When I was running my D&D Cyclopedia-powered campaign in Mystara, I pilfered something from WFRP's "Enemy Within" campaign: the roadwarden.
A roadwarden is an individual charged with maintaining the safety of the roads and its travelers. In Karameikos, I made them a combination of Texas Rangers and FBI Agents -- ensuring the safety of the roads and its travelers also meant looking after the caravans and their merchandise, as well as the businesses involved in trade.
All roadwardens were 3rd level characters from several possible character classes: the fighter, the elf, or the mage. Unlike WFRP roadwardens, they often traveled in pairs acting as the hands of the Grand Duke and the eyes of the Grand Duchess.
PCs were often asked to liase with these roadwardens, who would deputize one or more of the PCs to perform some service for the Karameikos.