Double trailers are 100% limited to only class A licence holders in Ontario. Not the AR, D, or any other class of licence can haul more than 1 trailer.
Now, that doesn't stop Bob with his class G from hooking up his 40' fifth wheel and his fishing boat behind it and heading off into the sunset, but as I often say here, it only matters when there's enforcement, which we're lacking.
This was my setup about 15 years ago.
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I'm fully legal towing it, I was at a legal length, and yes, I can even back it up because I learned how to pull commerical (Super-B) doubles back in the 90's. But you know how many times I got pulled over and asked to verify I actually had a class A licence, despite crisscrossing the province probably 20,000km inside a 5-6 year time period? Not a single time.
They are.
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Any vehicle over 21 feet long cannot legally be in the left lane on access controlled highways, trailer, no trailer, truck, car towing a trailer..pickup towing a trailer, doesn't matter.
But, again....I sound like a broken record, but.....enforcement enforcement enforcement. Or lack thereof.