Operation of buses is prohibited if:

Explanation

When the tyres have local damage exposing the cord layer.

A cut, split or local tear that exposes the tyre's cord prohibits operation of the vehicle regardless of tread depth. The cord is the tyre's skeleton and is what carries the internal air pressure and the vehicle's weight. Once the rubber over it has been torn and the cord is exposed to air and moisture, the strands corrode and part; the result is a sudden blowout, not a slow puncture.

The difference between the two is vital for a bus. A gradual puncture gives the driver time to slow down and pull over, but a blowout at speed pulls the steering to one side in a fraction of a second. In a vehicle with a high centre of gravity and dozens of occupants, that sudden pull can end in a rollover. That is why the law defines no tolerance here: an exposed cord means the tyre must be replaced, even if it was recently bought.

The exam point is that tyre damage and tread depth are two separate criteria. A tyre with 5 mm of tread but an exposed cord is just as unlawful as a bald one. By contrast, uneven engine running, however annoying, is not on the list of defects prohibiting operation and should not mislead you.

• Uneven engine running is an annoying technical problem, but it is not on the list of defects that prohibit a vehicle from moving. • Since uneven engine running is not a ground for prohibition, "in both cases" is wrong; only damage exposing the cord matters.

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