| I think Carbon749 is right.
As far as the one on the Pembury Road is concerned, I think the white lines are marked on both sides of the road so I've always worked on the basis that you could be 'got' in either direction. The camera certainly flashes if someone is speeding towards it and there are no other vehicles around.
I have heard stories of local bikers travelling towards it setting it off deliberately to use up the film.... how naughty! | 
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