Glare is an interference with visual perception caused by an uncomfortably bright light source
or reflection; a form of visual noise. 
 
There is a difference between Discomfort Glare and Disability Glare. Discomfort Glare causes discomfort, annoyance, fatigue and pain. Disability glare produces a reduction in the visibility distance of low-contrast objects.

There are many different sources of glare on American roads.  Misaimed headlamps, high-mounted headlamps, illegal out-of-spec (high wattage) bulbs, improper use of high beam and auxiliary forward , weather conditions and non-compliant HID retrofits are believed to be significant contributors to the problem.

Recent research indicates that headlamp aiming may be a much greater glare factor than HID, auxiliary lights and even higher headlamp mounting.  According to the AAA Foundation’s report, Countermeasures for Reducing the Effects of Headlight Glare (2002), as many as 50 percent of all headlamps on the road, or 110 million vehicles, may be misaimed.  Shock, vibration and wear and tear are the greatest contributors to headlamp misalignment.