First of all, I am not an attorney and cannot give you legal advice.
That being said, I can tell you how I approach all of these intellectual property issues. I ask myself the question, what is your risk of being sued? Sure, you want to consult with an attorney about these things but if you ask an attorney if you should do something or not, they will always tell you "no". There's less risk in not doing something than doing something, right? Sure, but you never get anything done.
So they highway sign is property of the government. The only government property you aren't allowed to shoot is that which is restricted as classified, usually military. You normally would have to jump a fence or sneak into a building to get access so you don't have to worry about it in this instance.
You also have to ask yourself: How Michael Moore gets away with shooting the logos of the corporations he is bashing? Well, you can shoot anything you want that is visible from a public place. You can get in trouble if you accompany it with narration making false statements of fact. But just shooting it, no. You aren't supposed to get in trouble, anyway.
Which leads us to our last bit: anyone can sue you for anything. Yes, they can - even if they don't have a legal recourse. There are some statutes in place to punish frivolous lawsuits but anyone can file a suit against anyone. There are a number of steps along the way before they can win a judgment against you. So they can file the suit but if it doesn't have merit, it will get slapped down by a judge. You just need to make sure it doesn't have merit.
If you use some common sense, consult with an attorney and follow some basic rules you can make you movies without fear of retribution, no matter how much people like or dislike your work.