Road conditions can vary enormously from day to day and region to region within California. Some roads like US 395 span the state through desert, lush valleys, high mountain passes, and snow-covered plains. You may need both snow chains and air conditioning in the same hour while driving through areas like the Owens Valley (it's happened to me). In mid-winter, you may even need snow chains on Interstate 5 (the main San Francisco to Los Angeles freeway) to get across the mountains only 40 miles from the palm trees of downtown LA (that's also happened to me, several times). Similarly, rain (and, particularly) fog can suddenly descend on you and the road, making driving difficult, dangerous, or suddenly impossible. High winds are also a common problem on the routes through mountains and passes.
Caltrans (the California Department of Transport) maintains an official Highway Conditions web page, and a recorded phone service for route information on (800) 427 7623. These resources are free and very useful for checking on everything from whether a road has been closed due to snowfalls to whether road works in progress on a particular highway are causing delays or not. This information is kept up to date 24 hours a day, 7 days a week. I often use Weather Underground for location-specific weather forecasts (the "Scientific Forecaster Discussion (NWS)" link on the results pages are a goldmine of useful information straight from the National Weather Service's mouth...), but there's a ton of other sites that do something similar as well.
The main problems you'll face in California are likely to be fog, rain, snow, and ice. These are touched on below.
Fog is common in places like the San Joaquin and Sacramento Valleys and much of the Northern and Central coast; unfortunately, the skills needed to deal with it are not. Large multi-car pileups in dense fog on the freeways happen at least annually, and it's common to see cars tailgating each other or speeding along at normal speeds in almost zero visibility (Britons will recognize this phenomenon). The coastal (advection) fog is most common during the spring and summer months; the Central Valley's tule fog tends to be more prevalent during autumn, winter, and early spring.
Caltrans maintains computerized fog warning signs on some freeways — pay attention to them, as some freeways (Interstate 5 comes to mind) can have quickly variable conditions, changing from one mile to the next (snow and fog to dust storms). It is often difficult to realize just how bad the fog or dust is until it's too late; typically, the worst accidents happen when you run into a virtual wall of fog or dust with no warning (this can happen frequently in the Central Valley). Visibility on the other side of the wall is zero — but you can't even see the fog or dust wall until you hit it. This phenomenon caused a 100+ car pileup, with more than a dozen deaths, many years ago on Interstate 5.
The majority of Californians live in places where it simply doesn't rain at all for six to nine months of each year; and when it does rain, not a lot actually comes out of the sky (except when it floods, of course...).
This means that when it finally does rain, no one can remember how to drive in the wet conditions. Most Californian drivers aren't used to the reduced visibility and the greasy, ultra-slippery roads, and accidents are far more common. Typically, few drivers bother to slow down, no one seems to realize that cornering is a lot harder, braking distances much longer, etc. The urban freeways are particularly bad for this — try to avoid them during and immediately after any rain.
Remember: rain is the (rare) exception in California; correspondingly, wet-weather driving skills people take for granted in wetter climates like London or Sydney are also the exception.
California may have a lot of hot deserts, but it also gets a surprising amount of snow, especially in the high country. Luckily, most snow falls on rural or semi-rural areas, so most drivers don't have to deal with this on a daily basis for long periods of time (the major exception is the whole Truckee / Lake Tahoe / Reno / Carson City area in Northern California and Nevada). However, if you're traversing the Sierra or high country for some reason (skiing at Tahoe or driving to Reno or Oregon, for example) you may encounter snow any time between early November and late May.
Caltrans does a pretty good job of keeping important mountain roads open during and after snow falls and blizzards. Donner Pass on Interstate 80, for example, is snow-plowed as much as is needed to keep it open all year round. Other roads, however, are closed for the season (e.g. State Route 120 through Tioga Pass), and you will not be allowed on to them at all. Many trans-Sierra routes like State Route 120 or State Route 4 are closed between Thanksgiving (end of November) and Memorial Day (May).
All major routes through the high country (the Sierra, the Tehachapis, etc.) have mandatory snow chain requirements signs on the side of the road; these are active during the colder months (during the warmer months they're turned away from the road). It pays to obey these (they're quite specific and easy to understand, and usually pretty reasonable), as you may be stopped by police or rangers and asked whether you're obeying them; in any case, you may have to obey them because the CHP may be enforcing them at a checkpoint next to the signs. More importantly, if you disobey them and have or cause an accident, or need towing, you can find yourself in financial and / or legal trouble.
