I.3 Planning considerations

This section deals with the planning of transportation infrastructure. In this context, the term “planning” means making informed decisions regarding the type of transportation infrastructure to be provided, and then choosing the most appropriate options based on a thorough understanding of the context within which the planned development will be implemented.

As part of the planning process, it is important to consider the neighbourhood layout and structure (refer to Chapter F) while also considering the type of project, the potential residents of the neighbourhood and the transportation trips, modes and network distribution. The features and requirements of the proposed project should always respond to the context within which the planned development will take place (Section I.3.2). The planning phase is concluded by considering different transportation options (Section I.3.3), specifically related to geometric design and then road pavements.

The transportation planning process starts with the definition of the study area, which is the area to be included in the transportation study. It is unlikely to be just the boundaries of the new project or neighbourhood. Transportation and traffic impacts can occur remotely from the development site. The careful definition of the study area ensures that a detailed understanding of the regional and local context of the project is developed, including the scale; the urban, rural or peri-urban nature of the study area; the socio-demographic status of the inhabitants; existing transport infrastructure and services; and other planned and existing developments in the area.

I.3.1 Characteristics of the proposed development

Decisions regarding transportation infrastructure and services need to be guided by an assessment of the characteristics of the proposed development and an understanding of the requirements or needs that will have to be met. Factors that should be considered are discussed below.

I.3.1.1 The nature of the proposed development

Various factors related to the nature of a development could influence decisions regarding the provision of transportation facilities and services. For instance, an extensive transportation master plan could be required for large neighbourhood development schemes. This is unlikely to be the case for smaller developments and improvement schemes for existing streets that are likely to be less complex, and, in some cases, a scheme layout is generally all that is required. Smaller projects may also not require detailed transportation design, as existing infrastructure might have spare capacity available. In these cases, the focus of the transportation planning would be to link into and integrate with existing systems. Large (or mega) projects may have to consider a range of transportation modes and require the inclusion of various public transport and NMT facilities. For mobility and access design purposes, population density and land use mix are critical considerations, as the efficiency of public transport facilities and services are dependent on user thresholds. Mixed use (i.e. a mix of residential and other land use types), mixedincome projects and projects that are primarily residential in nature would also need different approaches to the provision of transportation infrastructure and services. Similarly, inner city, infill projects would be different from, for instance, an informal settlement upgrading project. The nature of a project therefore needs to be clearly understood to make informed decisions regarding appropriate transportation options and facility and service provision.

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I.3.1.2 The residents of the area to be developed

Decisions related to the mode of transport and its capacity and coverage should be guided by information regarding the potential residents and users of the planned infrastructure and services. Traveller needs could be informed by various attributes of the neighbourhood inhabitants, including incomes, trip patterns for all modes to and from the neighbourhood, public transport dependence, car ownership levels and the needs of disabled persons. Usually, the detailed socio-economic characteristics of the residents of the new development are not known when a development is planned and designed. It can also be difficult to predict who and how many residents will make use of transportation infrastructure and services. However, it may be possible to make assumptions regarding the possible nature of the future residents and users of transportation by assessing the surrounding neighbourhoods or similar developments in comparable locations or contexts. It is important to consider the following:

  • The total number of residents that would have to be accommodated, taking into consideration that actual numbers may be higher than anticipated due to the fact that the provision of houses and services may attract more people than originally planned for.
  • The number of households and the types of housing to be provided in the development. This will have an impact on the transport demand and influence the modes used.
  • The range of residents with special needs that would have to be accommodated, e.g. people living with disabilities, including physical, dexterity and sensory impairment. Transportation infrastructure and services should, as far as possible, be accessible to all residents and users.
  • The age of residents and those that may use transportation facilities (i.e. gender ratios, age profile and size). An aging population might, for example, increase the number of public transport users who might require easier access to vehicles. Trip patterns might also differ (in terms of timing or destination) between the youth and the elderly. Other dimensions have been shown to influence travel choices such as gender, and occupation, which may be used for even finer segmentation of the beneficiaries.
  • Income and employment levels and spending patterns might give an indication of car ownership or dependence on public transport.
  • The number and location of schools, social and recreational facilities and retail facilities to be provided in the neighbourhood. This will inform the number of trips generated and the distribution of trips along the movement network. This will also give an indication of the opportunities for transport-oriented development.

