Can a bike use a primary road?

There is one 'no cycling' sign at the eastern approach to the Marylebone Flyover. I always assumed that meant the entire elevated section was off-limits (I invariably approach from the east), but I've never bothered to check. Perhaps that sign is a leftover from the motorway days, and perhaps they did change the traffic orders at the TfL takeover, although it would surprise me. When we did a ride out there some years ago (can't remember which one), someone accidentally went up on the flyover as he missed the left turn the rest of us had taken. I think he re-joined us at the bit where the elevated section is lowered again. For some reason, we stopped there for a while. I remember telling someone (perhaps John Ma3k) that hundreds of houses were knocked down for that piece of crap.