7. Network assessments and corridor/area studies

The system planning process emphasises the importance of a top-down, big-picture perspective, coupled with stakeholder engagement, for identifying transport strategies, plans and initiatives. It is important that such planning is also informed by complementary information such as the results of quantitative network assessments and corridor or area studies.

Quantitative analysis of data is a key input into the planning process. It can be used to draw out major conclusions, develop projections, test and compare the viability of options, and confirm or correct conclusions reached by intuition and dialogue alone.

7.1 Nature of assessments and studies

Network assessments and corridor and area studies analyse transport system performance at a broad, indicative level, sacrificing detail to gain breadth.[1] To be cost-effective and timely, loss of some detail is necessary, but the conclusions of broad studies are not substitutes for detailed assessments. It is preferable, although not essential, to undertake network assessments prior to component corridor and area studies.[2]

A network assessment can provide information to assist with:

  • Identifying problems at a high level
  • Categorising a network into sub-networks, including the corridors and routes to include in the network
  • Selecting priority corridors, areas, routes or links for close attention
  • Considering relationships between corridors and areas
  • Comparing across corridors, and across areas, to ensure that the allocation of funds is broadly in line with transport system objectives and government policies.

A corridor or area study usually considers a single transport corridor or area and provides information to:

  • Suggest infrastructure and non-infrastructure solutions for further investigation to develop into proposals for initiatives
  • Assist in formulating corridor or area objectives by testing the costs, benefits and effects of implementing different strategies, within various budget constraints
  • Explore cross-modal and upstream–downstream relationships between initiatives, including identifying situations where initiatives should be bundled together (e.g. a program of lengthening passing loops)
  • Provide data to assist analysts to estimate the costs and benefits of specific initiatives and to assist governments to check appraisals.

7.2 Data requirements

Network assessments and corridor or area studies are heavily based on quantitative analysis. Some potential data requirements are described in detail in NGTSM06, Volume 5, section 1.3. They include data on:

  • Population
  • Land use distribution – including existing and future areas and key centres designated for residential, commercial and industrial development and transport precincts
  • Economic activity
  • Transport infrastructure
  • Vehicle numbers and traffic composition
  • Flows of passengers and freight, including origin–destination estimates
  • Environmental and heritage considerations
  • Safety
  • Social factors.

7.3 Demand analysis and forecasting

Information about current and future demand for transport services is a key component of network assessments, corridor and area studies, and assessment of initiatives. Approaches to forecasting can be considered in three categories:

  • Extrapolation of past trends
  • Extrapolation relating the forecast variable (e.g. traffic) to one or more explanatory variables (e.g. population), usually through an econometric model
  • Judgment, including scenario analysis.

These methods are not mutually exclusive and can be used in combination. The choice of technique depends on data availability, resources and the validity of the extrapolation process.

Scenario analysis is a potentially useful tool when major change is occurring and the future is highly uncertain. Scenarios can describe a range of possible future circumstances and outcomes. In contrast, the traditional approach to forecasting focuses on a single forecast (typically based on one set of assumptions), which is unlikely to accurately predict ‘the’ future.

The purpose of scenario planning is to identify, and consider the potential impact of, several plausible ‘futures’; for example, different population assumptions. The alternative futures will typically involve significantly different challenges, risks and opportunities for individual activities and organisations. Scenario analysis helps transport practitioners and decision-makers to avoid the trap of thinking (and planning on the assumption) that the future is going to be just like the present, only a little bit more.

Scenario analysis makes the assumptions used in transport planning more explicit and facilitates better management of risk. It can contribute to improved understanding of factors likely to significantly affect transport and the interplay between these factors. Transport practitioners can then more effectively consider the implications for transport planning, increasing the likelihood that resulting strategies will effectively address future demands on the transport system.

Demand forecasting is discussed in greater detail in section T1 for passenger travel, section 4 of T2 and section 2.4 of NGTSM06 Volume 5 in relation to cost-benefit analysis, and section 3 of NGTSM06 Volume 5 in relation to rail transport.

7.4 Deficiency assessment

Deficiency assessment involves comparing the network and its components with specified benchmarks. It is a relatively simple way of obtaining an initial indication of possible problems in a network, corridor, area, or on a route or link.

