Most people agree that high-mounted turn and stop lights on trailers and truck bodies are a good idea because they tell drivers behind what the truck driver’s up to. That’s particularly true in heavy street traffic, when a flashing light cues motorists to react early by slowing down or moving over to avoid being delayed.

But sometimes lights are installed in a way that creates confusion, according to the Transportation Safety Equipment Institute, made up of firms that produce lighting and other safety devices.

TSEI reports that its member companies are seeing more requests from trucking fleets and trailer builders to add innovative lighting devices, and questions arise about their legality.  

The group says it’s actively working with regulators and the industry to ensure that everyone understands what's permissible in supplemental lighting.

Additional high-mounted auxiliary turn and stop lamps on trailers are fine if they comply with certain federal regulations governing lighting, says Brad Van Riper, chairman of the Government Relations Committee at TSEI and senior vice president and chief technology officer at Truck-Lite.

TSEI has summarized the criteria applicable to high mounted auxiliary turn and stop lamps: 

1.Required lighting devices are installed per the requirements of the standard (Federal Motor Vehicle Safety Standard 108 and sections 393.9 - 393.30 of Federal Motor Carrier Safety Regulations);

2.The installed supplemental or auxiliary lighting adds to or supplements the required lighting devices;

3.The supplemental lighting does not impair the effectiveness of the required devices; and

4.The supplemental lighting is not installed in a manner that would cause confusion (for instance, strobing or other supplemental red stop lamps in jurisdictions that do not allow them).

An example of an auxiliary stop configuration is shown below with the center marker lamps also serving as auxiliary stop lamps. Here they’re activated:

The corner clearance lamps can also double as turn signals if their clearance function continues when signals are off. This is done with converters working through dual-voltage LEDs or dual-filament incandescent lamps.

“It's critical that in meeting this request for better visibility during the stop function, we do not reduce the effectiveness of the primary function for the device at its particular location,” said Kristen Goodson, TSEI president and director of product management at Peterson Manufacturing.

Is supplemental lighting accepted by roadside inspectors? Yes, the institute says.

“TSEI learned that roadside enforcement is well versed in the limitation set by FMCSR 393.9 and only required lamps must be operational,” the group said in a statement. With respect to Part 393.3, roadside inspection officers consider three  guidelines that say a lamp may be used if:

1. It is not prohibited by regulation (such as amber tail/stop lamps, though amber turn signals are legal).

2. It is not inconsistent with regulations (such as purple/green lights)

3. It does not decrease the safety of operation (as referenced in FMCSA’s 393.11 Interpretation)

TSEI said that properly installed and working auxiliary turn or stop lamps are safety enhancements, and “their presence on vehicles should not negatively impact the good CSA scores of conscientious drivers and fleets.”

It added, “Being able to properly inspect and maintain the vehicle lighting and wiring system is one of the most important ways to ensure that vehicles operate safely.”