Previous writeups contain good advice on driving in snow. Winter tires have improved a lot even in the last decade, so today in 2017 they are even more effective than suggested above. They can save you a lot of grief. I totalled my Saturn SL1 on a hydro pole after hitting a patch of black ice while running on all-season tires, and since then I have scampered to put on the winter tires just as soon as daylight savings time ends. They are a worthwhile investment in road safety.
Not mentioned above is the importance of the windshield washer fluid top-up. You will probably need a lot of windshield washer fluid, not the summertime kind, but the real "good to -40 degrees" blue stuff. If you're going to travel a distance in any kind of traffic, you ought to top up every time you stop for gas, or for food, or to take a leak. If you run out of washer fluid on the highway, then the next semi-trailer you pass is going to give you a grit shower, and you are going to be driving blind. if that little orange light comes on, get more washer fluid in there at the very next chance.
Personally, I hate winter driving. I've been driving in Canada since I was 16, and I'm 52 now. I've driven in a lot of crappy winter weather. As a Canadian I'm supposed to be very stoic about it. But I put my dad's wood-panel station wagon in a ditch when I got disoriented in a white-out when I was 17. I stood my Dodge Aspen upright on its rear fender in a different ditch when I was in university. I spun its replacement, a bargain-basement Dodge Aries, through a harrowing 270 degree arc in a major intersection a few years later (and got away unscathed), and then about a decade ago I wrecked the Saturn. And I have been fortunate to come through a lot of other bad weather and mistakes, both mine and those of others. I hate winter driving. I am always, always afraid that old man winter will reach for me once again.
I've arrived safely home after yet another festive holiday season of multi-hour slogs through questionable weather in Ontario's snow belt. Mandatory festivities in late December are typically not a joy for me, and this year was no exception. Lake Ontario hasn't frozen over and the prevailing winds suck moisture liberally from the lake and then cross onto land, where they crystallize it into pure white misery, sprinkling it liberally from wherever I am to wherever I want to be.
Not being Finnish, I have only one word for snow, but I modify it with a lot of adjectives, many of which are unprintable in a family magazine. Of course there's also the continuum of slush, sleet, freezing rain, ice pellets, rime, graupel, ice, frost, and other forms of meteorological abuse which characterize the season. What they have in common is that they all suck to drive on, especially in combination, which is something that Ontario roads excel at providing.
A description of the common terminology used to describe winter road conditions seems useful here. Most provincial or state transportation departments/ministries have converged on a set of similar terms, and their web sites often offer a brief description of them. Here on e2 I can augment these bare-bones descriptions with more detail.
Bare roads are the gold standard of winter driving. (Bear roads, which are rough-hewn tracks made by bears to lure unwary travelers to their doom, are quite another story.) Bare roads come in two main varieties, "dry" and "wet". "Dry" is a relative term, not meaning warm and dry like a summer road surface, but more likely something closer to "with the frozen crud all scraped off and not glistening." You can drive on bare dry roads confidently, albeit with the awareness that the next hill or curve may bring something quite different. Bare and "wet" means there's no significant accumulation on the road surface, but the majority of the driving surface is damp, and likely to stay that way. With liberal applications of road salt by snow plows and trucks, you can encounter bare & wet at a wide range of temperatures. At a few degrees above freezing or lower, the driver must be alert to the threat of black ice in a shadow, on a curve, or on a bridge. Shadowed bridges at the end of a curve? The fool killer lurks there, awaiting the unwary who have crept up to a summertime cruising speed. And "bare & wet" is going to coat your vehicle with dirt, sand, salt, and other crap, so double-check that you topped up the windshield washer fluid—you're going to need it! And a bare road may still be bounded by massive snow berms pushed up by plows, so the snow is likely not all that far away.
This relatively benign sounding term conceals some difficult driving conditions. In basic terms, it means that there's one track (almost always the inner track, on the driver's side for we North Americans) which is "bare" as described above. The rest of the road surface will be obscured by ice, loose snow, packed snow, or perhaps congealed yeti ichor. The covered area typically includes the path that your passenger-side wheels travel, as well as the center-line and the shoulders. The visual landmarks you use to center your vehicle are often hidden, and your vehicle has uneven traction. As you drive you will find the vehicle needs constant correction as the conditions pull the vehicle to the side and then balance returns and you drift back the other way. Variants include "ice covered" which is what it sounds like, "snow covered" which means that the snow is fresh and/or loose, and "snow packed" which means it has been compressed by traffic but not yet scraped away by plows. Each of these variants offers different levels of traction, and they can often be found in exciting combinations along your route. You're going be clearing grime from your windshield here as well, although possibly not as much as with "bare & wet" or you may get some relatively clean snow mixed in. The biggest pain with "partly covered" is the confident driver who feels that the bare track is as good as a clear road, and wants you to drive that way too. He (probably) is going too fast, following too close, and is itching to pass. Even with 35 winters of driving under my seat belt, I still feel the added pressure when one of these folks looms up behind my bumper. But you have to just "Let It Go!" and drive at a safe speed.
