A11.13 - Checking growth constraint or variable matrix methods

A11.13 - Checking growth constraint or variable matrix methods

When to use

These checks are related to the procedures in appendix A3.3 and may be used to check the appropriateness of growth constraint or variable matrix methods for dealing with suppressed and induced traffic. The checks supplement the general model validation guidelines given in worksheet 8.

Suggested checks

Suggested checks include.

Method used Suggested information
The capacity of the do minimum network was upgraded
  • demonstration that the capital cost of do minimum improvements is less than 10-15% of the project option cost
  • indication of adequate capacity (see below).
A growth suppression technique was used (eg, matrix scaling, incremental matrix capping, shadow network, elasticity method on the do minimum)
  • indication of adequate capacity (see below)
  • details on the size and location of the suppressed travel
  • evidence, where feasible, of network performance before and after growth suppression
  • details of the methodology applied.
Peak spreading was used
  • evidence of current variations in peak proportions:
    1. within the study area, in the base year and historically
    2. between cities or across New Zealand.
  • based on this evidence, an indication that current traffic profiles in the study area are relatively peaked
  • forecasts of a decline in peak period speeds relative to the interpeak (because peak spreading is more likely to occur when peak speeds deteriorate faster than interpeak speeds).
A variable matrix technique was used (eg elasticity method on both the do minimum and project option)
  • indication of adequate capacity
  • differences between the do minimum and project option matrices
  • evidence of the convergence of the method (ie, stable estimates of costs and matrices), or other evidence to justify reliance on forecasts (see worksheet 8.4, part D
  • details of the methodology applied.

Checking capacity in the do minimum and project option

To check the do minimum and project option capacity, the following performance indices may be used. If the indices suggest congestion over large or significant parts of the network, judged on the basis of at least one hour of flow, then the network should be considered as congested. If, however, the congestion occurs only in the later years of the economic life of the scheme (which contribute very little to the BCR), these effects may be ignored where reasonable.

Performance indices Indicator of significant congestion
Level of service. Level of service E or F*.
Matrix feasibility. Network model is unable to achieve a stable realistic assignment.
Plots of link volume to capacity ratios or manual calculation of the ratio (see appendix A3.17). Ratios consistently higher than 1.0.
Link speed plots. Speeds consistently below realistic values (15-25 km/h) for links of average length.
Junction delay statistics. Delays consistently longer than
5 minutes per junction or queues 'blocking back' to upstream links.

* Level of service E occurs when traffic volumes are at or close to capacity, and there is virtually no freedom to select desired speeds or to manoeuvre within the traffic stream. Level of service F is in the zone of forced flow where the amount of traffic passing a point exceeds that which can pass it. Queuing, delays and flow breakdown occur at these flow levels. (Source: Austroads).