Sorry have to disagree with the above statement as cyclists should be on their side of the road or at least to their left and not in the middle of the road. They have a duty of care to other road users and themselves! Tractors travel at speeds higher than 20mph.If you are driving that fast round a rural bend that 2 bikes abreast cause you an issue you are sadly driving too fast.
I don't think 20mph is fast around a bend as you can easily see another vehicle coming from the opposite direction especially a tractor and brake according without having to stand on the pedals. I would think that many cyclists travel at speeds higher than 20mph.
As said it is only a minority of cyclists in our area that have no respect for other road users and they are probably from a town or city.