Most of the courses I rode on in the dim distant past involved an A road at some point. I looked out some start sheets today and the A roads I raced on were: A1, A4, A6, A13, A41, A45, A405, A414, A604 and the A1090. I felt safe doing so. It was much, much safer in those days because the traffic was lighter and the cars didn't go as fast (also starting early on a Sunday morning helped, as most folks were still in bed!).
Also, by and large, drivers were considerate and left plenty of room when overtaking you. I almost fell off on a roundabout once and the car behind me was considerate enough not to run me over... lol Of course, if we were out on a club run on a minor road we would reciprocate by dropping into single file to make it easier for them too. There was at least a bit of respect from all parties back then.
Times have changed considerably now in all areas of life. Not for the better.