Originally the word "street" simply meant a paved road (Latin: "via strata"). The word "street" is still sometimes used colloquially as a synonym for "road", for example in connection with the ancient Watling Street, but city residents and urban planners draw a crucial modern distinction: a road's main function is transportation, while streets facilitate public interaction. Examples of streets include pedestrian streets, alleys, and city-centre streets too crowded for road vehicles to pass. Conversely, highways and motorways are types of roads, but few would refer to them as streets.
The word street has its origins in the Latin strata (meaning "paved road" - abbreviation from via strata); it is thus related to stratum and stratification. Ancient Greek stratos means army: Greeks originally built roads to move their armies. Old English applied the word to Roman roads in Britain such as Ermine Street, Watling Street, etc. Later it acquired a dialectical meaning of "straggling village", which were often laid out on the verges of Roman roads and these settlements often became named Stretton. In the Middle Ages, a road was a way people travelled, with street applied specifically to paved ways.
The street is a public easement, one of the few shared between all sorts of people. As a component of the built environment as ancient as human habitation, the street sustains a range of activities vital to civilization. Its roles are as numerous and diverse as its ever-changing cast of characters.
Streets can be loosely categorized as main streets and side streets. Main streets are usually broad with a relatively high level of activity. Commerce and public interaction are more visible on main streets, and vehicles may use them for longer-distance travel. Side streets are quieter, often residential in use and character, and may be used for vehicular parking.
Circulation, or less broadly, transportation, is perhaps a street's most visible use, and certainly among the most important. The unrestricted movement of people and goods within a city is essential to its commerce and vitality, and streets provide the physical space for this activity.
In the interest of order and efficiency, an effort may be made to segregate different types of traffic. This is usually done by carving a road through the middle for motorists, reserving pavements on either side for pedestrians; other arrangements allow for streetcars, trolleys, and even wastewater and rainfall runoff ditches (common in Japan and India). In the mid-20th century, as the automobile threatened to overwhelm city streets with pollution and ghastly accidents, many urban theorists came to see this segregation as not only helpful but necessary in order to maintain mobility.
Le Corbusier, for one, perceived an ever-stricter segregation of traffic as an essential affirmation of social ordera desirable, and ultimately inevitable, expression of modernity. To this end, proposals were advanced to build "vertical streets" where road vehicles, pedestrians, and trains would each occupy their own levels. Such an arrangement, it was said, would allow for even denser development in the future.
These plans were never implemented comprehensively, a fact which today's urban theorists regard as fortunate for vitality and diversity. Rather, vertical segregation is applied on a piecemeal basis, as in sewers, utility poles, depressed highways, elevated railways, common utility ducts, the extensive complex of underground malls surrounding Tokyo Station and the temachi subway station, the elevated pedestrian skyway networks of Minneapolis and Calgary, the underground cities of Atlanta and Montreal, and the multilevel streets in Chicago.
Transportation is often misunderstood to be the defining characteristic, or even the sole purpose, of a street. This has not been the case since the word "street" came to be limited to urban situations, and even in the automobile age, is still demonstrably false. A street may be temporarily blocked to all through traffic in order to secure the space for other uses, such as a street fair, a flea market, children at play, filming a movie, or construction work. Many streets are bracketed by bollards or Jersey barriers so as to keep out vehicles. These measures are often taken in a city's busiest areas, the "destination" districts, when the volume of activity outgrows the capacity of private passenger vehicles to support it. A feature universal to all streets is a human-scale design that gives its users the space and security to feel engaged in their surroundings, whatever through traffic may pass.