The traffic-signal phase in which red and yellow appear together before green in the United Kingdom is the red-and-amber phase.
In the UK sequence, red is followed by red and amber together, then green. The combined display tells drivers that the signal is about to change to green, but it does not authorize them to move early. Drivers must wait until green appears and the route is clear.
This sequence differs from the common sequence in some countries, where red is followed directly by green. The UK’s red-and-amber stage provides a brief transition for preparing to move while preserving the red instruction.
Amber by itself has a different role: it normally means stop unless the vehicle has crossed the stop line or is so close that stopping might cause a collision. Signal meanings are defined by national traffic law, so drivers should not assume that every country uses identical sequences.