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Double white lines where the line nearest you is solid. This means you MUST NOT cross or straddle it unless it is safe and you need to enter adjoining premises or a side road. You may cross the line if necessary, provided the road is clear, to pass a stationary vehicle, or overtake a pedal cycle, horse or road maintenance vehicle, if they are travelling at 10 mph (16 km/h) or less.
Laws RTA 1988 sect 36 & TSRGD regs 10 & 26
How can you judge how fast a thing's travelling? (ignoring the fact you have a speedo on your car, but that would require you to slow down to the speed it was going, probably).
I just overtake cyclists if there's a solid white line and I can see far enough ahead to know nothing's coming. Even if they're going at 11mph