Age 64 is not "advanced in years", should be "old enough to know better".
With regards to the local area I think complacency has much to do with it and the unpredictability of the services. The previous incident was 621 running light engine.
Some of us are rail tragics and are on constant look out for trains, alas most of the community is not.
Your proposal regarding the signage has some merit but needs consideration for the legal implications. The signage could take the form of a permanent sign which folds down to display the message but folds up when not in use. However there are a lot of road crossing between Mt Barker Junction and Victor Harbor.
To fail to so mark just one crossing or forget to put out the sign could be seen as negligence by the operator.
In the old SAR Rules crossing keepers were instructed not to signal traffic over the crossing once a train had passed and presumably it was safe to do so, they were only to lower their stop sign and walk off the crossing. Responsibility rested with the road user. Active crossing do not show a "go" indication; the exception is when coordinated with traffic signals and this could be basis for an interesting legal challenge if there is some stuff up.
Also in the mix are crossings like that on Dunn Road Balaklava, the track is now disused but the signage (cross bucks) are still there; a sign is affixed to the post declaring the "Crossing not in use". From this one can extrapolate that all crossing are in use unless marked otherwise.
That said I don't consider marking any or all crossing regarding train operations a practical or legally sound proposition.
However a few strategically located signs on approach roads to the area reminding locals and visitors that all level crossings in the area potentially carry trains would be an advantage.
The application of Give Way or Stop signs at passive crossings clears up some legal implications regarding liability. I note that Steam Ranger are working on a recorder system which records speed, brake application and whistle/horn use. Is it too much to fit "dash cams" to locos and rail motors.