I should point out too swe didn't always have a lot of those signs. Out here they used to get BIG dogs that were over 80lbs to scare witnesses off, but witnesses then discovered the legal non weapon that scares the crap out of any guard dog...the umbrella.
I'd tell pet owners to try this at home but good greif, don't do it, your dog will be a shaking pissing wreck for about 5-10 minutes. I would NEVER do this. I have seen someone do it though and I was embarrassed and hoped nobody would be home.
It's rather simple:
1.See dog
2.Open Umbrella
3.???
4. Profit!
The only bad thing is, if you do get someone home, they will ask you what the hell you did to their dog. This will often result in a do not call anyway.
Only once was someone bitten and it was by a small terrier that refused to take no for an answer. A kid almost got hurt that time too. Crazy backwoods service! That isn't even mentioning the time they let someones horses out because they didn't close the gate! That is sometimes why private property signs are kept up, not just to keep people out, but to assure animals won't be let out.