Tunnel vision

Using a circle theorem

Triangle ABP is right-angled at P, because a triangle in a semicircle is always right-angled. 

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Both of the two smaller right-angled triangles share an angle with triangle ABP, so the three triangles are all similar.

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The scale factor between the yellow triangle and the red triangle is 3, so the horizontal side of the red triangle is 6$\times$3=18 m. So the total width of the tunnel is 20 m.

Using Pythagoras' Theorem

OP and OB are both radii, labelled $r$, so the distance from O to the far wheel of the lorry is $r-2.$

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Applying Pythagoras' theorem to the right-angled triangle formed, $$\begin{array}{rl}{r}^2=& (r-2{)}^2+6^2\\ \Rightarrow r^2=& r^2-4r+40\\ \Rightarrow 0=& -4r+40\\ \Rightarrow 4r=& 40\\ \Rightarrow r=& 10\end{array}$$ So the width of the tunnel is $20$ m.