An exercise to drive straighter.
Set three pairs of cones, wide enough to fit thru your vehicles easily.
No need for a tight clearance here.
Set them at D, X and G, down the center line, or in a straight line in your driving area,set 30 meters apart from each pair.
In the above photo, there are about 10 trips down this line indicated by wheel tracks left in the stone dust. You can see the first few trips indicated by the red arrows. Note how off center they are and how much they veer from straightness. The blue arrows indicate trips 6 thru 10. Note how much straighter they are and how much less they veer from straight.
This will help the driver feel what straight is *and* it will familiarize the horse with what it feels like to travel straighter and better balanced. Driving horses can not benefit from the guidance provided by a rider's legs. One way to help a horse is to repeat work in a way that is more efficient for them. A balanced horse does less work. Horses are evolutionarily wired not to work harder than they have to. Show them a way to work which requires less effort and they more readily will 'go there' themselves.