A 16-metre artic on an unlit road is mostly invisible from the side - a long dark gap between a lit cab and a lit tail. The lamps that close that gap are the side markers, and the ones that announce a wide vehicle’s full width are the end-outline markers. Both are defined and regulated in the Road Vehicles Lighting Regulations, and both fail trucks at the annual test when they are missing, wrong or dead.
The regulations define each lamp by its job. The side marker lamp: a lamp fitted to the side of a vehicle or its load and used to render the vehicle more visible to other road users - note ‘or its load’, because the load that overhangs the body inherits the marking duty. The end-outline marker lamp: a lamp fitted near the outer edge of a vehicle, in addition to the front and rear position lamps, to indicate the presence of a wide vehicle. Two different lamps, two different jobs: one draws the vehicle’s length, the other draws its width.
Those definitions anchor the fitment schedules: long vehicles - and trucks with trailers are the everyday case - must carry side markers along the flanks within the spacing the regulations set, and vehicles over the width threshold carry end-outline markers at the extremities, front and rear. The details live in the regulations’ tables, but the shape is simple: other road users should be able to read a truck’s dimensions from its lamps in the dark.
The annual-test and roadside failures in this corner of the regulations are rarely about trucks built wrong - they are about trucks kept wrong. The dead marker lamp: one LED out of a string of ten, easy to miss in daylight, an immediate defect in the dark - the driver’s daily walkround is the only reliable net. The trailer swap: the trailer from another fleet with a different lamp standard or damaged wiring, married to a unit whose driver assumes the lights work because the brake lights do - the coupling check is the moment to catch it. The load that outgrows the body: the overhanging or wide load that needed its own marking - the regulations’ ‘or its load’ clause makes the load part of the lamp question, not just the strapping question. And the modification: the body rebuilt, extended or converted with the lamp spacing nobody re-measured.
Side markers draw the length, end-outlines draw the width, and the Lighting Regulations require both on the trucks whose size demands them - including, where it overhangs, the load. The failures are maintenance failures: dead lamps, unchecked trailers, unmarked loads. Put the flanks on the walkround, and the rules look after themselves.
Marker lamps die quietly because nobody looks at the flanks in daylight: the brake lights get checked because someone stands behind, the markers get assumed. The fix is procedural and free: the driver’s daily check walks the full length on both sides - not a glance from the cab step - with the lamps on, and the trailer’s markers read as carefully as the unit’s. Fleets with workshops add the flank check to every PMI, where the failing LED string gets replaced before it becomes a roadside defect sheet. The examiner’s view is worth borrowing: stand fifty metres back in the yard at dusk and look at the outfit as other road users will see it - the missing marker announces itself instantly from that distance, and the exercise costs two minutes of a driver’s day against a prohibition and a test failure.
The lamps that draw the truck’s outline in the dark - side markers for length, end-outlines for width - are small, cheap and legally required. The failures are all maintenance: dead LEDs, unchecked trailers, unmarked overhangs. Walk the flanks daily, and the outline takes care of itself.
Most marker-lamp problems are trailer problems: trailers lead harder lives than units - swapped between fleets, parked in hedges, repaired by whoever was nearest - and their lamp strings show it. The disciplines that keep trailer marking legal are the coupling check (lamps proven, not assumed, every time a trailer joins), the fleet’s own trailer register with lamp condition noted at inspection, and the load question asked at every out-of-gauge job: does this load overhang the body, and if so, where are its markers? The regulations’ phrase ‘or its load’ turns the flatbed’s steel and the curtain-sider’s long pipes into lamp questions, and the answer travels with the load plan, not as an afterthought at the gate.
Markers are the cheapest compliance on the truck: the lamps cost pence, the walkround costs minutes, and the defect costs a prohibition. The fleets with clean lighting records are not lucky - they simply look at the sides of their vehicles every single day.