Strobe or rotating beacon-type emergency lighting is a requirement on all Minnesota Department of Transportation (Mn/DOT) maintenance and construction vehicles. There is an effort to replace the standard warning lighting with light-emitting diode (LED)-based lights in order to reduce cost of replacement and maintenance. LEDs are well-known for their long life and low current requirements, resulting in their use in a variety of applications including emergency vehicle lighting.

Photo of one of the LED strobes from Public Saftey Equipment used in this study.
However, concerns of LED-based systems include limited angularity, visibility under certain environmental conditions such as bright sunlight, and dimming capabilities during low light conditions. Because of these features, LED lighting has not been able to meet requirements for emergency lighting on maintenance and construction vehicles.

Top-view sketch representative of the LED lights tested in the Mn/DOT study.
The scope of this project is to review the strobe-type lighting currently used on Mn/DOT snow plows and LED replacements for that lighting. Included in the study is laboratory and field testing of the different light types in order to correlate objective and subjective test results. Lab testing included detailed optical measurements of angularity, brightness, and color for the standard and LED strobes. Field tests included visibility testing of different lights under typical driving conditions. Lights tested in this study were supplied by Whelen Engineering, Public Saftey Equimpent (PSE), and Federal Signal. The standard plow lighting currently used on Mn/DOT snow plows is a high-intensity discharge (HID) strobe made by Whelen Engineering.

Optical power measurement data - plots of intensity vs. rotation angle for each of the strobes measured are given.
The results of this study are generally mixed. First, it is clear from the data that the low power benefit of LED lights can be realized under specific conditions. For similar visibility conditions, the LED lights performed as well — or in some cases better — than the HID strobe. Viewed directly from the rear, side, or front of the plow vehicle, it appears that LED-based strobes can be made to be equally conspicuous while reducing the costs associated with high power, maintenance, and reliability issues.

One possible light configuration for improved LED strobe visibility.
However, the above benefits are limited by the problems associated with angularity of the LED lights. The LED lights were shown to be equally conspicuous as the standard strobe at well-defined angles only. The reduced conspicuity at off-angles is a direct result of the angular intensity variation of the LED devices due to the lenses used to increase the intensity at the front, back, and side of the plow vehicle. Decisions must be made regarding the requirements of visibility at off-angles in order to determine if LED lights can be made to meet the needs of Mn/DOT. If the expectation is that conspicuity comparable to the standard strobe is needed at all angles, LED lights must be designed to meet those needs. However, if conspicuity at limited angles is acceptable, then LED fixtures are already on the market that meets those needs.

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This work was done by Tim Vogt and Kenneth Miller of St. Cloud State University.