"This has been bothering me a lot - and I expect others are bothered by it as well – which is the brightness of the headlights. What are the rules regarding the brightness of the headlights? Are they enforced by police? How can I be sure that my headlights are also not too bright?"
Amen, Krishna. How annoying is it when you use the dimmer switch on your rearview mirror and are still getting blinded by the car, truck or SUV behind you?
There are no specific rules in Washington state about the brightness of headlights, according to Washington State Patrol spokesman Dan Coon. So, the state has adopted federal standards on this. The specific regulation is 49 CFR 571.108. It's by this standard that police will enforce the law.
Police mainly look for vehicles with novelty lights, or lights specifically designated for off-road, that are being used on the highways, Coon said. But he adds that it is very difficult for police to tell, just by looking, whether headlights that are the standard white or clear color are outside of federal standards.
Without getting into too much "inside baseball," here's what you need to know to make sure your headlights are OK.
"Original parts already meet the federal standards," Coon said. "Problems can arise when the original headlights or lamps are replaced. To ensure the standards are met, the replacement headlamp and equipment must be marked with the symbol 'DOT' either horizontally or vertically to indicate certification."
For those of you who want more specifics on this, see the bottom of this page.
Follow-up: A clarification to our last "I Wonder" question about school buses and whether you can pass them. The law states that you do not have to stop for a school bus if you are headed in the opposite direction AND the road has three marked lanes. Some readers asked us to clarify whether the center turn lane counts as a marked lane. Coon says yes, it does. He says if there is a center turn lane and you are in the lane furthest from the bus headed in the opposite direction, you don't need to stop. But, if you are in the center turn lane, you MUST stop regardless of which way you are headed. He also reminds you that children are not allowed to cross a road with more than two lanes. In those cases, they must cross at a designated crosswalk.
If you have a question (and yes, we would love to tackle topics other than traffic), send it to firstname.lastname@example.org. Please include your first name and where you are writing from so we can post it with your question. We will not post your last name or any other personal information.
Here is more from Dan Coon about headlight specifications:
The federal lighting standard is very complex and is difficult to interpret even for some manufacturers and lighting specialists. The standards do not set a single maximum intensity for all possible types and designs or headlamps because the types of lights and lighting systems designed by manufacturers make a single standard impossible. Rather, FMVSS No. 108 determines the maximum allowable light intensity for a light by its design and the type of lighting system being used. The maximum light output for a particular headlight is determined at a specific point in its aiming pattern. Other maximums apply at other points in the light's aiming pattern. Compliance is determined through a specific set of test procedures.
The actual performance standards are based principally on the standards developed by the Society of Automotive Engineers (SAE). FMVSS No. 108 and the SAE standards apply to all vehicles registered in the United States, regardless of the headlamp filament or light source. Stated simply, the maximum light output of headlamp systems, whether two-or four-light systems, is limited as follows:
1. Type 2 or 2A Lights—Upper beam limited to 20,000 to 75,000 candela per lamp. Lower beam limited to 15,000 to 20,000 candela per lamp.
2. Type 1 or 1A Lights—Upper beam limited to 18,000 to 60,000 candela per lamp.
A candela is the basic unit of measure of luminous intensity in the International System of Units. Although the candela has a specific technical definition expressed in terms of a specific frequency and power, in layman's terms it approximates the light output of a common candle. A 100-watt light bulb emits about 120 candela.