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The A37 is pretty nuts - hope it wasn't too bad. It's more frightening cycling on the A roads than in London for that reason you described. I get close passes from fuckers refusing to budge over the white line only to then see them move to the centre of the road to avoid a speed bump. It's baffling.
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I used to ride Bristol - Shepton Mallet frequently, mostly weekends though. However, I had many dicey encounters with careless and aggressive motorists. Some of the worst road users I've ever encountered on that route. There is a very pleasant country lane route out via Chew Valley Lake though. Speed bump fuckery really irks me, baffling is exactly the word.
I would say that this is your issue, and advise you to ride as unsafely as possible, forcing motorists to give you loads of time & space.
However, if it's country A/roads you're talking about, there's nothing you can do. It's their unmovable opinion that those roads are their dominion and they can use them as they wish.
I learnt this the hard way on the A37.