Truckers and rush hour commuters are far and away the biggest offenders these days. I try to avoid any truck or arterial routes, especially during the evening rush.
In Burlington, I have found Appleby line between the QEW and 407 to be hideous with the recent road expansion. Truckers are far and away the biggest offenders. Calling them professionals is an insult to the real pros like oh, hookers!
Don't blame Brampton, blame the licensing and drivers ed systems. My daughter just completed the Young Drivers course and not a thing was mentioned about driving in the vicinity of cyclists.