Two of the most common habits in driving - coasting down a hill in neutral to “save fuel”, and leaving the engine running while parked - are both addressed directly by the Highway Code, one as a safety failure and one as an outright MUST NOT. Both persist in professional driving because both feel like small efficiencies. Neither is, and for fleets they sit squarely in briefing-and-telemetry territory.
Rule 122 defines coasting as a vehicle travelling in neutral or with the clutch pressed down, and lists exactly what it costs:
Read that list at 44 tonnes. Engine braking eliminated means the service brakes inherit the entire kinetic energy of a loaded vehicle on a descent - and increased footbrake use reducing brake effectiveness is the fade sequence that ends in runaway. The gearbox is also the steering’s ally: out of gear, the vehicle’s response through bends is degraded precisely when a heavy vehicle needs its full composure.
The fuel myth deserves a direct answer, because it is the usual defence. Modern engines cut fuel delivery on the overrun in gear - a vehicle descending in gear under engine braking typically uses no fuel at all, while a coasting vehicle keeps the engine idling. The habit does not even achieve its stated purpose. What it does achieve is speed building downhill, brakes heating, and a driver one bend away from discovering what brake fade feels like in a loaded truck.
Rule 123 is two rules in one. You MUST NOT leave a parked vehicle unattended with the engine running. And you must not leave a vehicle engine running unnecessarily while stationary on a public road: generally, if the vehicle is stationary and likely to remain so for more than a couple of minutes, you should apply the parking brake and switch off the engine, to reduce emissions and noise pollution. The permitted exceptions: when the vehicle is stationary in traffic, or for diagnosing faults.
The unattended-engine limb is a security rule as much as an environmental one: a running, unattended truck on a public road is a theft invitation and an offence in itself. The unnecessary-idling limb is the depot and delivery-point issue: the cab idling through a 40-minute wait at a goods-in bay, the engine left on while the driver does the paperwork on the customer’s kerb. Both burn fuel, both generate exactly the local emissions and noise that clean air zones and angry neighbours exist about, and both are addressable by policy and habit.
Note the scope: the idling rule bites on public roads. Your own yard is yours - though the fuel arithmetic and the environmental duty argue for the same discipline everywhere.
The professional alternative to coasting is not complicated, but it is systematic: select the right gear before the descent, not during it; let the engine and any retarder or exhaust brake hold the speed; keep the service brakes for trimming rather than sustaining; and if speed does build despite all of that, brake firmly and early to bring it back under control rather than riding the pedal. Descent discipline is a trained skill, and it is the exact inverse of the coasting habit - control retained through the drivetrain instead of control gambled on hot brakes.
Two habits, two rules, and a measurable saving on the far side of both. Coasting and idling are the kind of compliance that pays for itself in brake linings and diesel before it ever meets an examiner.