Restriction on the construction of rumble devices6.
Where a rumble device is constructed in a road—
(a)
no vertical face of any material forming that device shall exceed 6 millimetres measured vertically from top to bottom of that face; and
(b)
no part of the device shall be more than 20 millimetres above the surface of the carriageway;
provided that a measurement shall be deemed to comply if it is no more than 5 per cent or 1 millimetre, whichever is the greater, more than the corresponding measurement in this regulation.