In 1940, the American auto industry standardized on the 7-inch circular sealed beam headlight. There were to be two of these hermetically sealed lens-reflector-bulb modules on each vehicle, a system that quickly became enshrined in Federal law as the only type allowed. This standardization solved some significant problems with the previous system of unsealed, non-standardized headlights — some used specialized reflectors, lenses, and bulbs that had no way to be aimed other than a wild guess — but it also put a hard stop to progress in automotive lighting in North America. Technological advancements in the field remained off-limits to American drivers for years — decades — after they became standard equipment for drivers in Europe and elsewhere. The first automotive halogen headlight bulb, for example, was released in 1962 after having been cooperatively developed by a working group of French, German, and Italian bulb and headlight makers. The headlights were filled with a halogen gas such as iodine or bromine and the inert gas let the filament run hotter, making the headlights more powerful, efficient, and compact than standard units. But they were banned in the U.S., where non-halogen sealed beam headlights (produced almost exclusively by a small number of American makers) remained the only type permitted until 1979. That's when halogenized sealed beams became available, but American drivers still couldn't see any better with the improved bulbs; automakers chose instead to use the extra halogen efficiency to reduce the headlights' wattage while keeping the beam performance above the legal minimum, enabling the use of cheaper, thinner wires and lighter-duty headlight switches.

In the early 1980s, the principles of aerodynamics were making car designs ever more slippery, and pressure was being brought on U.S. regulators to allow a headlight system more aerodynamic than the chunky, upright round or rectangular sealed beams. When regulators expressed receptiveness to the idea of changing the headlight laws, driving enthusiasts' and safety advocates' hopes were raised that American headlights would catch up to the state of the art in Europe. Instead, Ford devised a system of low-cost plastic aerodynamic composite headlights with the same beam performance as the down-rated halogen sealed beams, and lobbied successfully for the system's adoption as the new Federal standard. The 1984 Lincoln Mark VII was the first car offered on the American market since 1939 with replaceable-bulb headlights, and most of the world's automakers quickly provided this type of headlight on the models they sold in North America. These were frequently called "Euro headlights," since they looked like the model-specific headlights that had been common in Europe for many years. But the term wasn't accurate, since these new American-market headlights still produced the same beam pattern as the old sealed beams.

To this day, there remain two distinctly different headlight technical regulations globally: The American SAE (or "DOT") standard, which is mandatory in the US and allowed in Canada and Mexico, and the European ECE (or "E-code") standard, which is required or allowed in most every other country around the world. The main difference between the two standards is in the level of priority placed on controlling glare to other drivers on the low beam setting. The American standard permits a relatively high amount of glare from low beams and allows a fuzzy, gradual transition from bright to dark as the beam is cast down the road. The European standard strictly controls low-beam glare, and requires a sharp "cutoff" line at the top edge of the low-beam light pattern. Above the cutoff is the dark glare-control zone, and below the cutoff is the bright seeing-light zone.

That cutoff used to be horizontal all the way across the top of the beam, but in 1952 European manufacturers altered the shape of the cutoff to extend the low beams' distance reach: the cutoff stayed horizontal on the side of the beam facing oncoming traffic, but the cutoff on the curb side of the beam was angled or stepped upwards to throw light well down the road, illuminating road signs and pedestrians without glaring oncoming drivers. In addition to keeping glare to a rigidly controlled low level, the cutoff allows the headlights to be accurately aimed visually. The beams can be shone at a wall and set to the correct vertical and horizontal angles by judging the height of the cutoff and placement of the upstep. Traditional American beams don't have a cutoff, so they can't be aimed properly without the use of special aiming machinery.

This machinery required a standardized mounting pad to mate up to the headlamp — usually three protuberant nubbins built into the face of the headlights. As headlamps became thinner and canted backward into the bodywork, there wasn't enough room to add the locating nubs, so automakers began installing bubble levels on their headlamps. That added expense, and cost-averse automakers wanted to eliminate the aiming machine interface entirely. In 1997, American regulations were amended to permit headlights to produce low beams with a cutoff at the top. But it's not necessarily the same cutoff used in the international ECE standard; the US low-beam cutoff can be staggered like the current-day European beam, or it can be flat all the way across from left to right like a pre-1952 European beam. It can be on the left or on the right side of the beam. It can be narrow or wide, and it may or may not have an upsweep or upstep like a European beam.

So in America, low beam cutoffs are a relatively recent phenomenon and are not standardized across all headlights. As a result, some drivers find the cutoff difficult to get used to after years of experience with headlight that gradually faded into the distance. The sharp transition from light to dark created the sensation of short seeing distance — even though the visible depth is the same between headlights with and without a cutoff. The visible cutoff on the road surface also provides the driver with real-time, dynamic visual feedback of the available seeing range: "Here's where your light beam ends. If you can't stop in time, slow down!" That feedback isn't provided without a cutoff, so Americans have tended for years to overdrive the reach of their low beams more frequently and more severely than drivers in the rest of the world.

The differences in aim don't end with the single ECE vs. multiple U.S. cutoffs; American regulations call for all headlights to be aimed to the same angle no matter whether they're mounted down low on a Honda Civic or up high on a lifted Hummer. Under this system, vehicles with high-mounted headlights give their drivers a long seeing distance advantage, but headlights mounted up high produce a great deal of glare for drivers of lower cars. In the ECE system, beam aim angle depends on headlight mounting height; the higher the headlights are mounted, the lower they're to be aimed. This way, all drivers have roughly equal seeing distance and all vehicles produce roughly the same level of glare. In addition, ECE regulations require headlights with particularly intense bulbs — xenons and high-power halogens — to be equipped with automatic self-leveling systems that control the lamps' aim under all conditions. The beams are prevented from rising and zapping other drivers with glare when the vehicle is heavily loaded, suddenly accelerated, or driven over a rise in the road. They're also prevented from dipping and cutting short the driver's seeing distance under hard braking. Even the most powerful headlights are not required to have leveling systems under American regulations.

The controversy between the U.S. and every other developed nation isn't limited to low beams. Internationally, high beams are allowed to be twice as intense as they are in the U.S., where regulators point to tests made using outdated technology in 1975, with small sealed beams, as proof that more-intense high beams are neither technically feasible nor beneficial. And international ECE regulations require rear turn signals to emit amber light so other drivers can quickly discern them from brake lamps, while U.S. regulations permit rear signals to emit amber or red light — in fact, the brake lamp itself on either side of the car can be flashed to serve as a rear turn signal in the U.S. Since 1968, American regulations have required every vehicle to have amber front and red rear sidemarker lights and reflectors to provide side-on visibility at night; ECE regulations don't require sidemarkers, but vehicles have to have side turn signal "repeater" blinkers so drivers, bicyclists, and pedestrians can see the turn signal from the side.

There's room in both the American and international standards for very good and very bad lighting performance. Each of these two lighting systems has its advocates, and most of them claim their favorite system is superior. American makers who favor the SAE/DOT system say it is cheaper to comply with, gives drivers longer low-beam seeing distance and vehicles better side visibility, and contend that the ECE beams don't throw enough light upward to make overhead highway signs legible at night. Internationally, automakers that prefer the ECE system object to the high level of low-beam glare, relatively weak high-beam intensity, proliferation of aim procedures, red rear turn signals, and lack of aim control and sideward turn signal visibility in the SAE system.

As automakers scramble to produce cost-saving global cars with cost-saving global parts, it's not unthinkable that American regulators could be pressured to accept the system used by the other 181 countries in the world that use the European system. We'll look for the revised headlights to show up on the same cargo ships that contain our shipments of kilometer speedometers.