I have (accidentally) cycled on what used to be the A40(M) and M41. There are no warning signs on Gloucester Terrace and, if you take the right fork here, you have no escape.
208 Gloucester Terrace
https://maps.app.goo.gl/DTy1XjjrtTJw3A8h7
Ditto on that one.
Also accidental entry to the M40 itself at J7.
A1 - London Wall end of it
A2 - Crossed it in London
A3 - Short London stint
A4 - Bath, Avon causeway, etc. Candidate for a full length ride.
A5 - London
A6 - Peak District and Penrith on LEJOG.
A7 - Carlisle on LEJOG
A8 - Short section in Glasgow on LEJOG
A9 - Tain to Latheron, LEJOG
A10 - Liverpool St Station
A23 - London to Brighton (Veteran Car Route, only the trunk bit on the final run to Brighton, whereupon I was overtaking cars in lane 3!)
A27 - M3 terminus in Southampton following the A33 (multiplex).
A30 - London to Honiton at various points. Bits (inc trunk) on LEJOG. Old route almost in entirety (except Bodmin Moor).
A31 - M27 to Ferndown (when mostly closed), bits and pieces to A35.
A33 - Southampton, avoiding A303 at Popham.
A34 - Bicester to Oxford at rush hour, eek.
A35 - Most of the route between Southampton and Morecombelake.
A36 - Short bits near Bath and Wilton-Salisbury, the M27 spur at Ower
A37 - Dorchester to Shepton Mallet mostly ridden
A38 - Around Bristol.
A39 - LEJOG avoiding Bodmin Moor.
A40 - Oxford to London
A41 - Bicester
A43 - Through M40 J10 for Cherwell Valley services (miniscule)
A44 - Oxford-Chipping Norton
A46 - Dunkirk
A47 - LEJOG on outskirts of Nuneaton, 100 metres
A57 - Sheffield
A58 - Armley Gyratory
A59 - Blubberhouses - Bolton Bridge
A61 - Sheffield on LEJOG
A65 - While in Leeds
A66 - Whinfell Forest
A74 - Glasgow
A82 - Loch Lomond to Loch Ness
A87 - Isle of Skye
A99 - Latheron - JOG
Honourable mention for the A303.