Driving in snow

Today it snowed in Vancouver. Even though it wasn't much I had to learn how to drive in it fast! No I've never driven in snow before.
For your viewing pleasure here are a few tips to keep you safe:

1. The most important thing to remember: The chances of you getting safely to your destination are inversely proportional to your speed. That is the slower you drive the faster you'll get there.
2. Leave a few car lengths between you and the car in front. This will give you the required time to react to the changing situation, like someone cutting you off. Basically it's your safety margin.
3. brake just a bit earlier then you usually do. DON'T slam on your breaks. Your car will slide right into an obstruction ahead. Apply your breaks slowly.
4. If you drive a manual transmission don't coast in neutral. You'll end up losing traction too.
5. If you're turning (or whatever) and your car starts sliding, turn your wheels in the direction opposite of where the car is sliding. DON'T slam on your accelerator or breaks! Let your car right itself first.

Mainly driving in snow is about not losing traction. As long as your wheels are gripping you're all right.

I think that's all for now. Disclaimer: This worked for me, eh? Have fun, and drive carefully!

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Well Rancid_Pickle says first turn into the skid. I practiced skidding for fun earlier today, and now I can't quite remember whether i turned into the skid or not at first. I know on the way home I nearly missed a car because I started sliding to the right. But for me the sequence of actions was almost automatic - I straightened the car right away. Don't believe me though. Your best bet I suppose, is to practice (just like Mr R. Pickle says).

This applies to driving in rain and ice just as much as it does snow.

Traction is the friction that exists between your car's tires and the surface that your car rests on. Whenever you take an action with your car (accelerating, braking, or turning), you are using some of the traction that exists to accomplish this. If you take extreme action, you may lose traction. For example, floor the gas when starting and you may peel out and not go anywhere. Slam on the non-ABS brakes and your wheels will lock. Turn while you are going too fast and you will slide. Generally, these are undesirable actions.

As your car's speed increases, you need more traction to turn. This is because a faster car has more inertia in the previous direction that must be overcome. Therefore, it is easier to straighten out a slower moving car than a faster moving car.

One way to look at driving is as a series of control inputs. The driver has a limited amount of basic control inputs to the car: accelerating, braking, and turning. Any one of these control inputs uses traction. If you find yourself in a skid because of bad weather, it is important to use the smallest amount on control inputs to get yourself back on track.

Do not brake and turn at the same time as one needs all the traction available to be focused on stopping the car from going into bad places. Turn into the skid and do not jerk the wheel, but rather turn it slowly and deliberately.

All the textbook knowledge in the world won't prepare you for a skid as much as experiencing one with no consequences. While there aren't many (if any) places in the city, a sufficiently iced-over lake in a rural area will do for practice.

By the way, this is all my opinion and it is to be taken with a grain of salt.

Heropsychodreamer brings up a good point:

You will not be prepared for a skid until after you have been in one.

Most folks who have been driving for a few years will panic when they are in a skid (from some statistics given in my Driving Education class in High School). They will either hit the brakes as hard as they can, or they will steer in the direction they want to go rather than into the skid.

If the nose of your car starts pointing to the left, turn your wheel slowly clockwise until you feel the front wheels start to get some traction. Then slowly straighten your car by steering counter-clockwise until your vehicle is straight. For example:

1. Car is in skid. All motion is toward the top 
of your monitor.

             ___             ___
   F    -----___-------------___-----
   R    |                           |
   O    |                           |
   N    |                           |
   T    |    ___             ___    |
        -----___-------------___-----
            
          Car is sliding sideways.

2. Slowly turn your car's wheels into the skid.

             /\              ___
   F    -----\ \-------------___-----
   R    |     \/                    |
   O    |                           |
   N    |                           |
   T    |    /\              ___    |
        -----\ \-------------___-----
              \/              
          You will feel the front wheels start to
          rotate and grip. 

