Accidents and Breakdowns

The two most important things to remember here are: 1) join the CSAA or ACSC so that towing is reasonably cheap and easy; and, 2) try to avoid driving in a bad part of town so that you don't break down or have an accident there. Mobile phones also come in handy for breakdowns and accidents, of course, but if you're a foreigner they can be expensive to rent. Some of what follows assumes you don't have a mobile phone; if you do have one, use it instead of the various call boxes (but don't wildly call 911 just because your car is dead -- call the AAA or similar).

What you do if your car breaks down depends a lot on where it breaks down; in all cases, do your best to get the car off the road or at least onto the shoulder before you do anything to it. A few hints:

Wherever you are -- freeway or highway -- if you're stuck in the center of a busy road (e.g. on the center divider or in the traffic lane of a road without shoulders or dividers) don't leave your car and try to walk across the road if you're not absolutely certain you can make it across without being hit (on large busy freeways you have no chance at all of getting across untouched). If it's a truly busy road, someone will call the relevant authorities before long on their cell phone anyway, as you're bound to be causing traffic backups. Always leave your hood up as a sign that there's something wrong with your car if you're stuck in the center of the road.
Always use your hazard lights if they're working; and if you stay in the car keep your seat belt on. If you get out of the car, keep facing the traffic, and be prepared to run further down the center divide or shoulder in case vehicles appear to be heading straight towards you. It's unfortunately true that vehicles stopped in the center divider or in a traffic lane have a fair chance of being hit from behind by cars whose drivers aren't paying attention. This is particularly true for cars broken down on the various Bay Area bridges, where late at night the traffic is moving quickly and drivers are coming home from the clubs and bars....
On freeways, highways, and bridges with call boxes, if you can safely make it, use the nearest call box to get your car towed; the operator on the other end generally arranges this. Remember to tell the operator to get the nearest CSAA / AAA / ACSC tow truck if you're a member. You may have to wait several hours before the tow truck turns up.
In the San Francisco Bay Area, a joint CHP, Caltrans, and Metropolitan Transportation Commission effort called the Freeway Service Patrol operates a fleet of tow trucks on the busier urban freeways during the main commute times. These tow trucks continuously patrol the relevant freeways looking for stranded cars. They are a free service, and the drivers will help you change tires, jump start your car, etc., on the freeway or even tow you to the nearest CHP-designated place if needed.
On rural freeways and highways without call boxes, if you're a long way from any town, the best thing to do is wait for a CHP car to stop and help (they usually just arrange for towing). Make sure it's obvious that your car has broken down -- open the hood, for example. Someone may stop and offer assistance -- this can be dangerous (there have been several robberies in this situation), but it's usually OK.
If you're on an urban or suburban freeway or highway without call boxes, try to pull over as safely as possible, then see if it looks possible, and safe, to walk off an off-ramp or the side of the freeway to a pay phone to arrange a tow. If this isn't possible (e.g. you're in a bad neighborhood, or it's too dangerous to walk), you're probably best staying with the car, again making it clear that there's been a breakdown.
On any other urban or suburban road, try to secure your car and find the nearest pay phone to arrange a tow (or whatever).
Only call 911 from a mobile phone for a real emergency. Mobile phone 911 calls are handled from a central location which is currently grossly overloaded; the calls to 911 asking for traffic information, towing help, and the like are not appreciated by the 911 dispatchers (who nevertheless have to answer them).

If you leave your (broken-down) car on a freeway or highway long enough (where "long enough" varies greatly from road to road -- in some cases meaning several hours, others several days), the CHP may arrange towing in any case. You will be charged for this, and will need to contact the CHP to find out where your car has been towed to.

If you are involved in an accident, use your common sense to ensure that any injuries are not made worse, and that the vehicles involved are not causing further problems. Get the cars involved out of the way of other traffic if you can. If the accident is a bad one, involving any real injuries, get someone to call 911 immediately; otherwise call the local police or CHP. Some police departments (e.g. Berkeley) no longer respond to accident calls unless there's been significant third party property damage, real injury, or some sort of crime (e.g. the other car left without stopping); still, it pays to phone the non-emergency number to at least try to report the accident.

All parties to the accident must stop and offer up all license information to all affected parties and law officers. You should also try to get the names and addresses of any witnesses. You must also file an official report with the Department of Motor Vehicles (DMV) if more than $500 worth of damage was done, or where any injury occurred; this report should be made within ten days except if you were seriously injured. Of course your insurance agent will also want to hear about the problem, too.