Only time I was meant to use 51 there, it was flooded. I put together a route along the A1307 instead, but at the time it wasn't finished. If I'd been alone, that would probably have been Bar Hill to Swavesey on the A14. As it was, I was leading a small group in an audax, so I instead chose to ignore the closure, and be creative with what tarmac was there! The final act of that was riding 50m the wrong way up one of the slip roads to get from one line of cones to anotherYes, I agree. Don't let cycling be corralled into congested places or the gutter just because some Daffodil paints a bike symbol on it.
Still no need to make up speed limits that might deter people using the decent ones like National 51 St Ives to Cambridge, though.
The old A14 really was a treat on a tired bum, though