When approaching a stop sign at the entrance to an intersection or railroad crossing in California, you're required to bring your vehicle to a complete stop. That's for safety reasons, to stop you driving into traffic. If the road is marked with a limit line, you must stop there. Otherwise, the requirement is to stop at the entrance to the intersection or railroad crossing.
Tickets for rolling through a stop sign are quite high, averaging $238 in California. To get a conviction, prosecutors need to show that you didn't stop, or you stopped too late. You can fight the ticket if you didn't do either of these things, or you did them for justifiable reasons.
TL;DR (Too Long; Didn't Read)
To beat a stop sign ticket in California, you'll have to show that the traffic officer got it wrong and you stopped legally, that you couldn't see the sign, or you had to run the sign to avoid danger to yourself and other people.
Challenge What the Officer Saw
Was the officer in a position to accurately see where you stopped? Since your obligation is to stop at the marker line or intersection entrance, from a practical standpoint, you need to stop slightly before the line. If you drove conscientiously and stopped a fair distance from the limit line, the officer might not have been able to see whether you stopped or not.
Your first action, then, is to take pictures from where the traffic officer was sitting. Testify in court that you did stop and use the photos to show that the observation angle did not give the officer an accurate view. It's even better if you can find witnesses who can corroborate your version of events.
Prove the Sign was Obscured or the Limit Line Faded
Generally, the courts will not convict you for running a stop sign that you couldn't see. For example, it's not your fault if overhanging branches obscured the sign or someone had twisted it the wrong way. Similarly, you can usually fight a ticket if the marker line was too faded to see clearly, and this made you stop a little too far into the intersection.
A picture is worth a thousand words, so go back and drive the route, taking photos or video footage from 75, 50, 25 and 10 feet away. Show the pictures in court to prove that the sign or marker line wasn't visible until it was too late to stop.
You Missed the Stop to Avoid a Crash
Depending on the facts, you may be able to fight a ticket if your actions were justified. This might happen when, for example, you rolled forward to avoid another driver who was violating traffic laws, or you had to swerve to avoid a dangerous obstruction in the road. The key argument here is that you had to run the stop sign to avoid immediate danger to yourself and other people. This type of defense can be successful since it raises facts, instead of being based only as your word against that of the traffic officer.
You May Have an Out if It's a New Sign
Generally, you cannot challenge a stop sign ticket based on ignorance – it's no defense to say that you didn't know you were prohibited from rolling through a stop sign or you were not looking out for a sign. The one possible exception is that in which you run the route regularly, and the sign was newly installed. A sympathetic judge might let you off the hook if you travel the road everyday, and the stop sign was newly installed on the day you ran it. It's not a perfect defense, but it may be worth a shot if you're out of other options.