Australia

I have also driven extensively through mainly country WA and elsewhere and briefly worked as a truck driver in the NT, in my experience there is no consistency in understanding of these signals, as pointed out by the article, and I'm sceptical that it is consistently applied by truckies.

I do believe that there is an understanding among experienced truckies and some(a majority???) of other road users that there is a system but it is not universally understood by cara-fucking-van drivers, or apparently school bus drivers either.

As a car driver I have on at least 2 occasions had a truckie indicate it is safe for me to overtake(indicating L-L) in a situation that would have caused an accident. The only time it is safe to overtake is when you as the driver can be satisfied it is safe to do so. Relying on potentially ambiguous signals that are not universally understood is a literal accident waiting to happen.

I have been slightly obsessed with this since being quite outraged at a clear signal to pass being so dangerous and I've raised it with several truckies over the years, I think a small majority of truckies assume it's universally understood but a large minority never indicate to pass as it is dangerous/raises liability concerns. A clearly non random sample and I may have been asking leading questions...

You make a good point that for the truckies safety there is a need to indicate that it is unsafe but if people are misunderstanding this then I don't know what the solution is.

-- several edits for clarity --