How to condition the opening/stoping of a conveyor port?

I'm actually trying to simulate a crossroads, so I thought of using conveyors as roads but I'm having some troubles while conditioning the conveyors port at interactions.
Exp: While the conveyor A is occupied, close Conveyor B input.

Can some one please tell me how can I use triggers with a custom condition (true is written as default) ?

I can upload some illustrations of what I'm up to if my question is still not clear. Thank's.

Maybe think about the events that close and open the ports - rather than thinking about "while" a condition is true.

Or switch to task executors on networks and use the traffic light object.

Thank's for replying. Actualy I haven't thought about using task executers with traffic control lights, that would've been a nice idea! I tried first to make it work with task executors and avoiding collision options but it didn't go that well, so I just made it with triggers and sending delay messages for a conveyor to another.
Thank's again.