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Type of urban plan in which city streets form a grid

In urban planning, the grid plan, grid street plan, or gridiron plan is a type of city plan in which streets run at right angles to each other, forming a grid. Two inherent characteristics of the grid plan, frequent intersections and orthogonal geometry, facilitate movement. The geometry helps with orientation and wayfinding and its frequent intersections with the choice and directness of route to desired destinations. In ancient Rome, the grid plan method of land measurement was called centuriation. The grid plan dates from antiquity and originated in multiple cultures; some of the earliest planned cities were built using grid plans in the Indian subcontinent.

By 2600 BC, Mohenjo daro and Harappa, major cities of the Indus Valley civilization, were built with blocks divided by a grid of straight streets, running north–south and east–west. Each block was subdivided by small lanes. The cities and monasteries of Sirkap, Taxila and Thimi (in the Indus and Kathmandu Valleys), dating from the 1st millennium BC to the 11th century AD, also had grid based designs. A workers' village (2570–2500 BC) at Giza, Egypt, housed a rotating labor force and was laid out in blocks of long galleries separated by streets in a formal grid. Many pyramid cult cities used a common orientation: a north–south axis from the royal palace and an east–west axis from the temple, meeting at a central plaza where King and God merged and crossed. Hammurabi king of the Babylonian Empire in the 18th century BC, ordered the rebuilding of Babylon: constructing and restoring temples, city walls, public buildings, and irrigation canals. The streets of Babylon were wide and straight, intersected approximately at right angles, and were paved with bricks and bitumen. The tradition of grid plans is continuous in China from the 15th century BC onward in the traditional urban planning of various ancient Chinese states. Guidelines put into written form in the Kaogongji during the Spring and Autumn period (770 476 BC) stated: "a capital city should be square on plan. Three gates on each side of the perimeter lead into the nine main streets that crisscross the city and define its grid pattern. And for its layout the city should have the Royal Court situated in the south, the Marketplace in the north, the Imperial Ancestral Temple in the east and the Altar to the Gods of Land and Grain in the west." Teotihuacan, near modern day Mexico City, is the largest ancient grid plan site in the Americas. The city's grid covered 21 square kilometres (8 square miles). Perhaps the most well known grid system is that spread through the colonies of the Roman Empire. The archetypal Roman Grid was introduced to Italy first by the Greeks, with such information transferred by way of trade and conquest.

Although the idea of the grid was present in Hellenic societal and city planning, it was not pervasive prior to the 5th century BC. However, it slowly gained primacy through the work of Hippodamus of Miletus (498 – 408 BC), who planned and replanned many Greek cities in accordance with this form. The concept of a grid as the ideal method of town planning had become widely accepted by the time of Alexander the Great. His conquests were a step in the propagation of the grid plan throughout colonies, some as far flung as Taxila in Pakistan, The street grid consisted of plateiai and stenophoi (equivalent to Roman decumani and cardines). This was probably best exemplified in Priene, in present day western Turkey, where the orthogonal city grid was based on the cardinal points, on sloping terrain that struck views out towards a river and the city of Miletus.

