(b) encourages walking and cycling and safely provides for leisure activities such as strolling and jogging,
(c) creates a good neighbourhood in which to live, with a sense of place rather than of passage,
(d) is of pleasant appearance and scale, and is in harmony with the local natural environment,
(e) includes adequate parks and open spaces to provide the local community — both children and adults — with spaces for socialising and shared activities,
(f) is an extension of each abutting house and considers the link between each house and the street,
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(g) protects residents from undue noise from vehicles (Section 32.2) and neighbours, dust (Section 12.1.2) and air pollution (Section 32.3) from vehicles, and obstruction of natural light, ventilation, and vistas by nearby houses.
(h) provides adequate privacy,
(i) creates a feeling of local pride, social cohesion and identity, and (j) will be adaptable, in the face of future changes.
Amenity is somewhat less tangible and is defined as reflecting the convenient, pleasing, and agreeable features that raise being at a location above the level of social subsistence.
7.2.3 Design factors
The objectives of street design were listed in Section 7.2.2. The major road and street design factors which determine the details of the movement system in a residential precinct are related to land use, route layout, visual appearance, operating speed and the final design will depend on the relative weighting given to each of these factors at precinct and at suburb level.
(1) Land use within and adjacent to the precinct will mainly concentrate on the control of traffic generators (Section 31.3.4). They should be located with five key points in mind:
* Trip origins and destinations should be kept as close together as possible to minimise trip lengths (ideally within 400 m, see Section 30.2.2a), and to allow as many trips as possible to be made without recourse to the private car. Separating trips is less important than minimising them.
* Activities which generate high levels of pedestrian traffic should be located on only one side of an arterial street or, if at a major intersection, in only one quadrant of that intersection, unless all pedestrian movements across the arterial street can be controlled.
* Neighbourhood shopping-areas and car parks should be located on distributors or arterials.
* The provision of off-street parking (Sections 30.6.2) should be an integral part of the design of any major traffic generator. Parking exits and entries should be carefully located.
* Major generators creating continuous ribbons (or strips) of development should not be placed along arterial streets as they create continual traffic interruptions and a poor visual environment.
(2) Route layout (or network design) particularly involves defining the relationship between a residential precinct’s streets and the surrounding town or region’s arterial roads (Figure 7.1), in terms of the objectives in Section 7.2.2. At this interface, the layout should control and minimise connections between roads and streets at different levels in the road hierarchy (Section 7.2.5), particularly intersections between minor and major streets. The arrangement of the roads within the precinct is discussed in Section 7.2.4.
(3) The visual perception of a street will be largely determined by the chosen landscaping and alignment treatment (Sections 6.4 and 7.2.3). The need for the street’s traffic function to be unambiguous in terms of the various street classes shown in Figure 7.1 requires that each class of road should be sufficiently different in visual treatment to allow immediate identification of its position in the total network hierarchy. Vegetation and alignment both can dramatically influence visual perception.
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(4) Road design is largely determined by the assumed vehicle operating speed (Section 18.2.10). This speed depends on whether the street is in:
* a new area, in which case the chosen design speed (Section 18.2.6) will determine such factors as pavement width and street alignment (Figure 7.2 and Sections 6.5 & 18.3),
* an existing area, in which case operating speeds may need to be controlled or manipulated by traffic devices in the manner described in Section 7.3 and 18.1.4). Control of vehicle speed is the greatest single factor influencing the liveability of a street system. Residents perceive a greater degree of safety on the street and noise levels are kept at acceptable levels.
(5) Intersections can also be used to control vehicle behaviour. The devices listed in Section 7.3 can be used at intersections to provide pseudo-gateways into residential precincts or small towns to announce the changed environment to drivers. Typical gateways are distinctive entrance structures, speed signs, sharp bends, road humps and plateaus (Section 18.1.4), rough paving, road narrowings (Section 7.3), roundabouts (Section 20.3.6), channelisation, and pedestrian crossings. They may not so much reduce the speed by physical means as create a driver perception of a low-speed area.
