From ‘The Highways (Road Humps) Regulations 1999’:-

(a) each face of it across the carriageway of the highway in which it is
constructed is at right angles to an imaginary line along the centre of
that carriageway;
(b) it has a minimum length of 900 millimetres measured parallel to an imaginary line along the centre of that carriageway from the point where one face meets the surface of that carriageway to the point where the other face meets the surface of that carriageway;
(c) the highest point on it is not less than 25 millimetres nor more than
100 millimetres higher than an imaginary line parallel to the centre line of that carriageway connecting the surface of that carriageway on one side of the road hump to the surface of that carriageway on the other side of the road hump and passing vertically below that point; and,
(d) no vertical face of any material forming part of that road hump
exceeds 6 millimetres measured vertically from top to bottom of that face.

But there is no width restriction on speed cushions. So I have been
measuring them. In my area they are between 1,800mm and 2,000mm wide and the tops are around 1,200mm wide, giving chamfered edges of between 300mm and 400mm each side. That means that the tyres of virtually every car in the UK will be squeezed downwards and outwards if the driver traverses the humps by straddling them. And that explains the considerable damage that is occurring to the inner shoulders of car tyres, as can be confirmed by ever tyre fitter in the land.