The Etruscan people, whose territories in Italy encompassed what would eventually become Rome, founded what is now the city of Marzabotto at the end of the 6th century BC. Its layout was based on Greek Ionic ideas, and it was here that the main east–west and north–south axes of a town (the decumanus maximus and cardo maximus respectively) could first be seen in Italy. According to Stanislawski (1946), the Romans did use grids until the time of the late Republic or early Empire, when they introduced centuriation, a system which they spread around the Mediterranean and into northern Europe later on. as the best preserved example of the Roman grid. Outside of the castra, large tracts of land were also divided in accordance with the grid within the walls. These were typically 2400 feet (732 m) per side (called centuria) and contained 100 parcels of land (each called heredium). The decumanus maximus and cardo maximus extended from the town gates out towards neighbouring settlements. These were lined up to be as straight as possible, only deviating from their path due to natural obstacles that prevented a direct route. The Roman model was also used in Spanish settlements during the Reconquista of Ferdinand and Isabella. It was subsequently applied in the new cities established during the Spanish colonization of the Americas, after the founding of San Cristóbal de La Laguna (Canary Islands) in 1496. In 1573, King Philip II of Spain compiled the Laws of the Indies to guide the construction and administration of colonial communities. The Laws specified a square or rectangular central plaza with eight principal streets running from the plaza's corners. Hundreds of grid plan communities throughout the Americas were established according to this pattern, echoing the practices of earlier Indian civilizations. The baroque capital city of Malta, Valletta, dating back to the 16th century, was built following a rigid grid plan of uniformly designed houses, dotted with palaces, churches and squares. The grid plan became popular with the start of the Renaissance in Northern Europe. In 1606, the newly founded city of Mannheim in Germany was the first Renaissance city laid out on the grid plan. Later came the New Town in Edinburgh and almost the entire city centre of Glasgow, and many planned communities and cities in Australia, Canada and the United States. Derry, constructed in 1613–1618, was the first planned city in Ireland. The central diamond within a walled city with four gates was considered a good design for defence. The grid pattern was widely copied in the colonies of British North America.

In Russia the first planned city was St. Petersburg founded in 1703 by Peter I. Being aware of the modern European construction experience which he examined in the years of his Grand Embassy to Europe, the Czar ordered Domenico Trezzini to elaborate the first general plan of the city. The project of this architect for Vasilyevsky Island was a typical rectangular grid of streets (originally intended to be canals, like in Amsterdam), with three lengthwise thoroughfares, rectangularly crossed with about 30 crosswise streets. The shape of street blocks on Vasilyevsky Island are the same, as was later implemented in the Commissioners' Plan of 1811 for Manhattan: elongated rectangles. The longest side of each block faces the relatively narrow street with a numeric name (in Petersburg they are called Liniya (Line)) while the shortest side faces wide avenues. To denote avenues in Petersburg, a special term prospekt was introduced. Inside the grid of Vasilyevsky Island there are three prospekts, named Bolshoi (Big), Sredniy (Middle) and Maly (Small) while the far ends of each line cross with the embankments of Bolshaya Neva and Smolenka rivers in the delta of the Neva River. The peculiarity of 'lines' (streets) naming in this grid is that are each side of street has its own number, so one 'line' is a side of a street, not the whole street. The numbering is latently zero based, however the supposed "zero line" has its proper name Kadetskaya liniya, while the opposite side of this street is called the '1 st Line'. Next street is named the '2 nd Line' on the eastern side, and the '3 rd Line' on the western side. After the reorganization of house numbering in 1834 and 1858 the even house numbers are used on the odd numbered lines, and respectively odd house numbers are used for the even numbered lines. The maximum numbers for 'lines' in Petersburg are 28 29th lines. Later in the middle of the 18th century another grid of rectangular blocks with the numbered streets appeared in the continental part of the city: 13 streets named from the '1 st Rota' up to the '13 th Rota', where the companies (Rotte, рота) of the Izmaylovsky Regiment were located.

