As Kelly mentioned, the main roads are kept mostly clear - although some routes do require you to have chains with you. Weather can also change quickly and unpredictably in the mountains.
These might be of use to you, depending on your route:
Alberta Road ConditionsBC Highway CamerasMike.
P.S. I did a lot of work in northern Ontario and was also a field supervisor for a project covering ~40,000 sq km of eastern Ontario. Every one of our trucks carried spare gas, tow chain, jackall (come-along), water, blankets, shovels and more. The bottom line is, if you go off the road, how long will it take someone to find you (in the snowstorm that precipitated your leaving the roadway) - especially in an area without cellular coverage - and what will you do in the meantime?