This road sign:

Explanation

It prohibits the movement of tractors and self-propelled machines.

The sign is a circle with a red border and a black image of a tractor in the middle. In the family of prohibitory signs the reading rule is simple: a red circle with the image of a particular vehicle prohibits the movement only of that class, and all other vehicles are free. So this sign closes the road to tractors and self-propelled machines and imposes no restriction on cars and lorries.

The safety reason is the difference in speed. Tractors and self-propelled machinery move far more slowly than the general flow; on arterial streets, expressways and bridges that speed difference creates queues and risky overtaking and raises the likelihood of rear-end collisions. In addition, steel and tracked wheels damage the road surface and bridge decks, and mud and material spilled from these machines make the road surface slippery.

Worth memorising: read the image inside the circle as "the very thing that is prohibited" and avoid the inverted logic ("everything prohibited except this"); that logic applies only to signs qualified by a supplementary plate or an express exception.

• This description belongs to a different sign bearing the image of a car, which closes the road to all motor vehicles; here the image inside the circle is a tractor, so the scope of the prohibition is the same. • It inverts the logic: the sign does not except tractors but makes them precisely the target of the prohibition.

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