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Dynamic Transport Models

Dynamic transport modelling is a method of assignment developed by Gabites Porter Consultants to resolve problems with the standard assignment procedure. Generally, most assignment procedures make the following simplifications:

• All trips which leave their origin in the period will reach their destination during the period. The trip generation rates are usually calculated on the basis of including those trips that began in the period (or ended) in the period. It is seldom comprised of trips that began and ended in the period.
• That the destination choice and mode choice (the distribution and mode split stages) can be made on the basis of average travel costs over the period.
• That the route travel times for buses can be taken from the average times over the period.
• That the counted volumes on a link represent demand rather than supply.

In reality, none of these assumptions hold and they can create significant inaccuracies in congested networks. There are two main differences between standard assignment models and Dynamic assignment models:

1. Dynamic Models break flows trip matrices into smaller slices before being loaded onto the network. Each new slice of traffic is presented with traffic conditions created by the previous slice of traffic i.e. output time and distance matrices from the previous slice are used as a primer for the next slice of traffic. This also means traffic signal timings are re-optimised for each slice of traffic.
2. Dynamic models remove the requirement by traditional assignment procedures that every trip gets to the destination in the time period. This:
   • Enables Peak spreading to be evaluated; and
   • Provides the ability to capture and analyse the wave effect of traffic flowing through the network.
   • Ensures a more realistic level of traffic is using the network. By assuming all trips reach their destination tradition assignment procedures over estimate the amount of traffic on the network.

We have developed several such models in both New Zealand and Australia:

• North Shore City, New Zealand
• Wollongong City, Australia
• Nelson City and Tasman District, New Zealand

North Shore City, New Zealand

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North Shore is a city located to the north of Auckland City. In 2001 in had a population of 185,000 an extensive transport network, and substantial prospects for growth. Given its proximity to Auckland city, the main employment hub of the region, intercity links are very important. While there has been a gradual trend towards self-sufficiency the main transport problems for North Shore remain intercity capacity constraints. Specifically:

• Commuter trips into Auckland in the morning and back to North Shore in the evening; and
• Through traffic originating in areas north of North Shore using the north shore network to reach destinations in Auckland.

We have been involved in the modelling of North Shore's transport network since the early 1990's and have expanded the model's capabilities considerably. We have recently been involved in upgrading the

The North Shore model:

• Covers both the city and significant urban areas to the north and south;
• Uses dynamic assignment procedures;
• Contains a comprehensive coverage of public transport services; and
• Includes private and commercial trips separately.

Wollongong City, Australia

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Wollongong is a city situated to the south of Sydney with a population of 185,000. It has experienced significant growth over the past 20 years and has particular transport modelling requirements. Recent work GPC have been involved in includes detailed modelling of train passenger services in the city linking to the Sydney CBD.

We developed a TRACKS Transportation Model that:

• Covers the city up the urban boundary;
• Uses dynamic assignment procedures;
• Has a future model for 2031
• Contains a comprehensive coverage of public transport services; and
• Includes private and commercial trips separately.

Nelson-Tasman Model, New Zealand

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Nelson City and Tasman District are located at the top of the South Island of New Zealand. Nelson city has a population of 42,000 and the Tasman District has a population of 45,000. Most of Tasman's population however is located in the settlements of Richmond and Motueka which lie quite close to Nelson.

The area has experienced significant growth in recent times and this has resulted in a deterioration of traffic network performance. An urban growth strategy was completed in 2005 and predicted further growth that might exacerbate the traffic problems.

Given Nelson has limited opportunities for making improvements to transport infrastructure - it is bounded by steep hills on one side and the sea on the other - the area required transport modelling advice that could provide for the analysis of a full range of alternative solutions.

We were commissioned in 2004 to develop a dynamic assignment model for the area with capacity to test a number of roading, pricing, and public transport options. The model has since been used in different modelling studies:

• 2005 Nelson to Brightwater Corridor Study (Transit NZ, Nelson City Council, Tasman District Council)
• 2005 Marybank Traffic Impact Assessment (Nelson City Council)

The TRACKS Model:

• Covered Nelson City, Motueka, and Richmond;
• Used dynamic assignment procedures;
• Had a future models for 2011, 2021, and 2031
• Contained comprehensive coverage of public transport, cycling and walking trips; and
• Included private and commercial trips separately.