Most stop sign laws read something like this:
"The driver of any vehicle approaching a stop sign at the entrance to, or within, an intersection, shall stop at a limit line, if marked, otherwise before entering the crosswalk on the near side of the intersection. If there is no limit line or crosswalk, the driver shall stop at the entrance to the intersecting roadway or railroad grade crossing."
If you decide to take your case to trial, the government has to prove that you committed the violation. That means there generally must be proof of the following elements (parts that make up the offense):
Basically, to get a conviction, the government needs to show you either didn't stop or stopped too late.
Most statutes say you must stop at the nearest of the limit line, crosswalk, or entrance to the intersection. Practically, this means you must stop slightly before you reach one of these points. Some conscientious drivers, however, stop well in advance of where they have to. In these cases, an officer hiding behind a bush 50 feet down a side street might not be able to see whether the driver stopped or not.
To mount this defense, you'll first want to request a copy of the officer's notes. Typically, this involves making a written request to the police department or local prosecutor's office through a process called "discovery." (The notes, if obtained, can be used to verify where the officer was positioned when you allegedly ran the stop sign.) Next, go back to the scene of the ticket and take pictures from exactly where the officer was sitting. Especially if you can document a visual obstruction—and convincingly testify you did stop—this defense can be a winner. Of course, it's a big help if you can produce a passenger or other witnesses who can corroborate your version of events.
Generally, you can't be prosecuted for running a stop sign that was obscured or illegible. With a defense based on obscured or illegible signage, you essentially concede that you ran the stop sign but argue it wasn't your fault because the sign wasn't visible. This can occur if the stop sign is hidden by storm-blown branches, twisted the wrong way by kids, or obscured for any of a variety of reasons.
Pictures are worth a thousand words. Pictures taken from different angles or distances are generally more convincing than just an explanation. For example, if a stop sign is hidden from view by overhanging branches until a few feet before the intersection, take pictures from inside of your vehicle (with someone else driving) as the car approaches the sign. Snap one from a distance of 75 feet, and others from 50, 25, and ten feet away from the sign. (Label each photo with the exact distance from the sign.) In court, you can show your pictures to the judge and explain that they reveal the sign wasn't visible until it was too late to stop.
To video or not to video. In the past, many traffic judges were hostile to video evidence. Most traffic-court courtrooms weren't equipped with video players and monitors, so viewing a video evidence was inconvenient. However, smartphones and portable video players have made it much easier to present a video in court. So, if you feel your case hinges on the evidence in your video, it may be worth presenting it. Contacting and inquiring with the court clerk in advance of your trial is probably a good idea if you plan to use a video as evidence. Traffic courts are fairly informal with evidence, but you want to make sure you follow the court's procedures.
Ignorance is no excuse. It's not a valid defense to say you didn't know that what you did was illegal (rolling through a stop sign, for example). In other words, the law assumes that drivers know the rules of the road.
Generally, the fact that a sign was recently installed isn't a valid defense to running it. However, there's no question that it's easy to miss a new sign placed on a road that you travel regularly. So, some judges might be sympathetic and let you off the hook if you can show the sign really was recently installed.
Crosswalks and limit lines fade. If you are ticketed at a stop sign for stopping a little too far into an intersection, you may be able to beat your ticket by showing that the limit line or crosswalk was too faded to see clearly. Here again, a picture is truly better than a thousand words.
(Read about other options for dealing with a traffic ticket.)