I have ridden along that road on a charity ride and a weekend evening. Don't know what it's like during commuter hours on a bike, in a car you must have noticed the speeds...I have! I think some cars have gone in the ditch from what I remember.
I cycle one the other side of Earith and avoid those 60mph roads during commuter times if possible. People seem in a real hurry to shave 10 seconds off their journey at my expense. Not nice roads in the wind either as they are sooo open.
Only one way to find out!
There is the guided bus route(there is a little country track to Rampton off the GB route....then off to Cottenham?) if you want to take a bigger detour! (but mostly traffic free) Never tried it but looks like you might be able to get to Over via some of Long Homle drove near the Earith/ Shelford Rd junction.
Where does this country track start? I have done the road from willingham to rampton and is alright I think during commute hours and then on to cottenham has the bike path. I actually think the ride in the morning will be better being ealier then rushhour but the return will be smack in the middle and I know a pain in the ass