Night driving and the headlights coming the other way

Take one ordinary drive at night, from a lit town out onto an unlit road and back again, and watch the lighting decisions in the order they arrive. Nothing goes wrong. What is worth noticing is that at three separate points the driver is working with less information than the road is asking for, and only one of them feels like anything at all.

The journey has no particulars, because the particulars are the part that would have to be invented — no named road, no vehicle, no hour, no speed, no outcome. What it has instead is instructions, and every instruction in it is a quotation from a road authority: the United Kingdom’s Highway Code, Spain’s Dirección General de Tráfico, and the United Nations regulation that European headlamps are approved against.

Leaving the lit town

The first decision is already made, and it is made by law. The Highway Code’s lighting rule is a MUST: sidelights and rear registration plate lights lit between sunset and sunrise, headlights at night except on a road which has lit street lighting. The next rule says what kind of headlight the town gets — dipped headlights, or dim-dip if fitted, at night in built-up areas.

The DGT explains why dipped beam feels like so little light. Its description of the beam is short, wide and asymmetric, aimed towards one side of the carriageway in order to light that side more and not dazzle the driver coming the other way. The near edge of the road is lit because the beam was designed to give up the oncoming lane.

The unlit road

The street lights end, and the rule changes with them. The DGT’s account of when main beam belongs is a road whose lighting is insufficient — its working test is not being able to read a number plate at ten metres or make out a dark vehicle at fifty — at more than 40 km/h, and only where there is no risk of dazzling anyone.

This is also where the rule that governs night driving applies, and both authorities state it in one sentence. The Highway Code: drive at a speed that will allow you to stop well within the distance you can see to be clear. The DGT: set your speed so that you can brake and stop the vehicle within the lit space. Everything else here is a special case of that sentence.

The demanding word in it is see, not illuminate. The Highway Code makes corrective lenses a MUST for a driver who needs them to read a number plate in good daylight at twenty metres, and adds a rule for this stretch of road: at night or in poor visibility, do not use tinted glasses, lenses or visors if they restrict your vision.

The first oncoming car

A pair of lights appears a long way off, and the driver dips before it matters.

Both codes make this obligatory rather than courteous. The Highway Code’s rule is a MUST NOT: do not use any lights in a way which would dazzle or cause discomfort to other road users. The DGT requires the change to dipped beam when there is a foreseeable risk of dazzling, and settles the one case where the geometry is unfair — on a bend, the driver taking it on the inside should switch first, because their beam sweeps across the other’s eyes before the other’s sweeps across theirs. The Highway Code carries the same logic into an overtake: keep your headlights dipped until you are level with the other vehicle, then change to main beam if necessary, unless this would dazzle oncoming road users.

Dipping costs the dipping driver sight distance at the moment sight distance is scarcest. Neither authority discusses that trade. What they do instead is make the duty absolute and leave the compensation to the speed rule above.

The car that does not dip

Now the one this article is named for. A car comes the other way on main beam and stays on it.

The Highway Code’s instruction carries no exception: slow down, and if necessary stop, if you are dazzled by oncoming headlights. It is the same instruction, in the same words, that the Code gives for being dazzled by bright sunlight. The cause is treated as irrelevant — the response is not a judgement about the other driver.

The DGT supplies the part the Highway Code leaves out, which is where to put your eyes. Avoid looking directly at the light source and use the edge line of the carriageway as a guide, it says — the line on your own side, the side your dipped beam was aimed at in the first place, still lit while the middle of the picture is not. Its night-driving page puts it more generally: direct your vision to the least-lit part of the road, while reducing speed at the same time. Two authorities, written independently in different languages, reach the same instruction: eyes to the near edge, speed down, stop if that is what it takes.

What neither of them offers is retaliation. The Highway Code allows only one use of a headlight flash — to let other road users know that you are there — and says not to flash to convey any other message or to intimidate other road users. It follows this with the mirror-image warning for when you are on the receiving end: never assume that a flash is a signal inviting you to proceed.