7.2.4 Street systems
Within a residential precinct, cars, pedestrians and cyclists are separated wherever possible. Where this is not possible, the design should ensure that the behaviour of each road user is apparent to and predictable by other users.
The traditional street layout utilises the rectangular grid. However, it can create long through streets with frequent intersections. Two of the more adventurous layouts that aim to alleviate the effect of through traffic are:
* Radburn, which segregates vehicle and pedestrian traffic at the local level by giving houses separate access to networks of roads and paths, and
* Cluster housing in which the dwellings are clustered around culs-de-sac and landscaped spaces, with the rest of the development being left for communal use. This system provides all residents with freedom from through traffic and an enhanced outside living area.
Pedestrian-only areas (or pedestrian precincts), such as shopping malls, should be developed where possible (Section 30.5.3). However, thought must be given to the servicing of any adjacent premises — particularly shops. Part-time malls are one way around this problem, with servicing restricted to non-pedestrian hours.
The development of a residential street system is usually controlled by local government land-use (or subdivision) codes, building and planning ordinances, and market forces. With the best of intentions, all of these usually restrict design innovation. In addition, such past influences have often left many planning issues unresolved. For instance, many people in conventional residential precincts live in houses fronting onto arterials, sub-arterials, or precinct distributors, and thus with through traffic passing their door.
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7.2.5 Street hierarchy
The common approach to road hierarchy within a residential precinct has already been illustrated in Figure 7.1 where arterial and sub-arterial roads are the traffic carriers, with the distributor roads handling decreasingly lower traffic flows. The pattern outlined represents a tributary system characterised by a gradation of road types, with each lower road acting as a traffic tributary delivering traffic to roads higher in the hierarchy. The road hierarchy concept is used to:
(a) identify the organisation responsible for each road and street, (b) allocate funding priorities,
(c) set road and street design standards (Section 7.2.3),
(d) provide a rational basis for locating land uses that generate significant traffic (Section 7.2.3),
(e) allocate access control, traffic priority, and routing, (f) plan and provide traffic-restraint measures,
(g) provide drivers with assurance as to the traffic-related expectations of them and of other drivers, and
(h) operate measures to prevent driver behaviour that is inappropriate for the road being used, given its position in the hierarchy.
Unfortunately, road hierarchies and the associated road classifications are not always a useful concept in existing residential areas. This is because a large proportion of the existing road system services a mix of purposes, with many streets serving abutting houses also acting as distributors, taking traffic to other streets and with many precinct distributors and arterial roads having full roadside access from each abutting property. For example, the local distributor uneasily services its dual functions of traffic carrier and house-frontage function, a problem which persists in many current layouts.
Thus, despite the many words written over the years about road hierarchies, it is not surprising that this lack of real application exists, as the philosophy as originally expounded has a number of serious flaws. For example, the ‘classical’ concept of a gradual gradation of road types cannot be sustained within a residential precinct. Instead, new residential streets should be based on the principle that a road is either totally for local access or totally for traffic movement (Figure 7.3). This is quite feasible, as minor distributors can have total access control by careful street layouts and house sitings. The above criticism of road hierarchies applies only to existing residential precincts; a more positive view of their usefulness for considerations of wider areas and for major roads is given in Section 30.7.
In a residential precinct the through-traffic function is subordinate to its distributional, pedestrian, and social functions, and design speeds are kept low. This requires a sufficiently small spacing of distributor and arterial roads to provide adequate overall mobility. For example, for a low-density suburban housing development, arterial and sub-arterial spacings of the order of 1.5 km or grids of about 2.5 km2 in area have
been found to be effective, although intermediate distributors are also needed to service this system. These distributors should carry less than about 500 veh/d. Suburban population densities range from about 500 to 25 000 people/km2 with 3 500 as a common
average. At about 2.5 trips/day (Section 31.3.4.1), this gives between 1 000 and 60 000 veh/d, but less vehicle trips are usually generated by high-density areas. Each square kilometre enclosed can therefore be assumed to generate about 10 000 veh/d trips.
Section 7.4.1 shows that the environmental capacity of a street is about 3000 veh/d and so the above data imply that about three distributors (or arterials) are needed for each
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