The main development heading from the pit bottom, usually driven along the strike of the coal seam. Because it will carry heavy traffic and large volumes of air, a trunk roadway is at least 14 ft (4.3 m) wide and wide enough for two rail tracks. At intervals, crossheadings are excavated for opening out conveyor panels in the coal seam. Trunk roadways are usually driven in pairs for ventilation, storage space, and access.