The cyclist

Past the oncoming car, at the edge of the beam, a cyclist.

What that cyclist is required to be carrying is specific. The Highway Code: at night a cycle MUST have white front and red rear lights lit, and MUST be fitted with a red rear reflector, and with amber pedal reflectors if it was made after 1 October 1985. Spain’s cycling guidance requires the same white front and red rear position lights plus a non-triangular red rear reflector, and adds a reflective garment on interurban roads whenever lights are compulsory. Its one-line summary is hard to improve on: as important as seeing is being seen.

The pedestrian equivalent exists too. The DGT requires a pedestrian walking on the carriageway or the shoulder to carry an approved luminous or retro-reflective element, and recommends light-coloured clothing. The Highway Code tells a walker without a pavement which side to keep to so as to face oncoming traffic; the two codes name opposite sides, because traffic drives on opposite sides — a useful reminder that a rule of this kind is local by construction.

Here is the driver’s half, and it is why any of this is in a driving article. The DGT publishes one distance figure: a person wearing a reflective vest is visible at 150 metres. The Highway Code says reflective materials can be seen by drivers using headlights up to three times as far away as non-reflective materials. You cannot divide one by the other — different authorities, different things measured, and the quotient would look like a measurement without being one. What you can do is notice which side of the arithmetic you are on, because the person wearing nothing reflective is carrying none of that distance, and the distance is the only thing that was ever going to help.

The car that dips for you

The beam may have been dropping and lifting by itself all along. UN Regulation No. 48 defines an adaptive main beam as one that adapts its pattern to the presence of oncoming and preceding vehicles in order to improve long-range visibility for the driver without causing discomfort, distraction or glare to other road users. It then states what the sensor system must find: an oncoming motor vehicle at a distance extending to at least 400 metres, a preceding vehicle or vehicle-trailer combination at at least 100 metres, and an oncoming bicycle at at least 75 metres.

Read that list again for what is not in it. A pedestrian is not a detection requirement. The system is specified against things that carry lamps, and the cyclist is in the list precisely because a cycle at night is required to carry them. A person walking on an unlit verge carries whatever they happened to put on.

The regulation’s own answer to that is the stalk. It requires that it shall always be possible to switch the main-beam headlamps on and off manually and to switch the automatic control off manually — by a simple and immediate operation, with sub-menus not allowed. A requirement drafted that tightly exists because the driver is expected to use it.

Arriving

The street lights come back, the beam goes to dipped, and the drive ends without a story.

The section’s crash simulator, opened on a night pedestrian crossing mid-block, is where the sentence at the centre of this page turns into a number. Stopping within the distance you can see to be clear is an arithmetic claim: what you can see must exceed reaction distance plus braking distance. Two of the simulator’s published constants are worth holding in mind while you move the speed slider — a perception and reaction time of about 1.25 seconds for an unexpected hazard, and a friction coefficient of 0.7 for dry asphalt — because the tool cites both to their sources and neither improves because it is dark. Change only the speed, and read off the distance. Then ask whether your headlights were showing you that much.

What we cannot tell you

We cannot tell you how far your dipped beam reaches. No authority publishes a figure for a car: it depends on the lamp, its aim, the state of the lens and the road under it.

We cannot tell you at what distance a person in dark clothing becomes visible. We looked: the DGT publishes the figure for someone wearing a reflective vest and no comparison beside it, and the Highway Code publishes a multiple without a baseline. Two halves that do not join.

We cannot quote a manufacturer on its own automatic high beam, because the handbooks we tried refused the request, and we will not report what we did not read. The same happened to the United States rulemaking on adaptive driving beams, which is why the regulation quoted above is the United Nations one.

And we cannot give the exact legal wording that binds you, because the lighting rule differs by market and is stated in each market’s own traffic code. What does not differ is the instruction the two codes we read gave in the same words: when the lights coming the other way take your sight away, slow down, look to the near edge, and stop if you have to.