3. Slowly straighten your car until it is pointing 
the direction you are travelling.

We practiced skid control techniques in the parking lot of a large department store. I suggest you do this at night and in areas where there are no light posts or other cars. Make sure that the ground is iced and very slippery. Try to avoid going over 10MPH with a truck or Sport Utility Vehicle, as they tend to roll over during side skids. I-25 in Colorado (north of Denver) is littered every winter with the carcasses of SUV's that rolled over in skids. SUV's are not cars and they have a high center of gravity.

Get your car up to 10MPH. Turn the wheel quickly counter-clockwise, then hit the brakes until your vehicle starts skidding. Now practice the skid correction technique. When you are feeling confident, work on the other direction (steer right instead of left).

When you actually get some experience with skids, you will know how to react and how your car reacts. Skids take up a lot of space, and if you do not have space between you and what you're heading towards, all the experience in the world will not overcome the laws of physics. When the weather is bad, drive like you're 95 years old (except make sure your turn signals are off). I have already warned my daughter that she will learn these techniques when I teach her to drive. These techniques saved my ass and my car three times, including once on a highway in San Diego in heavy traffic and once on a mountain road with a rock wall on one side and canyon on the other.

I have one simple rule for driving in snow and ice:

It's not the v, it's the delta v.

For the physics impaired, this is basically what heropsychodreamer above said. You can go fast in the snow, as long as the road is straight, flat, there are no cars around you, and there's no wind. If you have to change the velocity of the car at all, that is the part that you need to do carefully and slowly.

In my opinion, the hardest part of driving in winter is dealing with all the other cars around you. If you're going a different speed than all the other cars around you, you will be forced to change your velocity, which is when you can get in trouble. You need a huge following distance in these cases, and also you don't want people tailgating you. If they are, then I guess you need to fix that (yes, sometimes, you may need to speed up in the snow to be safe.)

Other considerations are visibility and heat. If your car doesn't have a rear window defogger, it should. They're the law in most states anyway. You will need to run the defrosters as well, and in some cars, you need to have the air conditioning on as well to extract moisture from the interior to prevent the windows from fogging up. You want the car warm, but not too warm that you become drowsy. Also, use a good washer fluid that doesn't freeze easily--you'll need it to clean salt off the windshield to see properly.

And one more thing--brush all the snow off of your car before you start moving. Including the roof. There's nothing more annoying than being behind someone who has chunks of snow flying off their vehicle and crashing into your car. In New Jersey, it's illegal as well. Get a good snow brush, and use it. If you can't reach the top of your SUV, then you shouldn't be driving it in the snow anyway.

Driving in the snow can be stressful and intimidating. I must drive from Golden to Boulder (Colorado) every day to get to work and back. Normally it's a 40-50 minute drive, but in bad conditions it can take two hours. The highway I drive is state highway 93, which is only two lanes total most of the way, with a few annoyingly short passing sections. Due to its two lane nature I must deal with quite a bit of tailgating myself.

In good conditions, I drive faster than the average person. My el-cheapo Ford Escort handles great in good weather and is almost fun to drive in the twisty sections that make up the Front Range Highway. There are some downhill sections that allow me to reach high speeds with relative ease, making the car seem faster and ever slightly more fun to drive. There is often quite a barrage of wind (one permanent sign halfway through simply says "Gusty Winds May Exist") blowing, but in my low-profile Escort, it's barely noticeable. I've driven it in a Ford Explorer many times as well, and I feel much safer and confident in the Escort, in good weather or bad.