As discussed in Section F, layout and transportation planning should apply a user hierarchy where pedestrians are considered first. Most residents start and/or end their trips using an NMT mode. They therefore require the opportunity to use safe, direct and secure NMT routes to their destinations. The premise of this approach is that, if the most vulnerable user of the system is provided for first, provision for the rest (cyclists, public transport users, specialist vehicles like ambulances and finally, ordinary motor vehicles) will be far easier. This will also lead to a design that increases the attractiveness of walking, cycling and using public transport

I.3.2 Characteristics of the existing environment

Decisions regarding transportation infrastructure and services need to be guided by an assessment of the context within which the development will be located. Factors that should be considered are discussed below.

I.3.2.1 The physical location of the proposed development

Constraints and opportunities posed by the project site could influence the transportation infrastructure and service to be provided.

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(i) Topography

The topography of the project site is a key factor when making decisions regarding the direction, hierarchy and layout of the roads and streets. A site that is characterised by slopes has a significant impact on the provision of municipal engineering services, especially stormwater management. It could also affect the provision of NMT facilities, as the gradient should ideally not be more than 5% for pedestrian and cycle routes. Gradients also affect stopping distances that have to be provided for motorised vehicles, as well as road pavement design and appropriate road surfacing.

In addition, the structural design of roads may also be influenced by topography: where roads and streets cross height contours at an angle, or even perpendicularly, drainage design becomes a challenge and in such cases function rather than structure may require that a road or street be paved or provided with effective erosion protection. A sloping site could mean that additional costs would have to be incurred when constructing a street. The maximum gradient for the use of road surface asphalt paving is 14%. Streets that are steeper than this might have to opt for block or concrete pavement with accompanying support beams.

Topography also affects unpaved streets: In rolling and mountainous terrain there may be steep gradients that result in the erosion of street gravel and, in particular, erosion of their drainage facilities, with direct implications for their safety and functional use. A gradient of 5% is an average value above which erosion problems may occur on unpaved streets, and slopes steeper than this would warrant additional attention and protection. Gravels in the upper range of the Plasticity Index (PI) could effectively reduce erosion, but local conditions should be considered in the detailed evaluation.

 

Water drainage is an important factor to consider when assessing the topography of a development site. Water is one of the primary causes of premature failure, accelerated distress and reduced structural capacity of road infrastructure, and it is therefore essential that attention be paid to stormwater management from the outset (see Section L).

(ii) Climate

Climate has an impact on the structural design of road pavements. The meteorological environment is divided into macro-climatic regions with different moisture and temperature conditions. The moisture condition affects the weathering of rock and the durability of weathered material. The moisture conditions also affect the stability of unbound layers, depending on the drainage conditions, the surfacing layer integrity and the moisture content. On the other hand, temperature conditions largely affect the design of surfacing layers, particularly hot mix asphalt. Pavement designers should always consider climatic conditions and avoid using materials that are excessively water-susceptible or temperature-sensitive in adverse conditions.

(iii) Geotechnical characteristics

The in-situ ground condition of a site can sometimes necessitate the use of specialised construction methods or materials, or it can mean that certain areas of the site might not be suitable for construction. A proper preliminary soil survey should be conducted as the characteristics of the underlying subgrade will have implications for the structural design of the road. The specific considerations regarding the subgrade are discussed in Section I.4.2.5.

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Aspects such as the material depth and the classification of the subgrade material are addressed. However, the following questions need to be answered during the initial analyses of the local conditions to inform the layout and network configuration of the proposed development as well as the subsequent geometric and structural design phases:

  • What are the soil characteristics and quality? Classification of the subgrade material should be done.
  • Are there any aggressive chemicals or minerals present?
  • Is the site part of or close to a dolomitic area?
  • Was the site used for mining and exploration in the past?
  • Are there large rock outcrops on the site?
  • Are there gullies or other ditches on the site?
  • Is ground water present on the site?
  • What is the height of the water table?
  • Did dumping – legal or illegal – ever occur on the site?
  • Is the site subject to seasonal flooding?

