vcmi/config/roads.json

{
    "dirtRoad":
    {
        "originalRoadId": 1,
        "code": "pd", //must be 2 characters
        "animation": "dirtrd",
        "moveCost": 75
    },
    "gravelRoad":
    {
        "originalRoadId": 2,
        "code": "pg",
        "animation": "gravrd",
        "moveCost": 65
    },
    "cobblestoneRoad":
    {
        "originalRoadId": 3,
        "code": "pc",
        "animation": "cobbrd",
        "moveCost": 50
    }
}