Many of the earliest cities in the United States, such as Boston, did not start with a grid system. However, even in pre revolutionary days some cities saw the benefits of such a layout. New Haven Colony, one of the earliest colonies in America, was designed with a tiny 9 square grid at its founding in 1638. On a grander scale, Philadelphia was designed on a rectilinear street grid in 1682, one of the first cities in North America to use a grid system. At the urging of city founder William Penn, surveyor Thomas Holme designed a system of wide streets intersecting at right angles between the Schuylkill River to the west and the Delaware River to the east, including five squares of dedicated parkland. Penn advertised this orderly design as a safeguard against overcrowding, fire, and disease, which plagued European cities. Holme drafted an ideal version of the grid, but alleyways sprouted within and between larger blocks as the city took shape. As the United States expanded westward, grid based city planning modeled on Philadelphia's layout would become popular among frontier cities, making grids ubiquitous across the country. Another well known grid plan is the plan for New York City formulated in the Commissioners' Plan of 1811, a proposal by the state legislature of New York for the development of most of Manhattan above Houston Street. Washington, D. C., the capital of the United States, was planned under French American architect Pierre Charles L'Enfant. Under the L'Enfant plan, the original District of Columbia was developed using a grid plan that is interrupted by diagonal avenues, most famously Pennsylvania Avenue. These diagonals are often connected by traffic circles, such as Dupont Circle and Washington Circle. As the city grew, the plan was duplicated to cover most of the remainder of the capital. Meanwhile, the core of the city faced disarray and the McMillan Plan, led by Senator James McMillan, was adopted to build a National Mall and a parks system that is still today a jewel of the city. Often, some of the streets in a grid are numbered (First, Second, etc. ), lettered, or arranged in alphabetical order. Downtown San Diego uses all three schemes: north–south streets are numbered from west to east, and east–west streets are split between a lettered series running southward from A through L and a series of streets named after trees or plants, running northward alphabetically from Ash to Walnut. As in many cities, some of these streets have been given new names violating the system (the former D Street is now Broadway, the former 12th Avenue is now Park Boulevard, etc. ); this has meant that 2nd, not 1st, is the most common street name in the United States. An exception to the typical, uniform grid is the plan of Savannah, Georgia (1733), known as the Oglethorpe Plan. It is a composite, cellular city block consisting of four large corner blocks, four small blocks in between and a public square in the centre; the entire composition of approximately ten acres (four hectares) is known as a ward. Its cellular structure includes all the primary land uses of a neighborhood and has for that reason been called fractal. Its street configuration presages modern traffic calming techniques applied to uniform grids where certain selected streets become discontinuous or narrow, thus discouraging through traffic. The configuration also represents an example of functional shared space, where pedestrian and vehicular traffic can safely and comfortably coexist. In the westward development of the United States, the use of the grid plan was nearly universal in the construction of new settlements, such as in Salt Lake City (1870), Dodge City (1872) and Oklahoma City (1890). In these western cities the streets were numbered even more carefully than in the east to suggest future prosperity and metropolitan status. Hoddle Grid is the name given to the layout of Melbourne, Victoria, named after the surveyor Robert Hoddle, who marked it out in 1837 establishing the first formal town plan. This grid of streets, laid out when there were only a few hundred settlers, became the nucleus for what is now a city of over 5 million people, the city of Melbourne. The unusual dimensions of the allotments and the incorporation of narrow 'little' streets were the result of compromise between Hoddle's desire to employ the regulations established in 1829 by previous New South Wales Governor Ralph Darling, requiring square blocks and wide, spacious streets and Bourke's desire for rear access ways (now the 'little' streets, for example Little Collins Street). The city of Christchurch, New Zealand, was planned by Edward Jollie in 1850.

The term "town acre" (often spelt with initial capital letters) may have originated with Edward Gibbon Wakefield, who was involved in various schemes to promote the colonisation of South Australia in the 1830s, and, as founder of the New Zealand Company, the plans for Wellington, New Plymouth and Nelson. All of these towns were laid out on a grid plan, so it was easy to divide the land into plots of an acre (approximately 0.4 ha. ), and these became known as town acres. Adelaide was divided into 1042 Town Acres. Maps showing the divisions of the town acres are available for Adelaide, Nelson, and Wellington.

One famous grid system is in the British new town of Milton Keynes. In this planned city, which began construction in 1967, a system of ten "horizontal" (roughly east–west) and eleven "vertical" (roughly north–south) roads was used, with roundabouts at each intersection. The horizontal roads were all given names ending in 'way' and H numbers (for 'horizontal', e. g., H3 Monks Way). The vertical roads were given names ending in 'street' and V numbers (for 'vertical', e. g., V6 Grafton Street). Each grid road was spaced roughly one kilometre along from the next, forming squares of approximately one square kilometre. Each square and each roundabout was given its own name. The system provided very easy transport within the city, although it confused visitors who were unfamiliar with the system. The grid squares thus formed are far larger than the city blocks described earlier, and the road layouts within the grid squares are generally 'organic' in form – matching the street hierarchy model described above.