(iv) Landscape and ecology

The physical features of the landscape could have an impact on the layout of streets and the choice of pavement material. If the development is located in or near an ecologically sensitive area, there may be restrictions that may influence the layout of streets. Ensure that information is collected regarding the following:

  • The position of any telephone poles, overhead power cables, rock outcrops, water features, dongas, etc. that could restrict building work or may require approvals from various government departments.
  • Wetlands, surface water bodies or other ecologically sensitive areas on or near the site
  • Endangered or protected animal or plant species on or near the site.
  • Existing vegetation, especially trees, and whether they are deciduous or evergreen, indigenous or alien.
  • Natural features that may have cultural significance.

(v) Adjacent land uses and edge conditions

Adjoining properties have an impact on each other. Therefore, it is important to be aware of the land uses adjacent to the development site, as well as the edge conditions that affect the site. Some of the questions that need to be answered include the following:

  • What are the adjacent land uses and how could that potentially influence decisions regarding transportation infrastructure to be provided as part of the proposed development? In particular, surrounding land uses are relevant for public transport planning.
  • What are the local destinations (such as shops, schools, bus stops) that occupants of the new project will be wanting to access?
  • How can the new development best be linked to these to encourage walking and cycling?

(vi) Access to the site

Any development has to be connected to the surrounding area and to the settlement as a whole. The layout of streets and the provision of transportation facilities are influenced by the location of access points and existing footpaths and routes. The following questions need to be answered:

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  • What are the existing and potential vehicular, cycle and pedestrian access points to the site?
  • Are there existing footpaths / routes across the site? Where are these routes originating from and where are they going?
  • Can the existing footpaths / routes be accommodated in the new development?
  • Where are public transport facilities in the surrounding areas located in relation to the site? These may include commuter railway stations and services and other public transport facilities such as bus and mini-bus taxi terminals and stops.
  • Are there existing public transport routes through or near the site? Where do these routes intersect or join with the pedestrian desire lines and the shortcuts that people take?

I.3.2.2 Available infrastructure and services

Developments create additional demand for services and may therefore have an impact on existing transportation infrastructure and systems. It may be necessary to document and assess the existing transportation infrastructure and public transport networks and services, including NMT facilities. This information can be used as the starting point for the planning and design of new networks and facilities, or the upgrading of existing infrastructure and services.

The location of existing public transport infrastructure such as stops, terminals and ranks should be documented. Public transport operations and routes should also be identified. For example, information regarding rail services, train station locations and feeder services, bus and minibus taxi services and main routes, BRT routes and stations should be collected. Existing NMT facilities must also be identified and assessed. These may be formal and/or informal, for example to and from existing public transport facilities, at schools, hospitals and other social and commercial amenities.

I.3.2.3 Existing socio-economic features

The planning and design of a development have to be guided by the potential needs of the residents of both the new development and existing neighbourhoods. If the community that will move into the proposed development is known, it is critical to understand their needs and involve them in the decision-making process from the outset (see Section E). It is also important to acquire information regarding the socio-economic features of the neighbouring communities. This will provide some indication of the transportation infrastructure and services that have to be provided. The following questions should be answered with respect to the existing community (if known) and the adjacent neighbourhoods, especially those that are functionally linked to the development:

  • How many people live there?
  • What is the average size of households in the area?
  • What is the age profile of the residents?
  • What is the income profile of the residents?
  • What is the employment profile of the residents?
  • What types of housing are people living in?
  • Do residents have access to private cars?

I.3.3 Transportation infrastructure and road pavement options

The design of transportation infrastructure and services should cater for the travelling needs of individuals and the need for products to be conveyed. Transport users generally make decisions about destination, mode of travel, departure time, desired arrival time and route. The decisions take place within constraints that include budgets and the availability of supporting services and infrastructure in the network.

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It is important to provide prospective users of a transportation system with sufficient options to allow them to choose an option that would adequately satisfy most of their needs. However, when transportation planners and designers have to make decisions regarding the type, capacity, coverage and cost of transportation infrastructure, the needs of individuals have to be balanced with the needs of society. A critical consideration would be the potential impact of a chosen transportation option on the environment. In addition, the structural design of transportation infrastructure tends to be guided by restrictions imposed on it by geology, topography, design traffic, construction materials, and accessibility and mobility requirements.

