Subject: What is an actor?

> Can I ask you a question? is that possible for the
> traffic control system on another vehicle to be actor?
> because the traffic control system on one vehicle
> should communicate with the system on another vehicle.

Remember that an actor is a role, not an individual or even a system.  
Don't confuse instances with classes.

In addition, communication (such as sending a message) is not appropriate for a
use case.  A use case should be a higher-level notion.  For example, in a
Student Information System, registering for a course is a good use case.
Clicking on an icon or typing a key on the keyboard is not.

Finally, a use case can involve more than one actor as well as more than one
instance of the same actor.