Now it's getting nasty. Pavement may be peeking through in spots, but the road surface is largely obscured, and you're never quite sure if there's a layer of ice lurking under the snow. The snow may be fresh and loose, possibly from a fresh or active snow fall, but also possibly windblown snow from fields, even on a clear day. The snow may be packed down by previous traffic into a tight collection of snow pancakes, which are ready to betray your perception of solidity at any provocation. It may have solidified into ice. Like as not there will be a bit of each. As long as you have decent visibility and are cautious, slow and steady will get you through these conditions. One small advantage is there's typically not much road grit being thrown onto your vehicle, so constant windshield wiping may not be necessary. If visibility is poor you may not know that a curve is coming until you vehicle starts to leave the road surface, so make sure you're not going too fast so that you rue the discovery.
Drifts are ridges of snow created by the wind. While delightful to children and skiers, drifts are anathema to drivers. In fresh snow and poor visibility, it can be difficult to judge the height and depth of a drift until you're almost on top of it, and when you hit a big one you risk being stuck or losing control. I once hit a big drift during a sudden white-out and went into a spin. I wasn't going very fast but I couldn't tell how much I had rotated. I needed to get out of the car and determine my car's orientation by feeling for the edges of the road surface, all the while aware that another vehicle could happen by at any moment. That was pretty frightening. Drifting snow and poor visibility are my least favorite combination.
Visibility is considered as a road condition because you need to be able to see to drive. It seems I've already noded it, but for completeness, I'll summarize it here. Usually visibility is described as good (you'll be comfortable), fair (a bit hard to see but safe at reasonable speeds), reduced (which means safe only at low speeds), or poor (you'll be lucky to see an unexpected hazard before you hit it.) There's also the special variant zero, which means just what it says, an impenetrable wall of white. Zero visibility is a particular bugaboo of long trips, where conditions may suddenly worsen along a long and barren stretch of unfamiliar road. You can't drive but you can't stop either, for fear of encountering a truck as it comes through the back of your vehicle. This is the true white-knuckle drive, when you try to drive by extending psychic feelers down through the tires and by listening for subtle changes in road noise. Unlucky passengers will be tersely instructed to remain silent until a laneway or other refuge appears.
Police will typically close roads that are heavily drifted, dangerously icy, or otherwise impassable. Listening to local news radio to learn about closures and changing conditions is another must. However if a road is closed while you're on it, there's not much you can do. I have more than once surprised OPP constables by appearing on the wrong side of their highway barrier, wide-eyed and weary. "It was OK when I left Toronto!" I say, once I can unclench my jaw enough to speak.
Ontario has a lot of dedicated crews and equipment to keep the roads clear in winter. Large trucks with front plows and side wings patrol the highways, salt trucks coat the roads, and graders clear the residential streets. But there are a lot of roads and heavy snows or high winds can fill them in rapidly. And the crews are typically municipal, which means they plow up to an invisible boundary in the middle of God's Country and then turn around. This means that one contiguous stretch of highway may be cleared at different times, and to different standards, meaning that conditions can change rapidly from one county to the next. Very minor roads with no houses are often abandoned in winter, with "No Winter Maintenance" signs posted. Such roads are best avoided.
Planning your trip
Back when I was plunking cars in ditches, we didn't have on-demand access to weather radar from any coffee shop, but we do have it today. Checking conditions on each leg of the trip is just basic common sense. A delay or lay over is much better than a traffic accident. I've avoided a lot of stress by changing plans based on the radar map and real-time updates.
Why for fifty-three years I've put up with it now. I've got to stop winter from coming! But how?
If you live somewhere that snow doesn't accumulate, the Quebec ministère des Transports web site has some nice pictures of the various road conditions described herein. The Ontario version's not quite as nice, but the visibility pics are good, especially "Visibility is poor" which captures the thrill quite effectively.