Tractor trailers are THE biggest source of congestion on Toronto highways by far. Especially the 401, which connects Windsor/Detroit to Montreal, via Toronto. It's gotten somewhat worse since the province mandated they be governed to 110kph.
In my experience, semi truck drivers are as bad as the car drivers. In my three years of driving for the rental industry, I've seen truck drivers do downright terrifying things on the road. Aggressive driving, taking ramps too fast, loosing wheels. Once saw a semi trying to run an SUV off the road. No idea what pissed him off that much. More commonly, they refuse to share the road by following too closely (so other vehicles can't merge) and they refuse to keep right despite driving below the posted limit. They also filter onto local two lane roads to skip the weigh scales on the highway, causing huge backups in the morning. There's one road near me where it takes literally 15 minutes to drive all of 3km because of the trucks doing exactly that.
Not that car drivers don't pull the same stunts. They cause the majority of accidents. But trucks are much bigger vehicles so the danger/intimidation/stupidity factor is amplified. I think they get way too much credit for their driving ability. Maybe long haul truckers are better, but certainly not the local ones. Those places hire foreign and temp workers at low wages. Any idiot can get/buy an AZ license.
Personally, I'd like to see bigger investments made in rail infrastructure to get the long haul trucks off the road.