VOLUME 1 HUMAN SETTLEMENT PLANNING AND ... - CSIR

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GUIDELINES FOR HUMAN SETTLEMENT PLANNING AND DESIGNGUIDELINES ON THE ADAPTATIONAND CONVERSION OF MOVEMENTNETWORKS TO ACCOMMODATECHANGEAs indicated in the previous section, a range ofcontextual features will inform the configuration of asite‘s movement network. In particular, with regard tothe roadway component of the movement network,the actual and desirable level of car use and associatedmodal split of the site’s (and surrounding) populationwill inform the degree to which the system is open orclosed to through-traffic. This section discusses theanticipation of patterns of movement demand toinform the configuration of roadway systems and theadaptation and conversion of roadway systems toaccommodate changing modal split, and changingmovement patterns associated with dynamic spatialpatterns of land-use activity.The anticipation of patterns ofmovement demand within submetropolitanand local movementnetworksThe pattern of movement demand within a submetropolitanor local area is directly affected by thenature and form of the movement network. In thecase of a closed hierarchical roadway system, it ispossible to predict the volume and pattern of traffic(associated with a static spatial pattern of land use andmodal split) that will use each road in the system withsome degree of certainty, as the system presents nochoice of potentially equidistant routes between anytwo points. While it is not possible, in the case of amore open system, to predict the volume and patternof traffic with any degree of certainty - due to agreater choice of local and through routes - a moreopen system is more flexible and integrates the siteinto its surrounding environment better. There ishowever a trade-off between flexibility, integrationand cost. The ability of more numerous links within amovement network to accommodate a dynamic rangeof social, recreational, economic and movementfunctions over time, has capital cost implications - andin many instances greater road reserve width andstronger roadway pavement structure will be required.Every link in a network cannot therefore be designedto accommodate large increases in traffic volumes.To avoid the excessive road construction costsassociated with total flexibility and greater integration,it is necessary, within limits, to predict the possiblerange of movement demand conditions (or scenarios) amore open road network proposal may be expected toaccommodate. More specifically, the need is to identifythose links expected, in the short term at least, toaccommodate a wider range of functions and greatertraffic volumes. Without a reasoned estimate of thetraffic load a particular link or intersection will berequired to accommodate, it is not possible to makeinformed decisions relating to the design ofappropriate roadway cross-sections and pavements, orthe selection of appropriate intersection-controlsystems. The direction, volume and mode of movementgenerated and attracted by a proposed developmentwill be influenced by variables like household size andcomposition, areas of employment, levels of carownership, and use of public transport services. It isimportant that an understanding is gained of theextent of change which is realistically possible withinthese variables, so that the network can be designed toaccommodate such changes.Patterns of movement demand on vehicle-only links,as well as mixed-mode links expected to accommodatelarger volumes of traffic, can be predicted through afour-step modelling process, in which future trafficload, at a point in time (typically 15-20 years into thefuture), is assessed on the basis of a desirable, as wellas the existing, land-use pattern. It is important tonote that the modelling of future patterns ofmovement demand is sensitive to transport and landusepolicy. Extrapolations of current patterns ofmovement demand should therefore be tempered bythe settlement qualities to be created in a particularenvironment, and assumptions relating to the abilityof the public sector to manage transport-relatedmarket forces. The conventional “four-step” modellingprocess is dealt with in detail in the Department ofTransport’s TPG 7 and can be broadly summarised asfollows:• estimating the number of trips that will begenerated or attracted by each zone (often on thebasis of the anticipated population of the zonemultiplied by an average trip rate),• estimating the number of trips that will occurbetween different zones (often represented in theform of an origin-destination matrix, and a desirelinemap),• estimating the relative proportion of modesthrough which trips between zones will occur, andassigning the trips moving between differentzones, by different modes, to particular routes.Patterns of movement demand within a particular sitecan be crudely predicted through the following five-stepsimulation:• estimating the number of trips that will begenerated or attracted by each erf within the site(often on the basis of assumptions relating toaverage number of workers per household and perbusiness unit, and the average number ofschoolchildren per household);• estimating how many of these trips are local trips,and how many are into, and out of, the site;Movement networks Chapter 5.113
