Ask a workshop to name the lamps that matter on a truck and they will talk about headlights and brake lights. Ask an examiner the same question and the direction indicators come up fast, because they are the lamps other road users actually plan around. The rules behind them are more precise than most drivers realise, and they sit in two places: the Road Vehicles Lighting Regulations 1989 for the hardware, and the Highway Code for the hand on the stalk.
The Lighting Regulations do not leave the meaning fuzzy. A direction indicator is defined as a lamp on a vehicle used to indicate to other road users that the driver intends to change direction to the right or to the left. That definition does real work: because the lamp has a defined legal purpose, anything fitted in that position has to do that job and nothing confusing alongside it.
The same regulations define the hazard warning signal device as a device capable of causing all the direction indicators with which a vehicle, or a combination of vehicles, is fitted to operate simultaneously. Two things follow. The hazard function is legally built out of the indicators, not a separate system, so a truck with a dead indicator on one corner has a compromised hazard display as well. And on a combination, the unit and trailer are treated together, which is why a trailer connection fault that kills the trailer’s indicators is such a serious defect.
The regulations are specific about colour: amber light is the light of the direction indicator and the side marker lamp. That is why fitting clear lenses with odd-coloured bulbs, or cheap LED units that read white-ish in daylight, is not a styling choice but a compliance problem. If the lamp does not read as a proper amber indicator, the vehicle is not showing the signal the regulations describe.
Visibility is the other half. The rules for trailers carry a telling exemption: an older trailer need not carry its own stop lamp or direction indicator only while it is drawn by a vehicle whose corresponding lamps are visible to an observer six metres behind the trailer, loaded or not. The principle underneath that exemption is the rule for everything newer: the signals at the back of the outfit must be plainly visible from behind, whatever the trailer is carrying. A load that masks the trailer’s lamp cluster takes the whole combination out of that comfort zone.
There are also baseline fitment expectations. The regulations’ own tables show how seriously the law takes the device: even invalid carriages capable of more than 4 mph must be fitted with direction indicators and a hazard warning signal device. The smallest, slowest vehicles on the road are expected to signal properly. A 44-tonne artic hardly gets to argue.
Indicators fail quietly. A headlamp out announces itself to the driver on a dark morning; an indicator bulb out is invisible from the cab unless the dash tells the story, and on older dashboards the faster flash is the only clue. That is why the lights check in the daily walkaround matters, and why it has to include someone or something reflecting the rear of the outfit. The guide to maintaining roadworthiness treats lights as a core daily check item, and indicators are the lamps most likely to have died since yesterday.
The trailer swap is the classic trap. The unit’s indicators worked at the first-timer check, the trailer was dropped and another collected at midday, and the new trailer’s near-side repeater has been dead since last Tuesday. Nobody owns the defect until the driver does the tug-and-lights check on the new combination. The driver who skips it owns it instead.
The hardware being legal is only half the signalling story. The Highway Code’s signalling rules put the duty on the driver in plain terms: signals warn and inform other road users of what you are about to do, so give them clearly and in good time, and cancel the indicator when the manoeuvre is complete. An indicator left running down the motorway after a lane change is not a harmless quirk; it is a false signal telling every following driver you are about to move again.
Good time matters more in a long vehicle. The car behind needs the signal early enough to understand that the artic swinging left will first move right, and the cyclist alongside needs it before the turn begins, not during. The Code also reminds drivers to watch for the signals others give, and in a truck that means remembering how much of the following traffic can even see your repeater lamps.
Three habits cover most of it. First, the walkaround lights check, every driving day, on the actual combination about to move. Second, the mirror check after every signal: confirm the lamp has cancelled, because a self-cancelling stalk worn with age will happily lie to you. Third, treat the hazard device as the indicator system working all at once, because that is what it legally is; if the hazards look wrong, the indicators are wrong somewhere.
Indicators are cheap lamps carrying a heavy legal definition. They tell the world what a 44-tonne vehicle is about to do, and both the regulations and the Highway Code treat that promise seriously. The fleets that never think about them are the fleets whose drivers check, cancel and replace as a matter of routine.