Operation of buses is prohibited if:

Explanation

When the residual tread depth of the bus tyres is 1.6 mm.

The minimum permitted tread depth for a bus is 2.0 mm, so 1.6 mm falls below the legal limit and prohibits operation. The question's trap lies in the familiarity of that very figure: 1.6 is the limit for a passenger car and the mind immediately recognises it as "the legal figure". But the criterion is always tied to the vehicle type — passenger car 1.6, goods vehicle 1.0, bus 2.0 and motorcycle 0.8 mm.

Why is it stricter for a bus? Because tread depth determines how much water the tyre expels from beneath itself. With shallow tread, a film of water remains between the tyre and the asphalt and the vehicle effectively floats on water; at that moment neither the brakes nor the steering responds. A bus, with several tonnes of mass and a high centre of gravity, both has a longer stopping distance and, if it goes out of control, endangers dozens of occupants. The wider safety margin is the legislator's answer to these two facts.

Know the other option too: the coolant temperature gauge is a useful tool for looking after the engine, but it is not on the list of defects prohibiting a vehicle from moving. In this chapter, only the absence of a sound pressure gauge in an air brake system counts as a vital instrument forming a ground for prohibition.

• A faulty coolant temperature gauge is not on the list of defects prohibiting operation; unlike the air brake pressure gauge, this instrument is not a ground for taking the vehicle off the road. • Since the temperature gauge fault is not a ground for prohibition, "in both cases" is wrong and only the tread depth matters.

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