One 'quaint' medieval custom that might well apply to both fair revenue and tolls is that of 'farming'. This is not in the sense of agriculture, but in the (original) medieval sense that gives us the phrase 'faming out'. The deal is this:
The king or a major noble has the right to a piece of revenue - be it from a fair, tolls, a manor or what have you, but (s)he can't be bothered to collect it or simply wants to be distanced from it while not losing the revenue.
I missed all of the chat about tolls, lost as it was in the discussion about fairs. Exactly who can and cannot levy a toll over an international trail such as the Salt route or Genins trail is obvious. If you have enough men to enforce the imposition of a toll, then you can collect it.
Within the kingdoms, the first question to be answered is who owns the highways? I hope that this won’t be to boring, but, modern era highways are something I know lots about and I’ve always been fascinated by their history.
I’ve always assumed that Harn follows English highway laws, in many cases this is ‘custom & practice’ not statute. A few basic principles:
In all cases, highways within kingdoms are the Kings highway. Highways follow a defined (probably defined only by custom) route, are open for everyone, and grant a right of passage only.
The route of a highway is defined as the route people have customarily used. A new highway is created if people begin to take a detour to avoid a dangerous ford or dilapidated bridge and find a safer/quicker/better route. (IIRC it takes 20 years of unrestricted access to create a highway by these means). Once a highway exists, only the crown can stop people using it. A Lord can make it difficult for travellers to use a particular route, but he cannot lawfully stop them.
Anyone trying to avoid a toll by, using land that is not highway (by, for example cutting across cropland or pasture) is committing the crime of trespass. For a merchant, the delays incurred in taking wagons over unmade ground, plus compessation for loss of crops, grazing etc., almost certainly makes for more expensive delays than would paying the toll.
I would assume that on Harn, local landowners have a duty to maintain the highways through their demesme and, using Balesirs arguments, have the right to levy a reasonable toll on travellers for this purpose. Maintaining a muddy dirt track costs next to nothing, maintaining a bridge can be expensive. As others have said bridges are very good toll locations, after all, people have no alternative but to cross at them. Someone spent a lot of money building that bridge and they’re entitled to a return on their money.
To complicate matters, if you use English law, then the highways are actually owned by the frontagers (you own up to the middle of the road in front of your house). But you have no rights to do anything to prevent access and you may even have a duty to maintain the road for the public good.
Apologies, but I can bore for England on this topic.
Lots more if anyone is interested!