The transportation infrastructure and road pavement options discussed below essentially involve the geometric design of streets, and the types of road surfaces and pavement materials available. However, before particular options can be selected, various factors need to be considered. Some of these factors are outlined below before the possible options are discussed.

I.3.3.1 Factors to consider when choosing transportation and road pavement options

In addition to the aspects highlighted in Section I.3.1 and Section I.3.2 , various other factors need to be considered when decisions have to be made regarding transportation demand, needs and infrastructure options. Assessments may be required to gain an understanding of, for instance, the following:

  • The estimated number and nature of trips that will be generated by the proposed neighbourhood
  • The nature of existing traffic in the area surrounding the proposed neighbourhood
  • Planned developments, land-use changes and transportation infrastructure and services in the vicinity of the proposed neighbourhood

Based on the information gathered, the anticipated trip demand can be assigned to the transportation network. These aspects are briefly discussed below.

The estimated number and nature of trips generated by the proposed neighbourhood

Three distinct trip generation estimations need to be made, namely vehicle-based trips, public transport trips, and non-motorised trips. Estimates are usually based on traffic expected during a chosen peak hour or peak time. Traditionally, the morning weekday peak hour is used since this is normally the time period during which the highest number of person trips are made, and hence needs to be accommodated by the infrastructure. However, other factors may also have to be considered. There may be conditions peculiar to the development site, which may result in other forms of peaks, for instance travel may change seasonally due to holiday-makers visiting the area or leaving the area, or if special events are held in or close to the neighbourhood.

The information required to estimate trip generation includes the following:

  • The socio-demographic and economic characteristics of the neighbourhood. This information often provides an indication of the number, pattern and modes used for trip making.
  • The number of households, their location in the neighbourhood, and the type of housing that is to be supplied will have an impact on the transport demand and influence the modes used.
  • Traffic generators including education facilities, hospitals and clinics, shopping/retail areas, transport terminals (road-based, rail and airports) generate and attract high concentrations of trips and should be treated individually.
  • Land use types such as commercial, office or retail facilities may also generate particular trip patterns.

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  • Specific developments that could reduce motor vehicle trip making, such as transport-oriented developments.

A distinction should be made between trips that will have origins and destinations within the neighbourhood (e.g. trips made between home and school), trips that will be made from the neighbourhood to other destinations in the settlement (e.g. trips from home to work), and trips that may pass through the neighbourhood. In some cases it may also be important to gain an understanding of the distribution of trips according to purpose and the mode of transport (i.e. NMT and motorised trips, as well as the split between public and private transport). This will assist in planning services and infrastructure that will meet the transportation needs of those living in and around the proposed neighbourhood, particularly with respect to NMT.

The nature of existing traffic in the area surrounding the proposed neighbourhood

The estimated number of trips that the proposed neighbourhood will add to current motor vehicle traffic should be considered in conjunction with the existing traffic situation (‘background traffic’) and transportation demand. The ‘background traffic’ refers to traffic demand that would have materialised irrespective of the development project under consideration. The annual rate of growth of this demand should be taken into consideration.

In some cases a detailed quantification of the existing transportation demand may be required (including vehicular traffic, NMT and public transport). Some of the data required may be available from local, provincial and national transport authorities and transport agencies. However, it may be necessary to conduct transport surveys to supplement the demand data available from secondary transport data sources. The need for surveys, and the types of surveys required, would be determined by the availability of secondary data sources, the context and size of the proposed development, and whether or not transport simulation modelling will be required. Different types of traffic surveys can be employed, including traffic count surveys (usually conducted at intersections and on road links), public transport passenger and NMT surveys (to estimate the demand on current transport systems and/or at public transport terminals, numbers of passengers accessing public transport facilities by foot, etc.) and origin - destination traffic and passenger surveys (to determine the origins and destinations of vehicles, and sometimes passengers).

Planned developments, land use changes and transportation infrastructure and services

The potential impact on traffic demand and transportation infrastructure of other planned developments in the vicinity of the proposed neighbourhood should be carefully considered. The transportation infrastructure needs of these developments will affect the proposed development. Information should also be obtained regarding potential changes in land use and any latent land use rights for development (rights that exist but have not been exercised).