GUIDELINES FOR HUMAN SETTLEMENT PLANNING AND DESIGN• establishing trip directions on the basis of thelocation of major movement attractors (e.g. schools,employment and commercial centres) within the siteand in the surrounding area;• estimating the relative proportion of modes throughwhich trips in different directions will occur; and• assigning the trips moving in different directions, bydifferent modes, to particular routes.This simulation process can be conducted for peak andoff-peak conditions, to establish temporal variations inthe nature and pattern of movement demand. Thepatterns of movement demand identified can then beused to establish link volumes and turning movements.Given the uncertainty of future predictions of futureland-use development patterns and associatedmovement demand, and the need for movementnetworks to perform more than simply movementfunctions, it is important that the modelling processesdescribed above be used to test the likely consequencesof movement-network proposals, and not used as thebasis for formulating proposals (beyond minoradjustments to network configuration). Demand-drivenapproaches to movement-network planning and design,based on the traffic load forecast, run the danger ofeither entrenching existing inefficiencies andinequalities, or of simply basing proposals solely onpotentially erroneous predictions of futuredevelopment patterns.The management of changing patterns ofmovement demand within submetropolitanand local movementnetworksThe uncertainty of future predictions of movementdemandpatterns also makes it important to monitorchange (in terms of variables like traffic volumes,speeds and accidents) over time and, where necessary,to adapt or convert movement networks toaccommodate this change. It is important that initialmovement-network designs consider and facilitatepossible future adaptations. Such adaptations orconversions are essentially aimed (a) at managing or“calming” the increased volumes and speeds ofvehicular traffic associated with changing patterns ofmovement demand - more specifically to prevent largevolumes of high-speed, longer distance traffic fromcutting through quieter, predominantly residentialareas, and to slow traffic on roads that experience ahigh mix of pedestrian and vehicular traffic, and indoing so, (b) at maintaining the ability of links toaccommodate a range of movement and nonmovementfunctions. As mentioned earlier, anadvantage of multidirectional movement networks isthat a variety of traffic calming interventions can beapplied to convert the roadway network into a closedor speed restricted system which controls throughtraffic,while maintaining an open footway andcycleway network, and which still enables the roadwaynetwork to be converted back to an open systemshould this be required. The achievement of greaterflexibility therefore has implications for theoperational capacity of local authorities, in terms ofbeing able to monitor change, as well as for theconfiguration of movement infrastructure, and cost.Within local movement networks the ongoing trafficmanagementobjective is essentially to keep the speedof appropriate volumes of traffic low and, in doing so,to make the road as safe as possible for pedestrians.Traffic management, or calming, therefore takes twobasic forms. The first is the reduction of speed throughadaptations to the cross-section and horizontal andvertical alignment of the roadway. The second is thereduction of traffic volumes on certain roads throughconverting roadway network connectivity. It isimportant to note, however, that roadway adaptationscan also reduce traffic volumes by making the routeless attractive to through-traffic, and connectivityconversions can similarly reduce traffic speed.The introduction of these traffic managementmeasures are warranted when the monitoring ofpatterns of movement indicates that certain trafficspeeds or volumes along certain routes have increasedto levels that are not compatible with the range ofsocial and economic functions the route is required toperform (see TPG 14 on “traffic calming”).Internationally, speed reduction measures on localroads are typically deemed necessary when maximumspeeds exceed ± 30 km/h, and on mixed-mode arterialswhen maximum speeds exceed ± 55 km/h. Typically,volume-reduction measures are deemed necessary onlocal roads when traffic volumes exceed ± 600vehicles/h. At slower speeds, drivers have greateropportunity to perceive and react to a situation, thushelping to reduce the number and severity of collisions.Roadway systems can be adapted to manage trafficspeed, through the introduction of traffic-calmingmeasures as illustrated in Figure 5.1.10.Roadway systems can be converted to manage trafficvolume, through the introduction of traffic calmingmeasures as illustrated in Figure 5.1.11.Empirical studies on the effectiveness of trafficmanagement measures indicate that legal speed limits(in the form of “speed zones”) have little effect ondriver behaviour, and that it is rather the physical oroperational characteristics of the road that determinedriver behaviour. Traffic management measures shouldtherefore be self-enforcing. Studies suggest thatdifferent self-enforcing measures have variable impactson traffic volume and speed - apart from speed humpsand speed tables, “adaptation” measures tend to havea greater impact on traffic speed, than on trafficvolume, and “conversion” measures have a significantimpact on both speed and volume.14Chapter 5.1Movement networks
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Chapter 2A framework forsettlement-
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