Environmental impacts
Codes EN1 to EN3. Disruption does not only cost time. It costs emissions.
Traffic congestion and longer travel distances from rerouting increase greenhouse gas (GHG) and air pollutant emissions, such as NOx and PM2.5. These serve as a measure of the environmental impact of disruption.
The metrics
| Code | Metric | Applies to |
|---|---|---|
| EN1 | Increase in GHG emission (kgCO2e) | Short and long-term |
| EN2 | Increase in NOx emission (kg or μg/m³) | Long-term only |
| EN3 | Increase in PM2.5 emission (kg or μg/m³) | Long-term only |
Why EN1 applies to both, and EN2 and EN3 do not
The split is about how each pollutant does its harm.
Greenhouse gases are a cumulative global stock. Every additional kilogram counts regardless of how briefly the disruption lasted, so EN1 is measured for short-term events too.
NOx and PM2.5 cause harm through local concentration and sustained exposure. A single afternoon of rerouted traffic does not meaningfully change what residents breathe over time, which is why they are assessed only for long-term disruption.
Note the dual units
EN2 and EN3 are expressed as kg or μg/m³, and the choice is not cosmetic.
Kilograms measure how much was emitted. Micrograms per cubic metre measure what concentration people are actually exposed to, which is what health effects depend on.
Required input data
Environmental performance metrics are derived by combining agent-based model transport-activity outputs with appropriate vehicle-emission factors.
| Ingredient | Provides |
|---|---|
| Vehicle counts | How many vehicles |
| Vehicle-kilometres | How far they travelled |
| Traffic distributions | Where and when |
| Vehicle-emission factors | How much each kilometre emits |
Alvarez Castro (2024), for example, demonstrates a scenario-level CO2 calculation based on simulated vehicle-kilometres and external emissions factors.
Emission factors are external
The agent-based model supplies the activity; the emission factors come from outside it. Results are therefore sensitive to which factor set is used, and the source should be stated alongside any published figure.
Where this leads
Emissions do not disperse evenly, and that is what makes them a social question as well as an environmental one.
| Consequence | Metric |
|---|---|
| Pollution concentrates in particular areas, affecting the residents there | SO6 |
| Sustained exposure raises disease burden | SO7 |
| Both carry a monetary cost | EC7 and EC8 |