Commuting is periodically recurring travel between one's place of residence and place of work or study, where the traveler, referred to as a commuter, leaves the boundary of their home community. By extension, it can sometimes be any regular or often repeated travel between locations, even when not work-related. The modes of travel, time taken and distance traveled in commuting varies widely across the globe. Most people in least-developed countries continue to walk to work. The cheapest method of commuting after walking is usually by bicycle, so this is common in low-income countries, but is also increasingly practised by people in wealthier countries for environmental and health reasons. In middle-income countries, motorcycle commuting is very common. The next technology adopted as countries develop is more dependent on location: in more populous, older cities, especially in Eurasia mass transit (rail, bus, etc.) predominates, while in smaller, younger cities, and large parts of North America and Australasia, commuting by personal automobile is more common. A small number of very wealthy people, and those working in remote locations around the world, also commute by air travel, often for a week or more at a time rather than the more typical daily commute. Transportation links that enable commuting also impact the physical layout of cities and regions, allowing a distinction to arise between mostly-residential suburbs and the more economically focused urban core of a city (process known as suburban sprawl), but the specifics of how that distinction is realized remain drastically different between societies, with Eurasian "suburbs" often being more densely populated than North American "urban cores".