To clarify, the commuter route in question is a single carriage A road that connects two large UK cities. And this was at 8.30am!
I manage to bite my tongue when it's tractors (and there are enough of those that use the road in question). I remind myself that the farmer is doing an invaluable and often thankless job that keeps us all in food, and I don't know enough about agriculture to comment on whether or not it's absoutely essential that he drives his machinery at 20mph 5 miles along this road at 8.15am. I may silently swear a lot, but I try to give him the benfit of the doubt.
But horse boxes? Come on people. It's a hobby. I don't care if your darling daughter does need to be at a gymkhana with her pony, Princess, in order to win some lovely multi-coloured ribbons. Time your journey so that you are not on major roads at 8.30am, or don't be surprised if the 5 mile tailback of slowmoving traffic who are all trying to get to work starts to show a bit of aggression.