can you please advise me on this? - distance apart and is it measured from edge or centre? - wanted to fit a pair of flush mounts to my classic craven spaceman top box (width 18 inches)
Lrear indicators legaal separation width
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Sounds ok to me,
Some production bikes have the rear lights and flashers in one cluster.
isn't checked at MOT - see Jaffa's link - as MOT is more about maintained function than design.
However, position must still comply with RVLR to be legal on the road. See schedule 7 for indicators. I suspect for you, assuming bike >50cc first used 1st April 1986 or later, that means at least 240mm so your 18 inches should suffice.
They also need to be at least 300mm above ground, but no more than 1.5m, and visible to 80° outwards and 20° inwards - not only is it law, it's a good idea if you want them to be seen...