Stop people from parking in front of driveway?

Various cars repeatedly will partially block our driveway. They stay for at least an hour and we are unable to exit our driveway. However since police are slow they don’t show up until after the car leaves. So the car gets no consequences but I miss an appointment.

Since it’s not the same car every time or the same time of day, it’s impossible for us to resolve this interpersonally. I was considering putting up a sign, but I’m not sure what to say.

How can we ensure people stop blocking our driveway?