Tow Away signs on rentals

[Sam Leon]

How does those tow away sign work?  I have seen some signs that has the name and number of a nearby tow company mounted on buildings.  Are those signs obtained from the tow companies?  What is the process?

I am in the process of renovating a property that may take a few months and people near by know it's vacant so they freely park there and when I am there with hired help I don't have enough space.  Putting notices on the windshields help some but I need a sign to say I MEAN BUSINESS!!!