The deficiency assessment may focus on how transport system performance compares with performance benchmarks or on how the physical infrastructure compares with physical benchmarks.

Examples of performance benchmarks include:

  • Roads: vehicle numbers per lane per day, average vehicle speed, level of service, crash rates and accessibility indices[3]
  • Rail: transit times, contracted train paths, above-rail delays, below-rail delays, incidents, track availability and temporary speed restrictions.

The 2001 mainline rail infrastructure audit provides an example of a performance-based deficiency assessment (ARTC, 2001). ATC endorsed draft performance targets for the mainline rail network to identify potentially worthwhile upgrading investments. These targets are presented in Table 1.

Table 1: ATC draft performance targets for inter-modal trains
CorridorOn-time reliabilitya (per cent)Transit timeb (hrs)Train lengthc (metres)Double stack (conventional containers)

Melbourne-Sydney

75d

10.5

1500

No

Sydney-Brisbane

75d

17.5

1500

No

Melbourne-Brisbane

75d

29.5

1500

No

Melbourne-Adelaide

80

11.5

1500

No

Melbourne-Perth

80

80

56.0

56.0

1500 Adelaide east

1800 Adelaide west

No

Yes

Sydney-Perth

80

80

65.0

65.0

1500 Parkes east

1800 Parkes west

No

Yes

Adelaide-Perth

80

41.0

1800

Yes

Adelaide-Sydney

75d

75d

26.0

26.0

1500 Parkes east

1800 Parkes west

No

Yes

a. Percentage of inter-modal freight services (21-tonne axle loads and capable of a maximum speed of 115 kilometres per hour) arriving not more than 15 minutes after their scheduled destination time.

b. Average scheduled transit time for all inter-modal freight services (21-tonne axle loads and capable of a maximum speed of 115 kilometres per hour) on the corridor, i.e. terminal-to-terminal time making no adjustment for time zones.

c. Unrestricted length for interstate services, i.e. the train length up to which operators can operate any scheduled interstate services without reference to the track manager.

d. On-time reliability for these corridors is expected to be lower in the short term as significant investment is required to improve performance. Note: The term inter-modal trains refers to high performance trains in direct competition with road transport.

Examples of physical infrastructure benchmarks include:

  • Roads: load limits, lane widths, shoulder widths, design speeds, curvatures, gradients, intersections per kilometre and flood immunity
  • Rail: rail weights, speed limits, gradients, lengths of passing loops and height restrictions.

Deficiency analysis based on physical standards has an obvious role to play where the objective is to provide infrastructure at, or above, a certain minimum standard for equity reasons or network connectivity.

Deficiency assessment has several advantages. It is relatively straightforward to implement and can provide a preliminary indication of possible initiatives that may warrant more detailed investigation. The main limitation of deficiency assessment is that it does not take direct account of the economic worth of remedying a deficiency, which is driven primarily by costs and demand.[4]

Therefore, if the aim is to use deficiency assessment to help initial identification of initiatives that have economic worth (i.e. that are economically justified/warranted), benchmarks should be set at levels where intervention is likely to yield an acceptable CBA result. These benchmarks can be set on the basis of experience using the results of past appraisals of proposed initiatives carried out using CBA.[5]

7.5 Economic assessment

Deficiency assessment, using economically warranted justified/benchmarks (as discussed above), provides a very high-level indication of funding across the whole network that might be economically justified. It also highlights parts of the network, corridor or area where there is potential for economically advantageous new initiatives.

The next step in the assessment process is to specify the scope of these potential initiatives in hypothetical broad-brush terms[6] and subject them to conventional economic assessment (i.e. CBA). This can be undertaken at the network level, the corridor or area level, or at both these levels. Because a corridor or area is less extensive than a network, the CBAs in corridor and area studies can be undertaken at more detailed levels than the CBAs in network assessments. This additional level of detail makes it possible to use ‘what if’ scenarios to explore a wide range of options on both the supply and demand sides.

