Can I check the Castledawson A6 approach before starting a Derry journey?

Yes.

TrafficWatchNI lists an A6 Castledawson camera. It is on the important A6 corridor between Belfast and Derry, and can help before choosing whether to travel through the Castledawson area. The Park and Ride at A6 Roundabout is a separate facility from the traffic camera. A camera cannot tell you that a bus or parking place is available; use the relevant official service information for those details.