I notified Google of this, and to their credit:
Hi Lasers,
Your Google Maps problem report has been reviewed, and you were right!
You'd have to be crazy to cycle on this road. It's a 4-lane single carriageway with, in places, a 60mph speed restriction. It is completely unsuitable for cyclists, lethal in fact. There are no separate cycle lanes on the pavement, either. Cyclists should NEVER use this road.
We'll update the map soon and email you when you can see the change.
Thanks for helping us to improve Google Maps!