It will depend on the state you're in, or how strict the enforcement of bootleg laws are.
Directional signs are used along the highway directing traffic to an event, sale, or home for sale, and are often called "bootleg" signs, because they're typically considered illegal (the name comes from the Prohibition Era of the US' history when "bootlegged" liquor was sold across the US by illegal producers.
So, technically, these are illegal signs, although the name is now more associated with the genre of sign than whether or not they're actually legal or not.
The thought behind the use of these signs is that state employees are not working on weekends and are likely not going to do anything about these illegal signs which are gone by Monday morning when they drive to work. Out of sight, out of mind.
I have not heard of anyone having any issues with using bootleg signs, but I only know that in our area, they're not illegal anyway, so you can put them out whenever as long as you're not on the county or city or state right of way. And even then, people do it and the signs seem to be there for a long time, so I don't think in our area there's much enforcement.
Is a Sign Permit Required to Advertise Outdoors?
That depends on what type of outdoor advertising you're doing. Typically, if a sign is temporary, like a banner or small bootleg sign, no permit or license is needed, at least not in most areas. You'll want to check with your local sign police, er, I mean county or municipal building department to learn what your local code requires.
Now, if you're going to plant a billboard or a permanent business sign, then you're going to want to contact a reputable local sign company and get these answers because these companies are the ones that deal directly with the county and city and state building departments and so they're going to know where to find the information you need regarding your sign or other outdoor advertising.
Who Pays for those Signs Owned by Private Businesses?
One sure thing is that the Government won't pay for signs owned by most private business owners. If they did, there would be a plethora of signs on the highway, and it would be a mess and the government would go more into the hole than it already is.
There is an application process that you would need to go through with the state where you want your sign, and they only allow certain types of businesses to advertise on these signs, typically.
The types of businesses are those that provide traveler services such as fuel stations, restaurants, and the like. So, unless your business is of this type, first, you'll not be able to advertise on any Highway or Interstate signs, and if you are able to, you'll pay for the sign. In my experience, the pricing is generally reasonable and most good sign shops can make the sign for you if they follow the state or federal specs for the signs.