A Traffic Impact Assessment (TIA): also called a Traffic Impact Statement (TIS), evaluates how a proposed development will affect the surrounding road network, and is a common requirement for development approval in Australia.
When a TIA Is Required
Local planning schemes typically trigger a TIA based on:
- Development size (e.g. dwelling count, gross floor area, parking spaces)
- Land use type (higher trip-generating uses like retail, schools, industrial)
- Proximity to sensitive intersections, school zones, or classified roads
Core Components of a TIA
| Component | Purpose |
|---|---|
| Existing conditions | Road network, traffic volumes, crash history, existing intersection performance |
| Trip generation | Estimated vehicle trips generated by the development |
| Trip distribution & assignment | Where those trips travel to/from, and which routes/intersections they use |
| Intersection performance | Level of Service (LOS) at affected intersections, with and without development |
| Parking assessment | Supply vs demand against relevant planning scheme rates |
| Access design | Sight distance, turn treatments, internal circulation |
| Mitigation measures | Upgrades required to maintain acceptable performance |
Trip Generation
Trip rates are typically drawn from empirical databases (e.g. RTA/TfNSW Guide to Traffic Generating Developments, or jurisdiction-specific guides) rather than derived from first principles.
$$ \text{Trips} = \text{Rate (trips per unit)} \times \text{Development size} $$| Land Use | Typical AM Peak Trip Rate |
|---|---|
| Dwelling (house) | 0.4–0.6 trips/dwelling |
| Dwelling (unit/apartment) | 0.3–0.5 trips/dwelling |
| Supermarket | 5–8 trips/100 m² GFA |
| Office | 1.5–2.5 trips/100 m² GFA |
Level of Service (LOS)
Intersection performance is graded A–F based on average delay per vehicle:
| LOS | Average Delay (signalised, sec/veh) | Description |
|---|---|---|
| A | 0–14 | Good, minimal delay |
| B | 15–28 | Good |
| C | 29–42 | Satisfactory |
| D | 43–56 | Approaching capacity |
| E | 57–70 | At capacity |
| F | > 70 | Oversaturated, unstable |
Councils/road authorities typically require LOS D or better at critical movements post-development (varies by jurisdiction and intersection type).
Intersection Analysis Tools
| Tool | Typical Application |
|---|---|
| SIDRA Intersection | Signalised and unsignalised intersection modelling (widely used in Australia) |
| Micro-simulation (e.g. Aimsun, VISSIM) | Network-level analysis, complex corridors |
Access and Site Design
- Sight distance at the site access (see Stopping Sight Distance)
- Turn treatments (e.g. BAR/BAL, CHR/CHL per Austroads guidance) based on turning volumes and speed environment
- Internal circulation: swept paths for design vehicles (service vehicles, garbage trucks, fire appliances)
Mitigation Measures
| Issue Identified | Typical Mitigation |
|---|---|
| Intersection LOS falls below threshold | Additional turn lane, signal phasing change |
| Insufficient sight distance at access | Relocate access, speed limit review |
| Pedestrian/cyclist conflict | Refuge islands, dedicated crossing treatments |
| Parking shortfall | Increase supply or demonstrate demand management |
Practical Notes
- Trip generation rates are averages from surveyed sites: always sense-check against the specific development context (e.g. a supermarket anchored by a fuel station will generate more, shorter trips than the base rate assumes).
- "With development" LOS should be compared against a realistic future baseline (including approved but not-yet-built developments), not just current traffic: otherwise cumulative impact is understated.
- Mitigation measures need a funding and implementation mechanism (developer works, contribution, or council capital works): an assessment that identifies a fix without a delivery pathway is incomplete.