What are these rubber horizontal blocks in the tyre?

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These small horizontal rubber blocks show us the 1.6mm legal tread depth of the tyre. When the rest of the tyre tread is level with these markers it tells us your vehicles tyres are on the legal limit and that your tyres now need changing.

Although this is the minimum legal limit for the tyre it is advisable to change the tyres before you hit the minimum legal limit as the less tread depth on the tyre the less water can be dispersed from the tread which then reduces braking distances, grip and can cause aquaplaning in wet conditions.