most motorists don't look at the back of your truck at any time, not even for indicators when turning
How would you know if they do look for indicators? It doesn't help that the sign truckies give for "it's safe to overtake me now" is exactly the same as the sign for "definitely DO NOT overtake me right now, I'm about to turn right".
Anyway, you know what would really make roads safer? Fewer road trains to begin with. Almost all road trains would be far more effective as actual trains. They'd travel faster, carry more, and cost less to operate, and cost way less in road maintenance. All our intercity/intertown freight should be on rail. The only exception should be tiny communities under about 1,000 people, where freight trains can do the journey to the nearest >1,000 town and trucks can do the final leg.