If a vehicular accident is caused by a dangerous condition on the roadway, the injured victims could file a claim against the city or governmental entity responsible for the maintenance of that roadway.
In this particular case, it is important to look into when that stop sign fell and for how long it was not fixed. If that fallen stop sign was not fixed by a reasonable time, then the responsible governmental agency could be held liable for the injuries caused to the 78-year-old man.
When it comes to governmental entities or agencies there are time limits within which you need to file your personal injury claim. So, it would be in your best interest to contact a qualified and experienced Orange County auto accident attorney as soon as possible.