For the same reasons that it is an enjoyable drive in good weather, it sucks badly in poor weather conditions. The twisty downhill parts are terrifying. Where I could otherwise drive 65 and slam the brakes at the last minute during the curve if necessary, I must now drive 25 and approach the curve slowing down to 10 or 15mph, using the brakes before actually making the curve, as exemplified by heropsychodreamer's writeup. If the conditions are bad enough there may be a mile of traffic slowly coasting downhill, one car at a time, barely moving, sometimes so slowly that the speedometer doesn't even register it. During one occasion I slowly had to pump my brakes down a normally gentle curve that was just covered in pure ice and fresh snow. It was terrifying; I took it extremely slowly, so the car behind me started to pass me (illegally) and skidded way out of control, much to my amusement, front wheels a-locking. Luckily, they didn't quite roll off of the curve to the gentle hill below, but they sure came close.

I find that dealing with tailgaters is the primary problem when you want to drive safely. Sometimes I feel like I'm being forced into speeding by peer pressure, but it's far worse when it's bad weather and you know you can't go any faster, mainly because there is curve after curve approaching, and visibility is nearly zero. One time I was actually told to "Get off the road" by someone in a Suburban who ended up going a whole 4-5mph faster than I; I didn't, of course, but I wished him dead and sent bad Satanic vibes in his direction... Generally I drive 65-75mph in good weather, if it's bad enough I'll drive 15-35, usually 25 on average. Tough shit for the 5000lb Ford Expedition that wants to drive on the shoulder to pass all of the people inching down the rather scary downhill slope with a curve at the very end. If there's room, I'll make the gesture of moving to the side of the road to let people pass me, if they're especially obnoxious. Then I laugh as they storm ahead and find themselves behind another intelligent driver a city block ahead.

Every day after a bad snow I see cars in the ditch, often SUVs, and often Subarus, probably driven by (as I imagine them) over-confident yuppies that think four wheel drive makes it safe to go the speed limit when driving on two inches of snowpack. This gives me the strength to shake off absurd vehicular harassment that I can expect to experience again during the next storm. Usually I will simply call in and say "fuck this" if it's too bad, and so far my employer has been flexible. I don't like risking my life unnecessarily, especially if all I get in return is a day of work followed by another potentially fun drive on the way home! Unfortunately with my luck the weather always comes in during work, just in time to sabotage the ride home, giving me no choice but to drive. It feels like getting on a rollercoaster...

I don't see why you have a problem with driving in snow. It's kind of fun.

Let us get more specific

The points the above noders make are all correct, but ignore the difference between black ice, a thick layer of ice, snow, snow on ice, slush on ice and refrozen slush. (See Finnish words for snow; For the record, musta jää, kaljama, lumi, polanteet, sohjoa jäällä and röpelö respectively.) Each of them has completely different properties. The first snow of the winter - a thin layer of fresh, powder snow - affects the traction only a little. Ice, packed or untouched layers of snow and slush are the real danger, because they prevent the wheels from touching the asphalt.

First of all, the greatest danger is not crashing to the ditch or into a tree. That's what modern, safe cars can handle. The real danger is drifting to a lane going to the opposite direction, where the relative speed of impact is twice the speed of impact to a stationary tree, according to the Galilean transformation. No safety device can help if a 60-ton truck flattens your tiny tin can of a Sierra.

Types of ice and snow

In My Holy Opinion, thick layers of ice are the most dangerous. Lack of traction is not the greatest danger in them. They occur in the spring, when the snow melts in the day and refreezes in the night, compacting to hard, transluscent forms. They are hard, irregular in form and 2-10 cm thick. That's why they're dangerous. You can't crush them and drive over them: suddenly the road drives the car, not you! This scares the shit out of an inexperienced driver. To drive on a road with thick layers of ice, just drive where everyone else has driven. Blocks of refrozen slush form where no one drives. A lot of accidents happen when someone is trying to pass a car and hits this ice block between the lanes.

Then you have slush or freshly fallen snow on ice or compacted snow. Assume no traction. Braking distances are ridiculously long. You can get even 200 m from a highway speed of 100-120 km/h. The braking distance for big cars with a speed of 100 km/h will start to resemble the distances which trains need for a complete halt. Simply stop at each intersection where you have any doubts about visibility. If you're speeding through the intersection and another car comes doing the same, you can do nothing. Combine this with a dirt road and blocks of ice: crashing is 100% guaranteed, if you do not slow the car into a complete halt, until you know you're safe to go. A good '86 Toyota Camry was wasted by my mother because of this. I would've wanted it!

