What Is a Side Marker Light
A side marker light is a small lamp fitted to the side of a vehicle to make its length and width visible to other road users. Side marker lights face outward from the flanks of a vehicle, not forward or rearward, and their sole purpose is passive visibility. They tell drivers approaching from the side that a vehicle is there, how long it is, and roughly where its edges are.
Side marker lights are distinct from side repeater indicators. A side repeater flashes amber to signal a turn. A side marker light stays on continuously when the vehicle’s sidelights or headlights are active. The two serve different purposes, though some modern combined units, known as side marker lights with indicators, perform both functions in a single housing.
Typical vehicles fitted with side marker lights include agricultural trailers, flatbed trailers, HGVs, coaches, caravans, and motorhomes. Any vehicle or trailer above a certain length requires them by law in the UK, which makes side marker lights one of the most common lighting components on farms and haulage yards.
For a broader look at every lamp on a farm vehicle, see the Universal Vehicle Lighting guide.
Where Side Marker Lights Are Positioned on a Vehicle
Side marker lights sit along the flanks of a vehicle or trailer, spaced at intervals defined by UK lighting regulations. Their placement follows strict rules on distance from the front, distance from the rear, and spacing between each lamp.
Colour by Position
The colour of a side marker light depends on where it sits along the vehicle’s length:
- Front-facing side marker lights (within the front third of the vehicle): white or amber
- Middle side marker lights (along the central section): amber
- Rear-facing side marker lights (within the rear third): red
This colour coding gives other road users a quick visual reference for the direction the vehicle is travelling. A driver seeing amber side marker lights followed by a red one knows they are looking at the same side of a vehicle from front to back.
Spacing Requirements
On any vehicle or trailer longer than 6 metres, side marker lights must be fitted so that:
- The first side marker light sits no more than 4 metres from the front of the vehicle
- The last side marker light sits no more than 1 metre from the rear
- The gap between consecutive side marker lights does not exceed 3 metres (or 4 metres if the vehicle’s construction makes 3 metres impractical)
A standard 12-metre agricultural trailer typically requires 4 side marker lights per side to meet these spacing rules: 1 amber near the front, 2 amber along the middle section, and 1 red near the rear.
UK Legal Requirements for Side Marker Lights
The Road Vehicles Lighting Regulations 1989 (RVLR 1989) set out the legal framework for side marker lights in the UK. Schedule 1 of the regulations specifies which vehicles must carry them, what colour they must be, and where they must sit.
Which Vehicles Need Side Marker Lights
Side marker lights are mandatory on vehicles and trailers that exceed 6 metres in overall length. This captures most agricultural trailers, HGV trailers, coaches, and many livestock transporters. Vehicles under 6 metres are not required to have side marker lights, though fitting them voluntarily is permitted provided the lamps meet E-mark approval standards.
Exempt vehicles include certain military vehicles and vehicles manufactured before specific dates, but these exemptions rarely apply to working farm equipment built in the last 30 years.
Approval Markings
Every side marker light used on a UK road vehicle must carry an E-mark or e-mark approval. This marking confirms the lamp has passed ECE type approval testing for optical output, colour, and durability. Unmarked side marker lights from uncertified sources are not road legal, regardless of how bright or well-made they appear.
Penalties
Operating a vehicle on a public road without the required side marker lights is an offence under the RVLR 1989. Enforcement typically results in a prohibition notice requiring the defect to be corrected before the vehicle returns to the road. Repeat offences or serious deficiencies (for example, an entire side of a trailer with no working side marker lights) can attract fixed penalty notices.
For more on how lighting law applies to agricultural vehicles, see the lighting regulations section.
Side Marker Lights and the MOT Test
Side marker lights are checked during the MOT test on vehicles that are legally required to have them fitted. For cars under 6 metres, side marker lights are not part of the MOT because they are not required by law. For trailers presented for voluntary MOT testing, and for HGVs and coaches on their annual test, side marker lights are inspected.
The MOT tester checks 4 things on each side marker light:
- Presence: the correct number of side marker lights are fitted in the right positions
- Operation: each side marker light illuminates when the sidelights are switched on
- Colour: each side marker light shows the correct colour for its position (white/amber at the front, amber in the middle, red at the rear)
- Condition: the lens is not cracked, the housing is secure, and the lamp is not obscured by dirt or damage
A cracked lens that allows white light to escape where amber or red should show is a common MOT failure. Replacing a damaged side marker light before the test is straightforward and far cheaper than a retest.
LED vs Bulb Side Marker Lights
LED side marker lights last longer and draw less power than traditional incandescent alternatives. The performance gap between the two technologies is significant enough that most new trailers and commercial vehicles now leave the factory with LED side marker lights as standard.
