Riding on trunk routes is nuts. They're motorways by any other name, and whilst you have the right to ride there, why the fuck would you?
Because you want to get from A to B, and someone built a dual carriageway on top of the direct route.
No comment on the A30. It's off my patch. However, for the cyclist simply trying to get somewhere a long way away by a reasonable, short route, the options can vary.
Years ago I did North Herts to Cheshire (or back) as a day ride up the A6, a number of times. At that point little or none of it was dualled. A few years later I did some of it again and they had "improved" whole sections with no thought to cyclists.
Getting north up the A1 from here used to be a problem. There is a back-road route (sometimes there isn't), but someone slapped the dualled A1 in the way at Blunham, and blocked it. Cyclists had little option but to ride up the dual carriageway and turn right across it
It's been fixed by more recent improvements.
Further up, as the A1 has been "improved", a fairly continuous parallel old road has been left between Huntingdon and Peterborough. There's no reason to use the new road on a bike (you can't - it's motorway anyway). Further north, I'm not clear that there are parallel roads nearby.
When I rode to Staffordshire up the A5 some years back, again there was no nearby, direct alternative. I could have found a route around, but I was riding at my limit distance and didn't want anything that added miles. The A5 (then) was bearable, but some sections were not fun.
It's different if you're touring. If you're simply using the bike for travel, adding 10% for safety means that some places cease to be reachable in a day, especially as it probably adds another 10% of time in hills and navigation. Try telling a driver that they had to take every bypass, even where they knew that traffic was light and going through town would save 25% of time.
And that, as someone says, assumes that back roads are safer. If motorists are still doing 70mph but round blind corners, they may not be.