Last reviewed 27 March 2023

In this article, John Bullough and Mark Rea review some basic reasons for headlamp glare and foretell a brighter future when heavy goods vehicle drivers will see better at night and glare to oncoming drivers will become a thing of the past.

There seems little doubt that the driving public has become increasingly bothered by glare from vehicle headlamps. At least some of the complaints stem from developments in light source technologies over recent decades, from halogen filament to high-intensity discharge (HID) lamps, and now to light-emitting diodes (LEDs). But new light source technology is not entirely to blame. Mis-aimed headlights, particularly those with high mounting locations as seen on HGVs, are the primary culprit.

Dual functions of headlights

Headlamps are crucial safety elements on all vehicles, including heavy trucks. Many roads are not illuminated, so headlamps often provide drivers with the only available source of light for maintaining lane position and avoiding obstacles, pedestrians or other hazards. Conventional headlamps have two functions, a "low" or "passing" beam and a "high" or "driving" beam. The latter is designed for unlit roads without the presence of other traffic. High beam headlamps project high levels of light in a forward direction that would be very glaring to approaching motorists.

Low beams are designed to be used when other vehicles are present. They employ a sharp horizontal "cut-off" line above which there is little light, thereby limiting glare to oncoming traffic. Intensity is also reduced toward the roadway centre line on the right, in the direction of approaching traffic in the UK. Thus, to limit glare, most of the light from low beam headlamps falls within a "hot spot" below the cut-off line and slightly to the left. This design helps drivers see the roadway edge but does not provide the same visibility as high beams, which is especially important at high speeds (>60 km/h).

Evolving technology

For many decades, the reference condition for headlamp glare has been the halogen headlamp, which uses a filament to create warm-white (or "yellowish") light. In the 1990s, cool-white (or "bluish") HID headlamps (also referred to as xenon headlamps) were introduced. HID headlamps were more efficient than halogen headlamps, allowing greater light output while using less power. This "extra" output was usually directed to peripheral angles, presumably to improve visibility without causing glare to oncoming drivers. More recently, LED sources have overtaken both halogen and HID headlamp technology because they are even more efficient and, as an additional advantage, more durable.

Like HID headlamps, most present-day LED headlamps appear cool-white, but unlike HID headlamps they can be manufactured to appear any colour. Probably, most LED headlamps are cool-white rather than warm-white because studies have shown that roads illuminated by a cool-white source appear brighter than when illuminated by a warm-white source at the same light level. But the flip side of increased brightness of the road for drivers is that LED headlamps themselves will also appear brighter to oncoming drivers, thereby creating greater discomfort glare.

Aim — the critical factor

Although real and unaccounted for in certification and testing, the colour tint of headlamps, whether warm or cool, plays a relatively small role in causing glare. The much larger practical issue is headlight aim.

Aim is measured as a percentage, which means that if a vehicle is parked on a flat surface 10m from a vertical wall and the low beam cut-off line is located 10cm lower than the mounting height of the headlamp, the aim is described as 1% down because 10cm is 1% of 10m. In recognition of the interaction between mounting height and aim, the range of permissible aim values for heavy trucks is 0.5–3.25% down for mounting heights of 85cm or less, and 1.25–4% down for mounting heights greater than 85cm. The Vehicle & Operator Services Agency publishes guidance to drivers and fleet managers for checking and adjusting aim on heavy trucks (; search for "headlamp").

As already mentioned, low beam headlamps are designed to limit glare to oncoming drivers, directing light at an angle slightly below a line parallel to the surface of the road ahead. Glare experienced by an oncoming driver will occur, however, if a headlamp is inadvertently misaimed to be more parallel to the roadway surface. In fact, just 1° of upward mis-aim can shift a low beam pattern's hot spot from the roadway surface onto the windscreen of an oncoming driver. Even lesser upward mis-aim produces the same result when the headlamps are mounted higher, as they can be on an HGV. Poorly aimed and high-mounted headlamps can create a perfect storm of glare that some might attribute to the apparently brighter cool-white tint of the LED lighting technology. In other words, because that tint is so obvious relative to the halogen reference condition, some people blame the LED technology rather than the root cause which is, naturally, much harder to identify.

As a side note, downward mis-aim can also pose a safety problem because it restricts a driver’s forward visibility. A recent study of headlamp aim in the US revealed that most vehicles on the road had at least one headlamp mis-aimed, either up or down. Even a sizeable proportion of new vehicles at dealerships had a mis-aimed headlamp. The good news is that in the UK, HGVs are required to be checked for headlamp aim annually, so oncoming drivers are less likely to encounter mis-aimed truck headlights.

Help is on the way

An even newer headlight system technology promises to mitigate glare regardless of differences in colour tint, mounting height, or headlamp aim. In the past decade, adaptive driving beam (ADB) systems have been introduced; these systems always provide high beam lighting on and along the roadway, but never on the windscreen or mirrors of other vehicles. ADB systems employing cameras and software dynamically control the headlamps to maintain reduced light intensities toward approaching and preceding vehicles, eliminating the problem of mis-aimed headlamps. These new headlight systems are most commonly found on high-end passenger vehicles, but they can and should be utilised on HGVs as well, both for mitigating glare and improving the visibility of hazards for drivers.

ADB systems can go a long way to help drivers keep on trucking.