Ice on the road isn't that difficult, but I've witnessed an accident on the first day of season with icy roads. After I saw it, I tried driving in equivalent conditions in a parking lot with a 180 degree turn. The conclusion is that in a city, you can go almost any curve with 30 km/h, but you have to consider the steepness of the curve at 35 km/h, and 40 km/h is too much. Yes, it is this precise: only 5 km/h make the difference.

Snow is relatively easy to drive in. A compacted layer of snow has quite good a traction for turning. Untouched, fresh snow can be somewhat annoying, because you have to compact it as you go. Obviously the layer isn't perfectly even, so the wheels jerk into random directions. You have to correct these movements continuosly. Anything that's under the layer of snow is concealed from you. You cannot know in advance if there are forms of snow, ice or frozen slush hiding under the white blanket of newfallen snow. The road may drive the car. The speed limit is 35 km/h for a 90-180 degree street crossing as mentioned earlier. Drive slower in the turn, about 25-30 km/h, if the compacted layer is icy.

Black ice is said to be the most dangerous, because it's supposedly unpredictable. This is where Celsius degrees come handy. If the road is wet, subzero means black ice. Black ice shines in the headlights unlike water, which absorbs nearly all the light and only glimmers faintly. You can slow down and maintain a longer distance to the next car. Usually the road is covered in black ice only partially. So, if you lose traction, you have good chances of regaining it quickly. Then the car goes wherever the steering wheels are pointing at. If you have panicked and jerked the wheel, the result is exactly the same if the road was dry: the steering wheels are going sideways, and the car flips over.

Refrozen slush in a frost means that the traction is quite good. The car has to break the protruding forms of ice, which generates traction. Other cars will grind the ice into small pieces, so that it's like driving in a gravel road.

Technical considerations on the car

The car makes a big difference when driving in snowy weather. The more weight there is on the tyre, the more traction it gives. Hence the safest car has its center of gravity near the centerpoint of the car, so that the weight and the traction is spread evenly over the wheels. The car I have been driving, a '92 Volvo has its center of gravity in the front, so that the wheels in the back lose their traction easily. That makes the car dangerous, because it will start spinning in a curve. The front goes in the direction of the curve, the back goes to the forest. This can be corrected letting the car get a grip on the road first, then using the accelerator.

I haven't driven a car with the center of gravity in the back, but I think it will be even worse. The tyre has the most traction when it's going forward, i.e. not turning. The consequence is that when the steering wheels turn, the lose the traction even more easily that the back wheels. The car goes exactly to the same direction where it was going before losing the traction, according to the Newton's First Law of Motion. The driver becomes a passanger.

Front wheel drive means that if traction is lost, the car will go forward and cannot be controlled until the front wheels regain traction. They have the least of safety. You need some skill to drive a back wheel drive car in snow, but it's safer. In a curve, you can step down the accelerator, so that the back wheels will grind deeper into the snow and get a better grip. Rally drivers do this all the time, and it looks cool. Not to be done by women* or other timid persons. Four wheel drive needs even more skill, as you already noted. It stays on the road better, but when it starts to slide, it's harder to control. The safest is an "intelligent four wheel drive". It is normally a front wheel drive, but when it detects loss of traction in the front, it will connect the back wheels to the engine. This type can be too safe. When the car corrects the loss of traction because you speed to a curve, you easily forget the danger.

ABS is not the solution to all wheel-locking problems. It will make braking easier, but not idiot-proof. Any kind of braking -- including ABS braking -- can make the wheels to lose the remains of the traction it might have had. ABS braking is also braking, after all. Pumping the brake applies also to ABS. Keep the pedal down for one second, release for two, brake for a second again etc. In a non-ABS car, this prevents a total loss of control, but in an ABS car you may be able to drive through the curve without any incident.

