Driving Miami to Key West in February: how's the traffic?

Two years ago we stayed at Knight's Key in Marathon (Mile Marker 47) and drove down to KW twice during our stay. Don't know what weekend traffic is like as we were there mid-week. There are occasional stop and go areas along the way, but averaging about 45 mph is pretty typical. As tourists in the Keys you really don't want to drive too fast anyway. Keeping an eye on the road while seeing all that great scenery is difficult enough.