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 screen. The infrastructure-screen phase of the broader systemic method — rank candidate sites by SRS/IRR, after crashes are diagnosed by pattern.
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) | Crash patterns (movement & place, type) + road attributes |
| Stance | Reactive — scores what happened | Proactive — diagnose risk factors, screen network-wide |
| Weighting | Severity Index per record, summed | Risk-factor patterns; SRS / IRR screen |
| What it surfaces | Corridors where trauma has concentrated | Candidate corridors sharing severe-crash risk factors — some with little crash history yet |
| Output | A severity-weighted score, percentile-ranked into corridors | A prioritised screen: candidates ranked by SRS / 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.
What “systemic risk” actually means
Systemic risk is the term most often misread — and it is not a single metric, nor the same thing as an AusRAP star rating. It is a broader, agency-defined safety method for finding and treating risk proactively. Two of its phases are the ones a dashboard usually shows:
Diagnose reads crash records, but groups them by type and context — not by location — to find the recurring focus crash types and the road features behind them. Screen then finds the candidate sites across the network sharing those features, using the Star Rating Score and IRR, and prioritises them for treatment without requiring each site to have its own crash history. The Star Rating Score and IRR are an infrastructure screen used inside the method — not the method itself, which continues on into countermeasure selection, prioritisation, delivery and evaluation.
This is why a Network Safety Plan can show what looks like crash analysis under “systemic risk.” A Systemic Risk — Movement & Place view is the diagnosis input (crashes grouped by pattern); the Systemic Risk — Corridor / Intersection views are the infrastructure screen (candidates ranked by Star Rating Score / IRR). They are two faces of one method — and they read as genuinely “systemic” only when the screen is tied back to the diagnosed focus crash types, rather than presented as a bare star-rating output. FHWA’s systemic approach is consistent with this diagnose-then-screen logic — proactive and risk-factor based, not waiting for crashes to cluster — though it doesn’t prescribe these specific Australian tools.
The contrast with collective risk stays the same: it is the unit of analysis. Collective risk ranks one corridor by its own crash density; systemic groups crashes by type and pattern across the network and screens by risk factor. Crash history still informs the diagnosis — systemic isn’t crash-history-free — it simply doesn’t wait for each individual site to accumulate crashes.
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.