The operation of vehicles is prohibited if:

Explanation

In both cases; both a faulty pneumatic brake pressure gauge and a bus tyre tread of 1.6 mm prohibit operation.

In a vehicle with air brakes, the pressure gauge is the driver's only window onto the state of the air reservoir. This is exactly where air brakes differ from hydraulic ones: the pressure loss happens silently and gradually and gives no perceptible sign until the moment the pedal goes soft underfoot. With a faulty gauge, the driver only discovers on a downhill stretch or at a red light that the air was low. That is why the failure of this one instrument is enough on its own to prohibit movement.

The figure 1.6 mm is this question's classic trap. That figure is the limit for a passenger car, not a bus. The minimum tread depth for a bus is 2.0 mm, so a tread of 1.6 mm on a bus is below the legal limit and prohibited. Memorise the table precisely once: passenger car 1.6, goods vehicle 1.0, bus 2.0 and motorcycle 0.8 mm.

The logic of the stricter standard for buses is simple: greater weight means a longer stopping distance, and dozens of occupants mean greater potential casualties in a collision. Shallow tread also fails to expel water from beneath the tyre on a wet surface and sets the vehicle sliding on a film of water; in a bus that means losing control of several tonnes of mass.

• A faulty pressure gauge really is an offence, but it is an incomplete answer; choosing only this means you have overlooked the tyre tread fault. • A tread of 1.6 mm on a bus really is below the permitted limit, but it is not the only fault raised, and choosing it leaves the answer incomplete.

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