Simple as that.
Currently they are kind of useless since any player capable of making cannoniers (level 48) will have 2-3k of cannons already stocked. That makes buildings from the cannon production chain obsolete (especially considering the cost), so why not try to make any of them useful ?
One carriage per employed settler, building using such improvement can have a visual indication of it (background change of production screen or whatever you find suitable).
Discuss.