Asphalt is the black petroleum-based material used to pave roads, parking lots, and pathways. It consists of aggregate (crusite stone and sand) bound together with bitumen, a sticky substance refined from crude oil. Asphalt is the most common road surface in Canada because it is relatively inexpensive, easy to repair, and provides good driving conditions. However, asphalt roads require regular maintenance including patching potholes that form from freeze-thaw cycles and eventually resurfacing when the pavement deteriorates. Municipal public works departments spend significant portions of their budgets maintaining asphalt roads, a perpetual infrastructure responsibility.