MEDIOS DE ADQUIRIR LA PROPIEDAD

4.4.1 Overview of context-sensitive design

No road is designed or constructed in a vacuum. It could form part of an urban high-speed environment, or perhaps be a scenic drive in an area that attracts tourists. In short, the provision of the road takes place within some or other context. This context will directly influence the parameters of the design and inform the selection of the dimensions of the geometric elements of the road. Context-sensitive design (CSD) thus addresses safety and efficiency within the framework of the road’s natural and human environment.

Context-sensitive design asks questions first about the need and purpose of the transpor- tation project, and then equally addresses safety, mobility, and the preservation of sce- nic, aesthetic, historic, environmental, and other community values. Context-sensitive design involves a collaborative, interdisciplinary approach in which citizens are part of the design team. (Neumann et al., 1998)

This workshop signalled the development of the movement towards CSD in the United States. There are two main drivers of CSD. The first is that, starting in the 1970s, many coun- tries enacted legislation protecting the environment. Inevitably, designers with the concepts of mobility and accessibility deeply ingrained in their psyche ran into resistance from the public. Improvements sought by the designers that were were no longer unquestioningly accepted were

• Faster travel times • Greater safety • Reduced congestion • Less delay

Although roads were accepted as being necessary, they were also perceived as having a defi- nite cost in terms of invasion of the environment and disruption of community life. The sec- ond driver is that community values had gained a voice at about the time that the world was becoming increasingly litigious. Perhaps the two are not entirely unrelated. Communities had become more aware of what can be described as a ‘sense of place’. Design propos- als were weighed up regarding their impact on historical, cultural and community values. Where designs were in conflict with these values, proposals would be hotly resisted, if not rejected out of hand.

44 Geometric design of roads handbook

CSD is also known as context-sensitive solutions (CSS). This is because of the need for an outcome rather than merely a process. The road must be designed, built, maintained and operated, thus involving all the activities of a Transportation Department or Agency. It is also possible that the solution of a problem does not necessarily need a design component and could equally well be solved through the implementation of an operational measure. Many thus believe that CSS is to be preferred as being a more encompassing term.

4.4.2 the contexts of design

The most obvious contexts are the urban and rural environments. The urban environment is generally defined by ‘intensive land use for closely spaced buildings and structures and impermeable surfaces’. Traffic patterns are characterised by high volumes of vehicles moving at low speeds, whereas in the rural environment, traffic volumes are low and speeds high.

The high speeds of the rural environment require high standards of geometry. Gradients are low and radii of horizontal curvature high, as are the K-values of the vertical curvature. In consequence, cuts and fills could be high. The lower speeds of the urban environment make it possible to adopt lower design speeds and the accompanying lower geometric stan- dards. In consequence, roads can be held closer to ground level. This eases problems of provision of access to surrounding land uses.

In the rural area, drainage comprises mainly the transporting of water from the catch- ment area across the road reserve and drainage of the road reserve towards the surrounding area. In urban areas, the road reserve serves also as a drainage conduit of the surrounding land use. Having the road slightly below the natural ground level supports this drainage.

Rural intersections often are no more than simple bell mouths between two two-lane roads. Pastoral farming typically requires large farms, and in such areas intersections could be several kilometres apart. Agrarian farming would result in a closer spacing of intersec- tions but that would still be longer than that found in urban areas. Urban intersections fol- low each other in quick succession. Their spacing on urban arterials is typically of the order of 500 metres and, in residential areas, spacings of 100 metres and less are not uncommon. These intersections may be between multi-lane roads with medians, channelising islands and a high level of sophistication brought to bear on their signalisation.

In urban areas, the physical size of the various vehicles on the road starts to play a role because they are closer together. Stops and starts are the norm for urban traffic streams, with busses and semitrailers having rates of acceleration significantly lower than those of passenger cars.

It is not possible to endlessly supply more infrastructure to match the increase in vehicles on the road. In terms of the supply/demand equation it is now necessary also to address the demand side of the equation by the promotion of public transport. This requires that roads and streets must be bus-friendly with, where possible, relatively flat grades, generous kerb radii at intersections and careful location of bus stops. Passenger should not have to walk too far to get to the nearest bus stop, with a maximum of 500 metres being a useful target to aim for. This will obviously have an influence on the layout of the residential area.

4.4.3 the subcontexts of design

There are eight subareas within the primary contexts of urban and rural areas (Milton and St Martin, 2005):

• Urban centres

Fundamental design considerations 45

• Suburban corridors and nodes • Industrial corridors

• Rural town centres • Transitional areas

• Rural connecting corridors • Residential areas

They are described in the sections that follow, offering some indication of the features of these areas as they impact on parameters of the transportation facilities they require.

