I remember trying to use the old cycle paths alonside the A10 south of
the Cambridge roundabout (now an underpass where the A10 crosses the
North Circular Road) back in the 1970's

The problem was that whenever a side road joined the main road, the
give way line was between the cycle path and the main road. So
mototists joining the main road would shoot across the cycle path
without looking, and cyclists would have to look out at every junction.
I soon realised that it was far easier to cycle on the main carriagway
where I had right of way.

Add to that the fact that the cycle paths always vanished at big
junctions (like the Cambridge roundabout), and the incentive to use
them was zero!