White paint on the carriageway is the cheapest and most-read safety system on the road. For professional drivers, the white-line rules are doubly significant: crossings by large vehicles are disproportionately dangerous, the offences are camera- and report-detectable, and a driver who reads the markings properly gets advance warning of hazards before any sign or telematics alert. The Highway Code’s rules 127 to 132 are short; the detail in them is the detail that matters.
Rule 127: a broken white line marks the centre of the road - and when this line lengthens and the gaps shorten, it means there is a hazard ahead. That is the single most useful piece of road-reading in the Code. The paint is telling you the view is closing down: a bend, a crest, a junction, a hidden dip. The professional response is automatic - hazard lines ahead means lift, cover the brake, and reconsider any overtake. The same rule permits crossing the line to overtake or turn off only when you can see the road is clear: in a vehicle with a long wheelbase and a slow overtaking profile, “can see it is clear” is a longer demand than in a car.
Rule 128 covers double white lines where the line nearer to you is broken: you may cross to overtake if it is safe, provided you can complete the manoeuvre before reaching a solid white line on your side. Arrows painted on the road will tell you to get back on your side. The discipline is the completion window - start an overtake you cannot finish inside the broken section and you arrive, mid-manoeuvre, at a solid line it is now illegal to cross.
Rule 129 is the hard one: where the line nearer to you is solid, you MUST NOT cross or straddle it. Two exceptions only. First: to enter adjoining premises or a side road, if it is safe. Second: to pass a stationary vehicle, or to overtake a pedal cycle, horse or road maintenance vehicle travelling at 10 mph or less, provided the road is clear.
For truck and bus drivers the practical notes are these. “Straddle” is as prohibited as cross - running the nearside wheels on the line to give a parked car clearance is an offence unless an exception applies. The slow-road-user exception is speed-qualified: the cycle, horse or maintenance vehicle must be doing 10 mph or less. A cyclist moving at 12 mph is not within the exception - you wait behind them until the line breaks or the road ahead is clear enough to cross legally. That is uncomfortable with a queue behind, but it is the rule, and it is increasingly camera-enforced.
Rule 130: white diagonal stripes or chevrons separate traffic lanes or protect traffic turning right. Bordered by a broken white line, you should not enter unless necessary and you can see it is safe. Marked with chevrons and bordered by solid white lines, you MUST NOT enter except in an emergency. The chevroned solid-border areas are safety space, not spare lane - the hatched box ahead of a slip-road split, the protected right-turn pocket. In a wide vehicle it is tempting to borrow the hatched area on the approach to a junction; the rule leaves no room for it.
Rule 131: lane dividers - short broken white lines on wide carriageways - mean you should keep between them. For long vehicles this is an articulation discipline: the trailer tracks tighter than the tractor, and keeping the combination within the lane through bends is a trained skill, not a default.
Rule 132 completes the system with reflective road studs: white studs mark the lanes or the middle of the road; red studs the left edge; amber studs the central reservation on dual carriageways and motorways; green studs the edge at lay-bys and slip roads; and green/yellow studs indicate temporary lane layout changes, such as at road works. Night trunking, these studs are the road map in front of your headlights - the colour change at a slip road, the amber wall of the central reservation, the red line you should never be outside.
White-line offences by liveried vehicles generate public reports with photo evidence, and fixed penalties attach to the driver record that feeds your OCRS picture. Beyond enforcement, the markings are free safety information: the driver who reads lengthening hazard lines and slows before the bend is the driver who does not feature in your incident statistics. Put the system into induction, and the paint starts working for you.