Street width, or right of way (ROW), influences the amount of land that is devoted to streets, which becomes unavailable for development and therefore represents an opportunity cost. The wider the street, the higher the opportunity cost. Street width is determined by circulation and aesthetic considerations and is not dependent on the pattern configuration. Any configuration can have wide or narrow streets. Street length influences proportionately the number of street components that have to be constructed such as pavement, curbs and sidewalks, storm sewers and drains, light poles, and trees. The street length of a given area of development depends on the frequency at which streets occur which in turn depends on the length and width of a block. The higher the frequency of streets the longer is their total length. The smaller the block dimensions the higher the frequency of the streets. As the frequency of street increases so does the number of intersections. Intersections normally cost more than straight street length because they are labour intensive and require street and traffic signage. Pavement width influences the cost by affecting the amount of materials and labour required to provide a finished road surface. Pavement width is generally based on traffic engineering considerations and is not dependent on pattern configuration. As with the street width, any pattern can have wide or narrow pavements. Of all three factors that affect cost, street width, street length and pavement width, only street length is pattern dependent. An objective cost comparison would, therefore, rely on this variable with the full understanding that the other variables, though optional, can play a role. Traditional orthogonal grid patterns generally have greater street frequencies than discontinuous patterns. For example, Portland's block is 200 feet × 200 feet while Miletus' is half that size and Timgad's half again (see diagram). Houston, Sacramento and Barcelona are progressively bigger reaching up to four times the area of Portland's block. New York's 1811 plan (see above) has blocks of 200 feet (61 m). in width and variable lengths ranging from about 500 feet (152 m) to 900 feet (274 m) feet. The corresponding frequency of streets for each of these block sizes affects the street length. A simple example of a grid street pattern (see diagram) illustrates the progressive reduction in total street length (the sum of all individual street lengths) and the corresponding increase in block length. For a corresponding reduction of one, two, three and four streets within this parcel, the street length is reduced from an original total of 12600 feet (3840 m) to 7800 feet (2377 m) linear feet, a 39% reduction. Simultaneously, block lengths increase from 200 × 200 feet to 1240 × 200 feet. When all five blocks have reached the ultimate size of four street lengths out of total eight have been eliminated. Block lengths of 1000 feet (305 m) or larger rarely appear in grid plans and are not recommended as they hinder pedestrian movement (Pedestrianism, below). From the pedestrian perspective, the smaller the block is, the easier the navigation and the more direct the route. Consequently, the finer grids are preferred. Patterns that incorporate discontinuous street types such as crescents and culs de sac have not, in general, regarded pedestrian movement as a priority and, consequently, have produced blocks that are usually in the 1000 feet (305 m) range and often exceed it. As a result, street frequency drops and so does the total street length and, therefore, the cost. In general, it is not the street pattern per se that affects costs but the frequency of streets that it either necessitates or purposely incorporates. An inherent advantage of the orthogonal geometry of a proper grid is its tendency to yield regular lots in well packed sequences. This maximizes the use of the land of the block; it does not, however, affect street frequency. Any frequency of orthogonal streets produces the same packing effect. Orthogonal geometry also minimizes disputes over lot boundaries and maximizes the number of lots that could front a given street. John Randal said Manhattan's grid plan facilitated "buying, selling and improving real estate". This effect is accentuated with high traffic density in areas with commercial uses where speeds come to a crawl. Since the grid plan is non hierarchical and intersections are frequent, all streets can be subject to this potential reduction of average speeds, leading to a high production of pollutants. Greenhouse and noxious gases can be detrimental to the environment and to resident health.