The level of detail of CBAs at network, corridor or area levels is, however, superficial and considerably less than that of the rapid CBAs undertaken in the rapid appraisal of individual options and initiatives in Step 3 (see F3).

Undertaking superficial CBAs for a large number of potential initiatives on a network, corridor or area basis provides a more refined picture of likely future investment needs than deficiency assessment.

In undertaking these assessments, it is important to recognise the potential contribution of non-infrastructure solutions such as demand management, changes in land use policies, and improvements in freight logistics. Non-infrastructure solutions can be integrated into economic assessments (and superficial CBAs) by estimating the effects on transport demand and implementation costs, where these are significant.

The optimal timing of interventions is another key consideration in these assessments. The Bureau of Infrastructure Transport and Regional Economics (BITRE) has undertaken economic assessment of networks using an optimal timing criterion to identify potential investment needs where there is no budget constraint (see section 1.4 in NGTSM06, Volume 5 and Harvey, 1995). This methodology shows that a new construction initiative is justified if its economically optimal implementation time occurs in the past. The economically optimal time for an initiative to come online is the first year when the benefits forgone by delaying the initiative by one year exceed the saving in capital costs. At this time, the benefit–cost ratio (BCR) will be at least 1.0.[7]

With demand projections and potential initiatives, a preliminary time profile of economically warranted initiatives can be developed. Applying the optimal timing methodology usually reveals a backlog of initiatives that are immediately warranted, as their economically optimal times are in the past. The analysis can also be taken a stage further by specifying budget constraints, in the form of funding levels over long periods of time. The timing and mix of initiatives can be adjusted accordingly.

Network assessment and corridor or area studies also provide a framework to assess whether performance targets are feasible given technical, behavioural and other constraints. The studies can also indicate whether performance targets are achievable at an acceptable cost in terms of other objectives sacrificed. The feasibility of performance targets can be tested using the superficial CBA methodology with back-calculation.

The steps are to:

  1. Set a (quantitative) performance target
  2. Estimate the benefits from meeting that target
  3. Specify an acceptable cut-off BCR
  4. Divide the benefits by the BCR to obtain the maximum capital cost of an initiative that will be acceptable to meet the target.

Knowing the acceptable limits for initiative costs, planners can develop options that are more likely to pass detailed CBAs in Step 4 (see F4).

Finally, it should be noted that maintenance initiatives raise unique issues as they are difficult to put into a CBA framework. Section 1.5 in NGTSM06, Volume 5 provides information about deficiency assessment and economic assessment for maintenance initiatives.

[1] Obtaining complete knowledge of all potential initiatives would be an impossibly resource–intensive task. However, partial knowledge may result in failure to identify highly beneficial solutions and lead to wasteful allocation of resources to evaluate initiatives with poor prospects.

[2] As discussed above, system planning is an iterative process, with feedback between network planning and corridor/area planning. Provided that there is interaction, network assessments and corridor/area studies can proceed in any order or concurrently.

[3] Accessibility indices can provide benchmarks for comparing the relative disadvantages of different locations. These indices can also be used to compare initiatives in terms of the extent to which they promote accessibility. Estimation of accessibility indices for medical centres, educational facilities and centres of employment can assist with the investigation of social issues. NGTSM06 Volume 5 illustrates a simple accessibility index. Social indicators may also be helpful.

[4] For example, a deficiency that is expensive to remedy could generate an initiative with a disappointing CBA result. Conversely, a deficiency that is cheap to remedy, and economically warranted, might be missed. With respect to demand, deficiency assessments based purely on the physical infrastructure simply ignore demand considerations. Performance benchmarks related to volume–capacity ratio do take account of demand, but not the economic value of demand. A deficiency on part of the network having low utilisation might not be worth addressing, while an identical deficiency on a highly trafficked section of infrastructure could give rise to a worthwhile initiative.

[5] Lower benchmarks might be set for more highly utilised parts of the network and vice versa. This is particularly relevant if a categorisation system is in place for a network, and the category levels are strongly correlated with usage.

[6] Such as duplicating particular lengths of two-lane road or lengthening rail passing loops.

[7] For more information on optimal timing of initiatives, see the discussion of first-year rate of return in NGTSM06, Volume 5.