What if you disagree after being found at fault after an accident?

The decision of who is At Fault in an accident is generally decided by the police officer who is investigating the scene. If you don't agree with his/her decision you can always speak with their supervisor at the police department and give our side but it is very rare that these decisions are overturned. You only other point of appeal would be to your insurance company. The insurance companies take the police report as well as statements from both parties and make their decision as to who is at fault.