Caravan Exploit

Weather you send a replacement caravan or not you get the bonus, so instead of send a new caravan to the same city send it to a new city.  You might lose a little of the road bonus because it will take longer for the route to improve but it will give more trade routes bonus.

Or I might be mistaken.  That a caravan count for both cities,  But I think I have seen the trade route exist even after both caravans have been destroyed