Control a system by one slider??

Hello.

I need to control my system (my city)  from 1 slider (MAIN slilder).

But I have a problem because I have a city and some roads. And now I want to control the road and only show them when the MAIN slider reach a specific value, and until that value on the slider the road has to be hidden.

I don’t want to show the road the hole time, only when the 1 slider reach a specific value.

Any idea how to do that?