Everyone defines one. No one draws them together.
Search the literature and you find each concept explained on its own island. KiwiRAP pairs collective and individual risk. Austroads documents the Star Rating Score. A separate Austroads manual covers the Infrastructure Risk Rating. FHWA explains the systemic approach. The closest thing to a unified treatment is a dense government prose report — no single figure places all of them on shared axes.
That gap matters, because the confusion is practical: people read “collective risk” and “systemic risk” as the same data processed two ways, and they assume the Infrastructure Risk Rating sits on top of the AusRAP star rating. Neither is true. Below is the corrected map.
Two lanes, one destination
Every metric on this page exists to prevent fatal and serious injury (FSI). They split into a reactive lane that scores crashes that already happened, and a proactive lane that scores the risk built into the road itself. Both, in practice, rank corridors.
What feeds each one
Score the crashes that happened
Built entirely from historic crash records. It can only see danger after it has occurred.
- Collective risk — a severity-weighted score: fatal-and-serious plus minor-injury crashes, each weighted by a severity index, summed, then divided by years and section length. Percentile-ranked into corridors.
- Individual (personal) risk — crash rate: FSI per vehicle-kilometre travelled. The exposure-adjusted, traveller’s-eye companion.
Strength: pinpoints proven trouble. Blind spot: a dangerous road with no crash yet stays invisible.
Score the risk built into the road
Built from a coded survey of the road’s own attributes — no crash history required. It can flag a dangerous road before anyone is hurt.
- Star Rating Score (SRS) → 1–5 stars. The detailed iRAP/AusRAP model, ~78 attributes per 100 m.
- Infrastructure Risk Rating (IRR). A lighter NZ-origin model, ~9 attributes, its own log-product equation — a sibling of SRS, not built on top of it.
- Systemic screening. Uses those attribute-based scores to rank and treat risk factors network-wide.
Strength: finds latent risk early. This is where “systemic” lives.
Collective risk vs systemic risk
They are not the same dataset processed two ways. They sit in different lanes and answer different questions — yet both rank corridors.
| Collective risk | Systemic risk | |
|---|---|---|
| Data source | Historic crashes (FSI + minor injury) | Road / roadside attributes |
| Stance | Reactive — scores what happened | Proactive — scores the built road |
| Weighting | Severity Index per record, summed | Attribute risk factors (SRS / IRR) |
| What it surfaces | Corridors where trauma has concentrated | Corridors built to be dangerous, even with zero crashes so far |
| Output | A severity-weighted score, percentile-ranked into corridors | Corridors ranked by aggregated star rating score / IRR |
The punchline a crash-based view misses: on a real network safety plan, the worst-ranked corridor on the systemic page can carry zero FSI crashes — flagged purely because its star rating score says the road is built for harm. Collective risk would never surface it.
Siblings, not a stack
A frequent error is to picture a tower: AusRAP at the base, IRR layered on it, Star Rating on top. The evidence says otherwise. One road-attribute survey feeds two independent models. The only genuine “on top of” is the Star Rating banding sitting on the Star Rating Score.
The Star Rating Score bands into stars on the iRAP scale (vehicle-occupant thresholds shown):
Bicyclists and pedestrians use different (higher) bands, because their scores come from different crash-type equations. SRS is computed separately for each of four road-user groups: vehicle occupant, motorcyclist, pedestrian, bicyclist.
The Infrastructure Risk Rating is a parallel model
IRR was developed by Waka Kotahi (NZ) in 2016, inspired by the iRAP star-rating approach but with its own governing equation — IRR = log₁₀(R_SRS × R_SA × R_CW × ((R_LRH+R_RRH)/2) × R_LU × R_ID × R_AS × R_TV) — its own ~9 coded attributes, its own tool, and its own five risk bands (not stars). A trap to avoid: the R_SRS term inside that equation is a road-stereotype risk factor — it is not the AusRAP Star Rating Score. The names collide; the quantities are different. IRR shares a lineage with SRS; it is not derived from it.
Ranking a corridor systemically
At corridor level, a systemic risk view is typically sorted in descending order of an aggregated Star Rating Score — worst-built corridor at the top — with an Infrastructure Risk Rating shown alongside as a second infrastructure measure. Crash counts appear for context, but they do not drive the ranking. That is the whole point of the proactive lane: it orders the network by how the road is built, not by where people have already been hurt — the mirror image of collective risk, which ranks the same corridors by severity-weighted crash history.
One nuance worth keeping straight: the systemic approach begins with crash analysis too — grouping crashes by movement & place and crash type to find the recurring risk factors — before screening the network by infrastructure score. What makes it “systemic” rather than “collective” is the unit of analysis: by crash-type pattern across the network, not by a single location’s crash density.
References
- Austroads — AusRAP Road Safety Star Rating (SRS equation, attributes, road-user groups).
- iRAP — Methodology Fact Sheet #7: Star Rating bands (SRS-to-star thresholds).
- Austroads — Infrastructure Risk Rating Manual for Australian Roads (AP-R587A-19).
- iRAP — IRR origin: built on, but distinct from, the iRAP Star Rating methodology.
- KiwiRAP — Measures of Risk (collective vs personal risk formulas).
- Office of Road Safety (AU) — Local Government Network Risk Assessment Frameworks.
- FHWA — Why a systemic approach (reactive vs proactive).
- Austroads — AusRAP FAQs.
- iRAP — Methodology fact sheets index.