In his seminal 1982 study on livable streets that was conducted in neighbourhoods with a grid, Donald Appleyard showed that social networking and street playing degraded as traffic increased on a street. His research provided the groundwork for traffic calming and for several initiatives such as living streets and Home Zones, all of which are aimed at improving a street's social milieu. The amount of traffic on a street depends on variables such as the population density of the neighbourhood, car ownership and its proximity to commercial, institutional or recreational edifices. Most importantly, however, it depends on whether a street is or could become a through road to a destination. As a through road, it could sustain unpredictable levels of traffic that may fluctuate during the day and increase over time. A key characteristic of the grid pattern is that any and all streets are equally accessible to traffic (non hierarchical) and could be chosen at will as alternative routes to a destination. Cut through driving, or shortcutting, has been resisted by residents. Cities responded by making modifications to prevent it. Current recommended design practice suggests the use of 3 way intersections to alleviate it. The geometry of the normal, open grid is evidently unsuitable for protecting or enhancing the social environment of a street from the negative influence of traffic. Similarly, a 1972 ground breaking study by Oscar Newman on a Defensible Space Theory described ways to improve the social environment and security of neighbourhoods and streets. In a practical application of his theory at Five Oaks, the neighbourhood's grid pattern was modified to prevent through traffic and create identifiable smaller enclaves while maintaining complete pedestrian freedom of movement. The positive outcome of these changes reinforces Appleyard's findings and the need to reduce or prevent through traffic on neighbourhood streets; a need that cannot be met with a typical, uniform, open grid. The question of neighbourhood security has been a constant focus of research since Oscar Newman's work. New research has expanded the discussion on this disputed issue. A recent study did extensive spatial analysis and correlated several building, site plan and social factors with crime frequencies and identified subtle nuances to the contrasting positions. The study looked at, among others, dwelling types, unit density (site density) movement on the street, culs–de sac or grids and the permeability of a residential area. Among its conclusions are, respectively, that flats are always safer than houses and the wealth of inhabitants matters, density is generally beneficial but more so at ground level, local movement is beneficial, but not larger scale movement, relative affluence and the number of neighbours have a greater effect than either being on a cul de sac or being on a through street. It also re established that simple, linear cul de sac with good numbers of dwellings that are joined to through streets tend to be safe. As for permeability, it suggests that residential areas should be permeable enough to allow movement in all directions but no more. The overprovision of poorly used permeability is a crime hazard. The open, uniform grid could be seen as an example of undifferentiated permeability. A recent study in California examined the amount of child play that occurred on the streets of neighbourhoods with different characteristics; grid pattern and culs de sac. The findings indicate that the open grid streets showed substantially lower play activity than the cul de sac street type. Culs de sac reduce perceived danger from traffic, and thereby encourage more outdoor play. It pointed the way toward the development of hybrid street network patterns that improve pedestrian movement but restrict cut through driving. Similar studies in Europe and most recently in Australia found that children's outdoor play is significantly reduced on through roads where traffic is, or perceived by parents to be, a risk. As a result of this misperception of risk, children living in cul de sac communities are more likely to be killed by vehicles. This increased risk of death is due to multiple factors, including the families driving longer distances to reach their destinations, parents spending less time teaching their children to be as wary of traffic, and an increased risk of the parents accidentally driving over the children in their "safe" driveways and cul de sac streets. Traditional street functions such as kids' play, strolling and socializing are incompatible with traffic flow, which the open, uniform grid geometry encourages. For these reasons, cities such as Berkeley, California, and Vancouver, British Columbia, among many others, transformed existing residential streets part of a grid plan into permeable, linked culs de sac. This transformation retains the permeability and connectivity of the grid for the active modes of transport but filters and restricts car traffic on the cul de sac street to residents only.

