Motorway signals (see ‘Light signals controlling traffic’) are used to warn you of a danger ahead. For example, there may be an incident, fog, a spillage or road workers on the carriageway which you may not immediately be able to see.
Signals situated on the central reservation apply to all lanes. On very busy stretches, signals may be overhead with a separate signal for each lane.
Amber flashing lights. These warn of a hazard ahead. The signal may show a temporary maximum speed limit, lanes that are closed or a message such as ‘Fog’. Adjust your speed and look out for the danger until you pass a signal which is not flashing or one that gives the ‘All clear’ sign and you are sure it is safe to increase your speed.
Red flashing lights. If red lights on the overhead signals flash above your lane and a red ‘X’ is showing, you MUST NOT go beyond the signal in that lane. If red lights flash on a signal in the central reservation or at the side of the road, you MUST NOT go beyond the signal in any lane.
This applies until you pass another signal indicating that the lane is no longer closed, by displaying the word ‘End’ or a speed limit sign and you are sure that it is safe to proceed.
Laws RTA 1988 sect 36 & TSRGD regs 10 & 38