By: Gerald L. Ullman, P.E.
Nationwide, special flashing warning lights on construction, maintenance and service vehicle equipment are generally limited to the color amber (although some states use blue flashing lights on snow removal equipment). However, for the past few years, the Texas Department of Transportation (TxDOT) has begun allowing certain types of vehicles, those generally used in higher-risk activities such as maintenance operations or incident response and clearance, to display both an amber- and a blue-flashing warning light. TxDOT has legal authority to adopt policies regarding special flashing warning lights on construction, maintenance and service equipment in the state.
However, this use of a blue warning light has caused some concern within the state law enforcement community. Some officials worry that such practices degrade the effectiveness of the blue light for authorized emergency vehicle applications. Consequently, TxDOT contracted with the Texas Transportation Institute (TTI) to investigate the issues and possible effects of using flashing warning light colors other than amber for construction, maintenance and service vehicle equipment.
Color vision
Special vehicle flashing warning lights have two primary functions. The first function is to attract the attention of nearby drivers and pedestrians so as to alert them to the situation they are approaching or that is approaching them. Past studies have demonstrated the benefit of special flashing vehicle warning lights in obtaining a driver’s or pedestrian’s attention. The second function is to provide those drivers and pedestrians with information about the situation so that they can take whatever appropriate action is needed. Whereas detection/perception of a warning light is primarily physiological, information transfer is primarily cognitive. The sensory information received through the visual system is converted to something meaningful through a pattern recognition process. This process is highly dependent upon driver expectations developed through past experiences, how those experiences are coded in memory and the context in which the information is received.
Color plays an important role in the memory coding and pattern recognition process. Traditional wisdom has been to restrict certain warning light colors (primarily red, white and blue) to emergency vehicles so that motorists learn that these colors have a special meaning and need to respond appropriately to them. Meanwhile, amber has been assigned to maintenance and construction vehicle warning lights and to flashing beacons on signs and barricades or at intersections for general warning purposes under all conditions. Although it is appropriate to use warning light colors to convey a single message for emergency vehicles (i.e. high-hazard emergency situation approaching), it would seem desirable to distinguish between construction and maintenance activities that pose lower risk to workers and motorists, and those that pose higher risk.
The main question is whether such a distinction can be made on construction and maintenance vehicles without adversely affecting motorist perceptions and response to warning light colors used for emergency response vehicles (police, fire and ambulance). Research undertaken by TTI attempted to investigate these questions and is summarized in the next few paragraphs.
The research consisted of motorist opinion surveys and traffic studies to determine how motorists responded to a TxDOT maintenance truck outfitted with different-colored flashing warning lights.
Light/vehicle association
TTI researchers administered an opinion survey about flashing vehicle warning lights to a sample of about 150 Texas motorists. The survey consisted of two parts. In the first part, motorists were asked to assess how different warning light colors and color combinations affected their perceptions of the approaching hazard and their appropriate driving responses. Survey responses indicated that Texas motorists do interpret various special flashing warning light colors and color combinations differently.
Only slightly more motorists feel that some braking maneuver (either a tap or a gentle/firm application) is required for the amber/ blue combination than for the amber light alone. Conversely, most motorists believe a braking action is required for the red/blue and the red/blue/amber warning light combinations. The red/amber warning light color combination fell inbetween the responses for the amber and the red/blue/amber color combinations.
In part two of the survey, motorists were asked what colors of lights they expected to see on top of different types of vehicles. As would be expected, most motorists associate the amber warning light color alone with construction and maintenance vehicles as well as tow trucks. Meanwhile, motorists associate a red/ blue or a red/blue/amber warning light color combination with a police vehicle, and a red or a red/white color combination with ambulance and fire vehicles.
The results of the motorist opinion survey do suggest that motorists can and do associate different flashing warning light colors with different types of vehicles, and also associate different appropriate driving responses to them based on those vehicles. More importantly, though, motorists do not appear to focus strictly on any one color but rather on the specific color combination used to identify a specific vehicle type (i.e. construction or maintenance, police, fire, ambulance). This implies that it is possible to utilize other colors in combination with the amber warning light for special construction and maintenance activities without adversely affecting how motorists interpret certain other warning light color combinations used by emergency vehicles.
Adding to amber
The survey results described previously suggest that motorists do indeed perceive differences in certain vehicle warning light color combinations, and believe they should respond differently to them. To determine whether these perceptions actually translate into differences in driver behavior, researchers conducted a series of field studies on five urban freeway sections in Houston and San Antonio, Texas. In each study, TxDOT maintenance or courtesy patrol vehicles were outfitted with different vehicle warning light color combinations and placed, one at a time, on a shoulder next to moving traffic with the lights activated. To evaluate the effect that vehicle type itself has upon driving behavior, TTI was assisted in the research by officers with the Texas Department of Public Safety (DPS). These officers brought out their police cruisers (with their amber/blue/red warning lights) and allowed researchers to obtain data at the same locations as where the alternative warning light configurations on TxDOT vehicles were evaluated.
Camera problems did not allow speed data to be obtained during certain portions of the study. Vehicle speeds when the amber and blue light combination was displayed were significantly lower (5 to 6 mph) than when only an amber light was displayed at two of the five sites tested. At the other three sites, speeds were not significantly different between these two warning light configurations. Interestingly, no statistically significant differences were found in average speeds at any of the sites when the amber/blue/red warning light configuration was compared to the amber warning light-only configuration.
Perhaps equally surprising was the finding that the presence of the DPS vehicle parked on the shoulder with its lights flashing did not affect speeds any more than the TxDOT vehicles. No statistically significant differences were found between speeds observed when that vehicle was present and when the TxDOT vehicles with amber flashing lights were present. It also is interesting to note that all of the warning light configurations (including amber only) resulted in significantly lower speeds than those measured during a control (no vehicle or warning lights present) condition.
Another measure examined was the frequency of brake light activations for motorists approaching the various warning light color configurations.
At three of the four sites, the amber/blue/red warning light combination resulted in a higher braking percentage than the amber light only. The amber/blue combination also resulted in a significantly higher braking percentage (relative to the amber-only configuration) at one site. It also is important to note that at two of the three sites where data was available for the DPS vehicle, brake activations were significantly greater than they were when only the amber light was displayed on the TxDOT vehicle.
Researchers believe that the brake light activations may actually be a more pure measure of the potential impact of warning light color configurations upon driving behavior. Specifically, a definite trend towards increased frequency of brake activations was evident as colors were added to the basic amber light used on most service (construction, maintenance, utility) vehicles. Furthermore, the presence of other enforcement cues (i.e. the law enforcement vehicle) further increased this type of response by motorists. According to this data, the combination of amber and blue lights may have some incremental benefit above and beyond that of an amber light only. At the same time, this combination did not generate quite as many brake light activations as the amber/ blue/red warning light combination, suggesting that motorists can make a distinction between that particular color combination and others displayed. Although indiscriminate use of multiple colors for construction and maintenance vehicles would be undesirable and could lead to an eventual loss of credibility with the motoring public, selective application of blue (or possibly red) lights in conjunction with amber for certain work activities does appear to offer potential for improvement in safety.