It is important to know what transportation infrastructure is planned for the areas adjacent to and surrounding the proposed development. Liaise with the municipal, provincial and national transport authorities and transport agencies to understand planned new roads, public transport services and facilities and non-motorised transport facilities within the area. The proposed timing and phasing of these improvements should also be considered. They would normally go ahead whether the new neighbourhood is developed or not, but the new project may change the time lines and priorities assigned to these improvements.

Assign the future trip demand to the network

Based on all the information gathered, the future trip demand needs to be assigned to the various links and intersections making up the networks. This would allow for the demand flows on the links and at intersections to be estimated, and the required capacity of the links and intersections to be modelled. The trip assignment process ismost commonly undertaken using simulation software that provides estimates of vehicle queues and delays at the intersection during the modelled period.

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The resulting vehicular demand and capacity relationships across the network will determine the levels of service of the future network. Road-based vehicle levels of service in urban areas are most commonly influenced by the intersection layouts and controls, and the level of service is defined by the average delay experienced by a driver on an approach to the intersection. If the level of service is not initially satisfactory, the transport planner must introduce roadway and/or intersection improvements to alter the demand-supply relationship and hence establish acceptable levels of service. The analysis will also determine the type of intersection control that is required, i.e. stop, traffic circle or signals. In all cases, the provision for NMT at intersections is critical.

The roadway and intersection cross-sections and land requirements are determined as part of this process. The cross-sections must also cater for NMT and public transport vehicle infrastructure. If on-street parking is to be provided in certain areas, this must also be included in the cross-section. NMT and motorised vehicle conflicts should be avoided as far as possible when designing the intersections and NMT roadway crossing points.

Levels of service for NMT and public transport facilities are commonly determined by traveller densities in confined spaces such as stops and terminals, and by passenger flows in restricted spaces such as walkways and bridge crossings. Types of control at the NMT and motorised vehicle traffic conflict points must be carefully considered to ensure safety and convenience.

The arrangement of NMT and traffic at generators such as schools and hospitals must receive specific attention. At schools, the conflicts between vehicles and pedestrians and cyclists must be identified and carefully planned to ensure the maximum levels of safety. At other facilities such as hospitals and large retail facilities, individual vehicle access, egress, NMT and parking studies may be required.

 

Transportation simulation software

Transportation simulation software is used as a tool to assist transport planners in simulating the transport demand and supply relationship to determine suitable levels of service in the peak periods. Deciding on the most suitable software depends on the nature of the proposed development, the simulation needs of the transport system and the needs of the transport planner. Several software platforms are available to transport planners, and most reputable platforms are suitable for use. However, there are important issues that planners should consider when deciding whether to use a software platform, and if so, what type of software is most suitable. Transportation simulation can be done at essentially three levels:

  • Macro-level models: These applications are for large area networks, typically regions or metropolitan areas. They are complex models that require substantial data sets for their development and are developed by metropolitan and provincial transport authorities. The transport planner may be dependent on such a model to obtain sub-area traffic and passenger demands.
  • Meso-level models: These models are network-based and used to simulate smaller areas. They are commonly used for urban settlement trip simulation, and can simulate detailed intersection operations.
  • Micro-level models: These models are most commonly used to simulate traffic intersections. They can be network-based or simulate isolated intersections. Transport planners are advised against using isolated intersection simulation models in urban areas with complex road networks.

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I.3.3.2 Geometric design options

The purpose of geometric design is to shape the visible, physical components of streets and roads to enable the convenient, economical and safe movement of goods and people, regardless of their mode of transport, and in such a way that the negative impact on the environment and surrounding communities are minimised. The geometric design of streets and roads should support the objectives described in Section I.2.2 and contribute to the creation of neighbourhoods that accommodate all people, including those with disabilities.

The design of streets and roads influences, and is influenced by, the surrounding environment, and it is directly linked to the character of a neighbourhood. It is therefore important to also consult Section F, as the information about geometric design provided here is complemented by the information on neighbourhood layout and structure provided in Section F.

