Red is for Danger
One of the first lessons novice cyclists are taught is not to cycle past a parked car closer than the width of a door. Doing so risks the driver opening their door into your path, with serious consequences if you collide with it or are pushed off into traffic alongside.
Cambridgeshire County Council are reinforcing this message by painting the danger zone red and adding cycle logos to warn cyclists to keep out. Unfortunately, as can be seen above, some cyclists still don't get the message.