beamwidth and interference

We are thinking of using the Canopy @ 5 Ghz to create a point to point data connection that will carry police data. The two points are about 1 mile appart and one point is on a mountain top about 800 feet above the second point. The issue that the chief is raising is the possibility of interference. To respond to that, my question is: based on the beam width of the two units (both with reflectors or dishes) how close would an interfering station have to be to affect our system? Let’s assume the interfering unit is on the same frequency and what ever else to make it the worst case scenario.
Thanks Walt