Rural crime is literally highway robbery - generally, gangs of bandits engaged in robbing villages and merchants as they work or travel from place to place. It is slightly distinguished from urban crime in that it doesn't normally confine itself to a 'criminal underclass', and instead targets ordinary citizens more directly.
Like urban crime, rural crime is based fundamentally off the population a gang of brigands terrorizes - usually, most rackets target areas of about 10,000 people. Too much larger than this will require subordination of some sort, often by adding a lieutenant who runs part of the gang.
Rural crime also requires the maintenance of actual armed brigands - a minimum of a dozen or so brigands for every ten thousand people terrorized, though more brigands makes surviving the inevitable response more likely. Every section of 8-15 brigands costs about 50 fb and is generally equal to or a little more competent than militia in their quality.
Rural crime earns three percent of the population targeted - (0.03 x Population) in firebirds annually. This can be stepped up by hiring more brigands - every additional gang allows you to increase your percentage by one on a successful Survival, Tracking or Region Lore roll. You can attempt to increase your percentage by one every few months, starting a few months after the additional brigands have been brought on board.
Establishing a gang of highwaymen requires the brigands to operate in an area for a few months, and then a successful Survival, Tracking or Region Lore roll - the number of VPs indicates the percentage, to a maximum of 3 for one section of brigands, 4 for two and so on.
It is also possible to have water-born rural crime - piracy, either of the high seas or riverine varieties. The costs to get into piracy are higher than the costs to get into banditry, though the mechanic is similar. Maintaining enough riverboats to affect about 25,000 people's worth of trade costs about 250 fb/yr. A single pirate sloop costs about 500 fb/yr, and affects the trade of about 40,000 people.
Effects of Rural Crime
Rural crime immediately affects the income of the lands targeted - every percent of rural crime income decreases the income from those lands by two percent. Note, however, that this is not population-independent, so that if a large lord has brigands terrorizing a valley containing 10,000 people, he only loses six percent of his income from that valley, not from his entire fief.
Additionally, rural crime affects the income of guilds in the area, particularly the Muster and Charioteers. They see a similar percentage decrease from their service contract income in those lands as do the lords. If the criminals are taking care to avoid robbing guilders, they see only half of their rural crime income.