4.4.3.1 Urban centres

The central business districts of large towns and cities are typical urban centres. They are relatively small in relation to the urban areas they serve and are primarily the location of commercial developments and offices. Urban centres include housing, small parks, retail space, recreational opportunities and public transport termini. The employment opportuni- ties offered by urban centres result in high concentrations of traffic converging on them in the early mornings and similar volumes leaving in the late afternoon and evening. Parking is normally off-street in parking garages but short-term parking may be provided on-street.

These centres should be pedestrian friendly, with wide sidewalks, and provide facilities for parking of bicycles. Shelters should be provided for users of public transit. Traffic control would usually be by signalisation.

4.4.3.2 Urban corridors

The major streets within urban corridors often are dual carriageways, with side streets and alley ways serving loading areas, but may also include multi-lane undivided streets. Very often and depending on the volume of traffic to be served, urban corridors take the form of one-way pairs of streets. High-density residential buildings may be located within these cor- ridors, which may also include local concentrations of small shops and municipal libraries and other services as well as community centres.

Urban corridors usually have to accommodate a variety of modes of transport, including • Passenger cars

• Freight vehicles of all sizes

• Public transit, usually busses but possibly including light rail • Cyclists

• Pedestrians

Intersection are usually signalised subject to warrants for signalisation being met. Bus stops are frequent and possibly at intervals of 500 metres or less.

4.4.3.3 Suburban corridors and nodes

Suburban corridors differ from urban corridors insofar there is a greater preponderance of passenger cars in the traffic stream. Very often, shops have their own parking areas and access may be via short frontage roads separated from the major road by an island generally referred to as an outer separator.

Roads may have a median, which, where there are many left-turns to off-street parking, may be replaced by two-way left-turn lanes.

46 Geometric design of roads handbook

Bus routes are normally located on these roads so that bus shelters should be provided at stops. Pedestrian crossings should be provided, preferably in close proximity to these bus stops.

4.4.3.4 Industrial corridors

Industrial corridors have high morning and late afternoon peaks as workers arrive at and then leave their workplaces. Bus volumes as well as cycle and pedestrian volumes are fairly high at these times, with the latter two being low at other times. During the course of the day, the percentage of truck traffic in the stream would tend to be high, with many of these trucks being semitrailers. Industrial corridors are typically truck routes and the geometry of the intersections should be designed to accommodate these vehicles. The major routes are generally arterials with signalised intersections.

4.4.3.5 Rural town centres

The centres of villages and small towns in a rural environment are typically host to a variety of activities such as business, shopping and recreation. Government offices at all three tiers of government, from state to local, could be found at these centres along with other services such as post offices and police stations. Town halls and other public places such as librar- ies would be located at the town centre. Traffic volumes would be medium but with high volumes of pedestrians and low volumes of buses. Buses would principally provide a link to nearby towns and cities.

Reserve widths tend to be narrow, and some village centres are considered to be historic in nature. There could be a strong argument in favour of vehicle exclusion in these areas, with the road network being to human scale rather than to vehicle scale. This suggests that geometric design would have to be very sensitive to local community values.

4.4.3.6 Transitional areas

At the outskirts of rural towns there may be transitional areas of various types. These areas would be located on roads leading to other towns and villages and could be either residen- tial or commercial in nature. If residential, two possibilities arise: residences could be up- market on large stands or shacks occupied by people who are probably unemployed. The up- market transitional area would tend to have a preponderance of passenger cars whereas shack dwellers would rely on walking or cycling. Because the through route could also serve as a truck route, it would be desirable, in the case of shack dwellers, to provide cycle tracks and pedestrian paths that are well clear of the through route.

In developing countries, shack dwellers often make use of animal-drawn transport, typi- cally donkey carts. These carts often have the same width as a passenger car but travel at no more than a fast walking pace. Because of the speed differential between them and the motorised vehicles, providing paths for them that are clear of the through route would be desirable.

Commercial development would tend towards light industry and small shops. Parking would generally be outside the limits of the road reserve.

4.4.3.7 Rural connecting corridors

These are roads linking rural towns to each other or to urban areas. Design has to take due cognisance of rural land use patterns. Agrarian farming results in high seasonal fluctuations in traffic volumes in the form of farm to market trips whereas pastoral

Fundamental design considerations 47

farming does not show these spikes in demand. Intersections are typically widely spaced and have a simple layout. Depending on the topography and hence the alignment of the road, speeds could be high. The safety of the relatively infrequent cyclists and pedes- trians on these roads thus suggests that paths close to the reserve boundaries should be considered.

4.4.3.8 Residential areas

Residential areas are served by collectors and local streets. Pedestrian volumes are low but children playing in streets should not be unexpected. Traffic calming measures should be brought to bear on the design of residential streets. Traffic calming comprises reduction in traffic volumes as well as reduction in traffic speeds. Traffic volumes can be kept low by ensuring that residential streets do not offer a convenient alternative to congested arterials. Traffic speeds are generally kept low through the application of speed humps and chicanes. These traffic calming measures are generally a case of retrofitting an existing grid pattern of streets but in a ‘green fields’ development the same end results can be achieved without the application of essentially artificial measures.