 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 recommended that a slower flash frequency be used, because there was better response to the longer flash durations than to shorter flash durations. A flash pattern such as a double flash or a pattern similar to a rotat- ing beacon will provide an appearance that enables vehicle identification and should improve response. A rotating beacon will provide the appearance of flashing, and when two beacons are used, it is rare for them to appear synchronized.

OCR for page 1
4 Steady Light It is recommended that, if a steady (continuous burning) light is used on the vehicle in order to meet the federal vehicle lighting requirements, it should be used only as a supple- ment to the flashing-light systems. Because regulations are subject to change, the most recent federal statutes (e.g., Federal Motor Vehicle Safety Standards [FMVSS] 108) should be con- sulted when designing a lighting system. Steady lights have many other vehicle uses, such as clearance indicators, brake lights, and vehicle headlights, and thus 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, sides, and back of the vehicle. The lighting system should be positioned such that the light does not cause excessive glare to approaching and passing drivers. The light should also be placed away from the tail lights of the vehicle to allow those lights to be seen (i.e., mounted high on the vehicle above the typ- ical eye height of other drivers) and in a manner that will outline the vehicle (i.e., on either side of the vehicle and on any portion of the vehicle which extends beyond the lane such as a plow blade or a trailer extension). Retroreflective Tape It is recommended that retroreflective tape be used as a supplement to a flashing warning- light system. This material can be used to identify vehicle shape, but should not be used as the only notification system on the vehicle. Effective-Intensity Requirements 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 apparent to other drivers. The research showed that the nighttime and daytime requirements are different and may require two alternative warning-light systems, or a method of automatically attenuating the daytime levels in the presence of dark conditions. The effective-intensity measurements used in this research are based on the Form Factor method for calculating effective intensity. The recommended values represent the total light output limits for the warning-light system. A higher effective intensity may be required for vehicles that are primarily used in urban and visually complex environments.