Lifespan
An LED side marker light has a typical operating life of 30,000 to 50,000 hours. An incandescent bulb side marker light lasts 1,000 to 2,000 hours under normal conditions. On an agricultural trailer that runs sidelights for 4 hours per day during autumn and winter, an LED side marker light lasts over 20 years before needing replacement. The bulb version needs replacing every 1 to 2 seasons.
Power Draw
A single LED side marker light draws approximately 0.05A to 0.15A at 12V. An incandescent side marker light draws 0.3A to 0.5A at the same voltage. On a trailer with 8 side marker lights, the total LED draw is under 1.2A compared to 3A or more for incandescent. This lower current draw reduces load on the towing vehicle’s electrical system and minimises voltage drop over long wiring runs.
Voltage Options
Side marker lights are available in 12V and 24V versions. Most agricultural trailers, caravans, and light commercial vehicles run 12V systems. HGVs and larger plant typically use 24V. Some LED side marker lights are rated for both 12V and 24V operation (often labelled as 10 to 30V multi-voltage), which makes them suitable for trailers that may be towed by different vehicles.
Road Legality
Both LED and incandescent side marker lights are road legal in the UK, provided they carry the correct E-mark approval. Fitting an LED side marker light that lacks E-mark certification, even if it functions perfectly, makes the vehicle non-compliant.
How to Choose and Fit Side Marker Lights
Choosing the right side marker light depends on 3 factors: vehicle type, voltage system, and mounting style.
By Vehicle Type
- Trailer side marker lights: the most common application. Agricultural flatbeds, grain trailers, and livestock trailers all need side marker lights if they exceed 6 metres. Surface-mount LED units with self-adhesive or screw fixings are the standard choice.
- Side marker lights for trucks: HGVs and rigid trucks use larger, brighter side marker lights, often with integrated reflectors. 24V units are typical.
- Caravan side marker lights: caravans over 6 metres need them. Compact, low-profile LED units in white housings are popular for aesthetic reasons.
Mounting Styles
Side marker lights come in 3 main mounting configurations:
- Surface mount: the lamp body sits flat against the vehicle panel. Two screws or adhesive pads hold it in place. Best for flat-sided trailers and box bodies.
- Bracket mount: a metal or plastic bracket spaces the lamp away from the chassis rail. Used where the vehicle side is not flat or where clearance from wheels and mudguards is needed.
- Flush mount: the lamp body recesses into a hole cut in the panel, sitting flush with the surface. Common on coaches and motorhomes for a clean finish.
Wiring
Side marker lights wire into the sidelight circuit so they illuminate whenever the vehicle’s parking lights or headlights are on. On trailers, this means connecting to the appropriate pin on the towing socket (pin 4 on a 7-pin socket, or pin 4/R on a 13-pin socket). Each side marker light needs a positive feed and an earth, either through a dedicated earth wire or via the mounting bolts to the chassis.
For agricultural use, all wiring connections should be waterproofed. Heat-shrink crimp connectors or IP67-rated junction boxes prevent corrosion from rain, mud, and pressure washing. A side marker light that works perfectly in the yard but fails on the road is usually the result of a corroded earth connection.
IP Rating
For agricultural trailers and farm vehicles, choose side marker lights with a minimum IP67 rating. This rating confirms the unit is dust-tight and can withstand temporary immersion in water. Trailers that spend time in muddy fields or go through farm washdowns need this level of protection. IP69K-rated side marker lights withstand high-pressure jet washing and are the best choice for livestock trailers that require regular disinfection.
Browse side marker lights at Agri Lighting for E-marked LED and bulb options suitable for trailers, trucks, and farm vehicles.
Keeping Side Marker Lights in Working Order
A side marker light requires very little maintenance. LED units, in particular, are fit-and-forget components for most of their service life. The 3 most common issues are:
- Cracked or broken lenses: caused by impact damage from branches, loading equipment, or reversing into obstacles. Replace the entire unit rather than attempting a repair, as a damaged lens compromises the light’s colour output and waterproofing.
- Corroded connections: farm vehicles are exposed to moisture, chemicals, and salt. Check wiring terminals at the start of each autumn and reapply dielectric grease to prevent corrosion.
- Dim or flickering output on incandescent units: usually a failing bulb or a poor earth connection. LED replacements eliminate the bulb failure issue entirely.
A quick walk-around check before any road journey takes 2 minutes and catches failed side marker lights before they become a roadside prohibition or an MOT failure.
For related reading on rear-facing compliance lighting, see the guide to rear fog lights.