The Traffic Signs Regulations and General Directions 1994

Flashing beacons

42.—(1) A beacon—

(a) showing an intermittent amber light and placed in combination with a temporary sign within the meaning of regulation 41 or the sign shown in diagram 562, 610, 7001, 7009, 7010, 7012, 7013, 7019, 7020, 7021 or 7022, and in compliance with the requirements in paragraph (2); or

(b) showing an intermittent blue light and placed by a constable or a person acting under instructions (whether general or specific) of the chief officer of police in combination with a sign shown in diagram 606, 609, 610, 616, 633, 829.1, 829.2, 829.3, 829.4 or 7105, and in compliance with (c) and (d) of the requirements in paragraph (2),

shall convey the warning that drivers of vehicles should take special care.

(2) The requirements mentioned in paragraph (1) are—

(a) the peak intensity of light emitted by the lens or lenses of each such beacon shall be—

(i) if the period between individual flashes does not exceed of a second, not less than 100 candela on the principal axis of the relevant lens;

(ii) if the period between individual flashes exceeds of a second, not less than 2000 candela on the principal axis; or

(iii) if the period between the cessation of a double flash and the start of the succeeding double flash exceeds of a second, not less than 1000 candela on the principal axis;

(b) each lens shall be of such a shape and size that the perimeter of its area projected horizontally onto a vertical plane shall be capable of lying wholly inside a square having sides of 200 millimetres in length and wholly outside a square having sides of 100 millimetres in length;

(c) the height of the centre of the lenses from the surface of the carriageway in the immediate vicinity shall be not less than 800 nor more than 1500 millimetres; and

(d) the rate of flashing shall be not less than 40 nor more than 150 individual or double flashes per minute.