Hmm, Let's try this here:

Route::filter('department', function($route, $request)
{
     //remember, Auth::user() is used to call the current user (default) and give access to its data (kind of like calling data from your User model), not the methods within your user model
     //might want to read this: http://laravel.com/docs/security#authenticating-users
     $department_id = Auth::user()->id; 
     if(!Request::isMethod('post'){
        //here I use 'User::' because this would be the correct way to access a model method from this file
	if (!User::canAccessDepartment($department_id) && $request->segment(2) != "create")
	{
                return Redirect::to('/')->with('notice', 'Error!');
        }
     }
});

Written from thoughts so apologies if it's not sound syntax or logic, but it should work for your situation.