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Whatever you read, Siberia will teach.

Special Thanks and Olympics for: CentrX, Catchpole

* the women I know tend to be preoccupied with this "getting from point A to point B" thing.

Successful handling of an automobile on snow and ice depends on two factors: skill and equipment. All the previous writeups in this node have emphasized the first. But no matter how good a driver you are, your car's performance on frozen surfaces will be far from optimal if its tires aren't designed for the task.

Simply put, if you plan on doing any serious driving on snow, you need to get yourself some snow tires. If you live in an urban area and the only winter driving you do occurs on flat, city streets which are quicky plowed and cleared when snow falls, you might be able to get by with the driving techniques described above. Even then, though, a sudden blizzard could easily leave you in a ditch. But if you'll be driving during the snowy season on highways, steep inclines, irregularly plowed country roads, or even (especially!) roads where snow melts and re-freezes into ice, a set of snow tires is a must.

Sure, all that's been said about steering out of skids is important. But wouldn't it be better to avoid skidding in the first place? A four-wheel drive vehicle will help you drive through heavy snow, but as many SUV owners can attest, four wheel drive doesn't mean four wheel stop. And antilock brakes won't do you much good if your tires won't grip the surface you're trying to stop on. Snow tires, unlike "all-season" tires, are built specifically for these conditions, and I can tell you from personal experience that they're far more effective than anything else except snow chains, which are immensely impractical and must be removed as soon as roads clear.

A car on snow tires can be driven in clear conditions in the ordinary manner without any great loss in performance. On snow, the snow tires's thick, widely-spaced treads will improve traction and stopping. Even better, get snow tires with metal studs built into the tread. These will decrease your stopping distance on ice dramatically, and provide much securer handling under extreme conditions. The only concern here is legality - many jurisdictions ban studded tires to prevent supposed damage to roads (though I think their case is overstated). Without studs, though, snow tires are legal everywhere, and with studs, they're legal in most areas where you'd need them at least in the winter months.

Since snow tires are typically removed every fall and spring, you'll have to pay for tire mounting and wheel balancing twice a year if you only have one set of wheels. The expense can add up over the years. If you've got an older vehicle, it's less expensive to find an extra set of wheels at a salvage yard, have your snow tires mounted on them, and simply change between the two sets whenever you need to. An added benefit is that you can always keep a full-sized spare tire with you and not have to deal with the stock "donuts" if you get a flat.

If you don't take my advice and get a set of studded snow tires (if you'll be driving in a place where you'd need them), at the very least buy a set of chains and keep them in your trunk for emergencies. They're inexpensive, and they may be your only way to get your car out of a bad situation. Many high-elevation highways are impassible in winter on ordinary tires.

The Finnish company Nokian is known for making the best snow tires (this is perhaps not a coincidence), though they tend to be on the expensive side. Kumho makes snow tires almost as good as Nokians, for a significantly lower price. Most of the American manufacturers' products don't perform as well, with the exception of Firestone's trendy "Blizzaks," which can be quite pricey.

As for why I'm placing such an emphasis on tires, last winter I drove from Montana to San Diego in my 1987 Volvo 240. The weather along the trip was unexpectedly extreme, and I encountered one of the worst blizzards I've ever seen while crossing the mountains in Utah along Interstate 15. Dozens of vehicles were stuck at the side of the road, including a good number of fancy 4WD rigs. A Suburban about 100 yards ahead of me skidded and collided with the concrete median barrier. With studded tires on the wheels of my rear-wheel drive Volvo, I made it through the mountain passes at a reasonable speed without even the slightest loss of traction.

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

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.

Partly covered

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.

Snow covered

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

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

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.

Road closures

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.

Winter maintenance

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.

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References

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.