Street networks of old cities that grew organically, though admired for being picturesque, can be confusing for visitors but rarely for the original inhabitants (see plan). Similarly confusing to visitors are the plans of modern subdivisions with discontinuous and curvilinear streets. Change of street orientation, particularly when gradual or arbitrary, cannot be "mapped" in the mind. Impasses, crescents or cul de sacs frustrate the traveler especially when they are long, forcing an arduous retracing of steps. Frequency of intersections, however, becomes also a disadvantage for pedestrians and bicycles. It disrupts the relaxed canter of walking and forces pedestrians repeatedly onto the road, a hostile, anxiety generating territory. People with physical limitations or frailties, children and seniors for example, can find a regular walk challenging. For bicycles this disadvantage is accentuated as their normal speed is at least double that of pedestrians. Frequent stops negate the speed advantage and the physical benefit of bicycling and add to frustration. Intersections are not only unpleasant but also dangerous. Most traffic collisions and injuries occur at intersections and the majority of the injuries to pedestrians crossing with the right of way. A dilemma arises from trying to meet important planning objectives when using the grid: pedestrianism, cost efficiency and environmental responsiveness. To serve pedestrians well, a rectangular configuration and high frequency of streets and intersections is the preferred route, which the orthogonal grid geometry provides. To reduce development costs and environmental impact, lower frequency of streets is the logical path. Since these two design objectives are contradictory a balance needs to be struck. Such balance has been achieved in leading modern projects such as Vauban, Freiburg and Village Homes, Davis. Both score high in pedestrian and bike mode share and, at the same time, in reducing negative development externalities. Their layout configurations represent a fusion of the classic grid plan with recent street network patterns. Examining the issue of walkability, a recent comparison of seven neighbourhood layouts found a 43 and 32 percent increase in walking with respect to a grid plan and conventional suburban layout in a fused grid layout, which has greater permeability for pedestrians than for cars due to its inclusion of dedicated pedestrian paths. It also showed a 7 to 10 percent range of reduction in driving with respect to the remainder six neighbourhood layouts in the set, an environmental benefit.

Perceived and actual safety play a role in the use of the street. Perceived safety, though perhaps an inaccurate reflection of the number of injuries or fatalities, influences parents' decision to allow their children to play, walk or bike on the street. Actual levels of safety as measured by the total number of collisions and the number and severity of injuries are a matter of public concern. Both should inform the layout, if the street network is to achieve its optimum use. Recent studies have found higher traffic fatality rates in outlying suburban areas than in central cities and inner suburbs with smaller blocks and more connected street patterns. An earlier study found significant differences in recorded accidents between residential neighborhoods that were laid out on a grid and those that included culs de sac and crescents. The frequency of accidents was significantly higher in the grid neighborhoods. Two newer studies examined the frequency of collisions in two regional districts using the latest analytical tools. They investigated the potential correlation between street network patterns and frequency of collisions. In one study, cul de sac networks appeared to be much safer than grid networks, by nearly three to one. A second study found the grid plan to be the least safe by a significant margin with respect to all other street patterns. A 2009 study suggests that land use patterns play a significant role in traffic safety and should be considered in conjunction with the network pattern. While all intersection types in general reduce the incidence of fatal crashes, four way intersections, which occur regularly in a grid, increase total and injurious crashes significantly. The study recommends hybrid street networks with dense concentrations of T intersections and concludes that a return to the 19th century gridiron is undesirable. Stringent adherence to the grid plan can cause steep inclines since the topology of the land is not taken into account. This may be unsafe for drivers, pedestrians and bicycles since it is more difficult to control speed and braking, particularly in winter conditions.

One of the greatest difficulties with grid plans is their lack of specialization, most of the important amenities being concentrated along the city's main arteries. Often grid plans are found in linear settlements, with a main street connecting between the perpendicular roads. However, this can be mitigated by allowing mixed use development so that destinations become closer to home. Many cities, especially in Latin America, still successfully retain their grid plans. Recently, planners in the United States and Canada have revisited the idea of reintroducing grid patterns to many cities and towns.