The signs are advisable but the actual ASL is enforcable as motorists have to stop at the first stop line. Rule
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Not quite and thereby comes the difficulty in enforcement.
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Advanced stop lines. Some signal-controlled junctions have advanced stop lines to allow cycles to be positioned ahead of other traffic. Motorists, including motorcyclists,
MUST stop at the first white line reached if the lights are amber or red and should avoid blocking the way or encroaching on the marked area at other times, e.g. if the junction ahead is blocked. If your vehicle has proceeded over the first white line at the time that the signal goes red, you
MUST stop at the second white line, even if your vehicle is in the marked area. Allow cyclists time and space to move off when the green signal shows.
The problem being that unless the driver is actually observed entering the ASL they can always claim they had already crossed the first line when the lights went red.