Geometric design involves the three-dimensional design of the road or street, and it is guided by various factors, including the characteristics of the natural environment (e.g. topography, vegetation, soil conditions) and built environment factors such as the nature of the environment alongside the street or road (e.g. houses, shops, street cafes, open space). Critically, not only should the needs of those making use of motor vehicles be considered, but also of those making use of non-motorised transport and of the sidewalks (pedestrians, cyclists, those with disabilities etc.). The key is to have a thorough understanding of the context within which the street or road will be embedded. Furthermore, it is important to adopt a holistic approach when making decisions regarding the geometric design of streets and roads. A range of disciplines may have a role to play, including engineers, planners, urban designers, landscape architects, architects, environmental scientists and hydrologists.

In essence, the geometric design of streets and roads involves the following three key aspects:

  • Alignment (horizontal and vertical)
  • Cross-sections
  • Intersections

These aspects are briefly outlined below. Each aspect involves a range of interlinked components that are described further in Section I.4.1.

(i) Alignment

The horizontal and vertical alignment of streets and roads is closely linked to the layout and structuring of a neighbourhood as discussed in Section F, and the information provided in Section F.4.1.4 should specifically be referred to. More information is also provided in Section I.4.1.5.

Horizontal alignment
The horizontal alignment of a road or street is essentially the route that it follows if viewed on a map. From a geometric design perspective, it involves the design of the curves and straight sections (tangents), guided by certain calculations, to ensure the safety and comfort of the users of the street or road. The process to determine the horizontal alignment of a street or road is iterative in nature since various factors need to be considered, including the local topographical and other characteristics of the physical environment, as well as the layout and structuring options outlined in Section F.3.3 and design considerations discussed in Section F.4.

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Vertical alignment
Streets and roads include sections that are sloping up or down, and sections that are commonly referred to as flat. This line that a street or road follows up and down a vertical plane is referred to as vertical alignment. As with horizontal alignment, factors that need to be considered include the topographical and other characteristics of the physical environment, as well as the layout and structuring options outlined in Section F.3.3 and design considerations discussed in Section F.4.

(ii) Cross-sections

The cross-section of a street or road has to accommodate moving and parked vehicles, non-motorised vehicles such as bicycles, pedestrians, wheelchairs, engineering infrastructure and utilities such as stormwater drainage, water, electricity, communications and sewer trenches (also see Section F.4.1.5). In some cases, it is also required to provide space for more than just movement-related activities such as formal and informal trading, socialising and recreational activities for children. Neighbourhood streets are often regarded as public open space and may have to fulfil a range of functions (see Section G.3.3.2).

The cross-section (also referred to as the road reserve) may comprise all or some of the following components:

  • Traffic lanes for motorised vehicles (including dedicated lanes for high occupancy vehicles such as buses)
  • Cycle lanes (they could be incorporated on the outside of traffic lanes, or they could be located on the verge, either as dedicated cycle lanes or combined with pedestrian walkways)
  • Parking lanes (between the outside traffic lane and the verge, or embayed in the verge area)
  • Verges (the area between the edge of the outside traffic or parking lanes and the road reserve boundary)
  • Sidewalks (the part of the verge that accommodates pedestrian traffic, usually paved)
  • Median islands (the area between the inner edges of the inside traffic lanes of a divided street or road)

The aspects to consider when designing cross sections are discussed in Section I.4.1.

(iii) Intersections

Roads and streets form a network, and it is inevitable that different sections of the network will meet (connect or cross) at certain points to create intersections. The movement of motorised and non-motorised transportation (including pedestrians) is restricted at intersections, and this could lead to congestion and accidents. It is therefore essential to ensure that intersections are able to deal effectively, efficiently and safely with all types of traffic. Intersections can take on various forms, including three-legged (commonly known as T-junctions), four-legged and multi-legged intersections. Different types of intersection control can be employed, for instance signilisation, multiway stop or yield, mini roundabouts, traffic circles.

Pedestrian crossings are critical parts of intersections. They should make it possible for all people, regardless of age, to cross a street safely and conveniently, including people with physical, dexterity and sensory impairments, wheelchair users, people pushing prams, etc.

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The Neighbourhood Planning and Design Guide
Creating Sustainable Human Settlements

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ISBN: 978-0-6399283-2-6
© 2019