The main chain requirements are usually one or more of the following, in increasing order of severity; the "R" codes are used by Caltrans, as explained in their comprehensive Chain Controls web page:
- Snow chains must be carried. This is actually not a real requirement, only an advisory, but there are several signs around the state that make it look like it's required.
What this means is that there's no real problem on the road now, but that there's the possibility of a new snowfall or ice, so keep a suitable set of chains in your car ready for use. In any case you should always carry chains in the higher mountain areas except during the summer months.
- R1: Snow chains required; snow tires OK.
This basically requires all cars without snow tires (even 4WD cars without snow tires on all four wheels) to put snow chains on before proceeding any further; you'll often have to pass a checkpoint where the CHP or Caltrans check for this. This at least allows normal cars with good tires through without putting chains on. Even if you have snow tires, most vehicles must still carry chains under these conditions (this may also be checked at a checkpoint).
In California, a snow tire is pretty much any tire that the manufacturer has certified "M+S" (mud and snow) or similar on the sidewall, and / or that the CHP or Caltrans thinks will pass muster. A lot of modern tires with good all-weather tread patterns will do the trick here; if in doubt, put chains on anyway, or ask at the checkpoint ahead of time.
- R2: Snow chains required; 4WD with snow tires OK.
This is the most common requirement, basically restricting chain-free travel to 4WD vehicles with snow tires on all four wheels. Once again, you must still carry chains even with 4WD, and you may have to pass inspection at a checkpoint.
- R3: All vehicles snow chains required.
This is pretty obvious — there's a lot of snow and ice ahead, and no one's going to get through without putting chains on, 4WD or not. This doesn't happen very often — in my experience roads are usually temporarily closed rather than made R3 — but plan for it even if you've got an AWD or 4WD vehicle.
One problem with the Caltrans snow chain requirements signs is that they don't always tell you when it's OK to take the chains back off. This is usually pretty obvious, but it pays to be cautious.
There are always pull-over areas near the chain requirements signs on freeways and major highways; these are used to put your chains on. Smaller county roads might not have adequate signs or pullout space for chain installation, however. People driving into hilly subdivisions with 2WD cars in snow should consider putting chains on before entering the subdivision, just in case they end up being needed in an area with no pullout space. Plenty of traffic problems are caused by cars stopped in traffic, on a hill, for chain installation.
On busier routes, there may be a bunch of professional chain installers at the pullouts, who will put your chains on for you for a fee. Avoid these guys unless you really need them — modern snow chains are really pretty easy to put on. There's no excuse for not knowing how to put chains on your car; if you've rented a car and a set of chains, make sure a) they're the right chains for the car; and b) you know how to put them on the car before you have to put them on at the pullout. Chains can usually be rented or purchased from service stations and outdoors supply places in the mountains, and bought from places like Kragen Auto Parts, etc., in the cities, but (of course) the closer you get to where you have to put them on, the more expensive renting or buying chains is....
On some of the busier routes, when the weather is particularly bad Caltrans or the CHP operate convoys of vehicles in each direction to make sure no one gets lost and the single available plowed lane is utilized properly. Alternatively the traffic is also sometimes "metered", i.e. the CHP only lets through a certain number of cars each hour in each direction.
(Special thanks to Tom Tilley for additions to this section).
Every year there are some pretty spectacular accidents and pile-ups due to ice, especially "black ice". Black ice is the invisible and very slippery ice that usually thinly coats the entire surface of a road. It's particularly prevalent in the Central Valley during winter and the Sierra late autumn to early spring.
Black ice often forms on the top of bridges and causeways, and is virtually invisible. You won't know it's there until you've hit it, at which point it's probably too late to do much except use all your ice-driving skills to keep control, slow down, and try to avoid hitting anyone else. The best defensive measures are to be extremely careful when approaching bridges, causeways, or other stretches of road that aren't embedded on firm ground (the ground keeps things warmer so that, in California at least, the road itself doesn't normally freeze too often if it's actually touching ground). You should also be careful after freezing rain, or during and after heavy fogs in mid-winter.
The long causeway between Davis and Sacramento on Interstate 80 is a notorious spot for black ice accidents during winter. This part of the freeway is almost ideal for it: it's elevated, so that it freezes much easier than the surrounding land; it's right over a large stretch of water that provides handy airborne moisture; and, above all, it's a nice long, straight, heavily-trafficed section of freeway where people are accustomed to driving 75 M.P.H. before dawn on their way to a day's skiing. Multiple vehicle pileups are almost inevitable here, year after year.