Where weather meets transport
This is the step the rest of the framework exists to enable.
On their own, the two components produce useful but incomplete answers. The hazard model knows which roads flood. The transport model knows who was driving. Neither knows what happens when you put those two facts together, and that intersection is where the actual damage lives.
What the intersection reveals
Overlaying the hazard in space and time onto the transport simulation shows how a specific event, such as an extreme rainfall event, hits the system from both sides:
| Side affected | What it looks like |
|---|---|
| Supply | Flooded roads, closed links, reduced capacity |
| Demand | Delayed trips, uncompleted trips, journeys never started |
The second column is the one that is hard to get any other way. A closed road is visible on a map. A cancelled hospital appointment is not.
Turning impact into numbers
Once the disruption is identified, performance metrics quantify it. The IMP assesses impact from three perspectives, because a single headline figure hides more than it reveals:
| Perspective | Asks | Example |
|---|---|---|
| Economic | What did it cost? | Costs arising from cancellations and travel delays |
| Social | Who bore that cost? | Equity analysis across population groups |
| Environmental | What else did it cause? | Increases in polluting emissions |
Why three, not one
Two interventions can look identical on cost alone while distributing that cost very differently across communities. Reporting the three dimensions separately keeps that difference visible, and it is usually the difference that decides which option is defensible.
In detail
The full metric set, organised under the POESE framework covering physical, operational, environmental, social and economic impacts, is described in Performance metrics.