Scenario testing gives planners the foundation for land use and infrastructure integration, and the evidence to understand the implications of land use changes, before committing to them.
Every zoning change, density uplift or new growth front is a 50-year funding decision.
The infrastructure costs that follow from those decisions are locked in long before the first development application arrives.
By the time infrastructure issues do surface, the land use decisions driving them are also already locked in.
When it comes to implementation, many strategic plans have been undermined by infrastructure challenges. While we know what good integration of land use and infrastructure looks like, the challenge is that:
We’re not fully understanding or grappling with the implications of land use decisions early enough to change direction.
We’re using Development Assessment to solve issues that should’ve been addressed earlier.
Scenario testing (particularly with urban growth models) gives us a way to see and understand those consequences at the strategic planning stage, and then commit to the best decision.
What does scenario testing actually involve?
Scenario testing is a method used to explore how different possible futures or situations might play out, and how decisions or systems would perform under those conditions.
Effectively, it’s the process of asking “What if?” and examining the consequences.
In the context of planning for new homes, scenario testing usually involves identifying potential changes to the land use pattern (zoning, building heights, densities, etc) through multiple scenarios to:
- identify which one delivers the most housing supply
- while balancing liveability considerations, and
- the ability to service those homes with the necessary infrastructure.
Using an urban growth model to quantify what those changes produce gives a consistent, ‘rules-based’ structure that allows for interface with infrastructure models.
An example could be:
1.
Calculate the resulting number of homes, population and employment
2.
Test those outputs against infrastructure network capacity
3.
Identify what new or upgraded infrastructure would be required
4.
Cost the infrastructure implications under each scenario
The model doesn’t make the decision. It shows what each option costs, what works and what doesn’t before anyone commits to a preferred outcome.
If the tools exist, why isn’t this happening more?
Building and testing scenarios as part of strategic planning activities is not new. But I feel that we don’t do it enough. When it is done, it’s often without a strong evidence base or the structure and consistency of a rules-based urban growth model.
Four barriers often get in the way of using urban growth models in scenario testing:
Expertise
Urban growth models are highly technical work with complex systems. Fewer planners have hands-on experience with them.
Complexity
Planners and decision makers can be ‘scared off’ by the complexity of the modelling task.
Time
Calibrating, preparing inputs and running models takes time. Project timelines often don’t allow for multiple ‘runs’.
Cost
Establishing and maintaining an urban growth model is a major investment for local government.
While these are real constraints, the cost of not testing is higher. When land use decisions outpace infrastructure planning, local governments fund the gap or pass it on through charges.
What good scenario testing needs
1. Evidence
A shared data foundation that all parties work from.
Why this matters:
Infrastructure providers often work from different projections and assumptions. We can’t align decisions if we don’t start from the same place.
What this looks like:
Common forecasting, consistent assumptions and repeatable methodologies. Less time debating the data, more time discussing what it tells us.
2. Iteration
Multiple rounds of testing where land use and infrastructure shape each other.
Why this matters:
A single pass isn’t enough. Integration doesn’t happen by simply declaring “land use leads” or “infrastructure leads.”
What this looks like:
Land use shapes infrastructure demand; infrastructure capacity shapes land use. Multiple rounds let teams test shifts and refine strategies.
3. Feasibility
Costing infrastructure implications of each scenario before committing.
Why this matters:
Every density change and new growth front carries long-term financial consequences that are invisible in a static plan.
What this looks like:
Reveals funding envelopes, lifecycle costs, network impact magnitude, sequencing needs and whether current settings support the growth pattern.
What happens when we skip this step?
When scenario testing doesn’t happen at the strategic planning stage, the integration task gets pushed downstream:
- No decision gets made, due the lack of comfort.
- Land gets rezoned for outcomes that can’t be affordably serviced
- Infrastructure issues surface during development assessment, not before
- Negotiations become protracted and expensive for local government and developers
- Infrastructure charges rise, with the local government wearing the cost of any shortfall from capped charges
Scenario testing makes those risks visible early enough to plan around them and take the best option available.
Making space for this work
As planners, we need to use the urban growth modelling tools at our disposal at the strategic planning stage, not leave integration to be resolved in Development Assessment, or worse still, end in decision-making paralysis.
Every rezoning decision that outpaces infrastructure planning creates costs someone must carry. The earlier we test, the fewer surprises we get later.