The number that does the heavy lifting
Authorities say speed is "a factor" in roughly one in four to one in three fatal crashes. Victoria's TAC puts it at "one-in-four (25%)." But read their own definition:
"Speed, both excessive speed and inappropriate speed for the conditions… was a likely contributing factor in one-in-four fatal crashes." — Transport Accident Commission (Victoria)
That single number is doing three different jobs at once:
- Illegal high-range speeding — the reckless outlier everyone agrees is dangerous.
- "Inappropriate speed for conditions" — which can be, and often is, at or below the posted limit (90 in fog in a 110 zone).
- Speed as an aggravator — a crash caused by fatigue, alcohol or failure-to-give-way, where speed only worsened the outcome.
The missing statistic There is no published Australian figure that isolates "crashes caused by a driver exceeding the posted limit on an otherwise safe road" — the precise behaviour that low-level speed cameras target. Governments simply don't report that cut. The absence of that number, while "speed is a factor" is quoted everywhere, is itself the finding.
Note: crash-causation factors are not mutually exclusive — one fatal crash can be coded speed + alcohol + fatigue — so the published shares do not sum to 100%. Sources on the Sources page.
The limit isn't "the safe speed"
For a century, traffic engineers set limits using the 85th-percentile speed — the speed at or below which 85% of drivers naturally travel on a road. Set the limit there and you target genuine outliers. Set it well below, and you criminalise the safest, most typical driver — and manufacture a permanent, predictable stream of "offenders."
"85th Percentile Speed means the speed at or below which 85% of all vehicles are observed to travel under free-flowing conditions past a nominated point." — Main Roads Western Australia, Speed Zoning policy
Under the "Safe System" doctrine, Australian road authorities are steadily moving away from anchoring limits to how people actually drive, toward limits based on survivable impact speeds — which in practice means setting many limits below the 85th percentile. That may or may not be good safety policy. What it undeniably does is widen the gap between the legal limit and normal driving — and every km/h of that gap is billable.
Austroads' Guide to Road Safety Part 3: Safe Speed (2024) reframes limit-setting around the Safe System rather than observed behaviour. The US FHWA has likewise moved (2023 MUTCD) to discourage rigid reliance on the 85th percentile. See Sources.
The tolerance you're not allowed to know
How far over the limit before you're fined? In most of Australia, that's a secret — and legally, there is no guaranteed buffer at all.
| Jurisdiction | Published tolerance |
|---|---|
| Victoria | 2 km/h fixed (tightened from 3 in 2019); 3 km/h or 3% mobile published |
| NSW | Not published confidential |
| Queensland | Not published confidential |
| SA, WA, Tas, ACT, NT | Not published confidential |
Only Victoria publishes a figure. Everywhere else, the exact tolerance is treated as operational secret and, in law, any amount over the limit is an offence. A system that fines you at 2 km/h over — inside the error margin of many car speedometers — is not targeting danger.
Source: RACV — Enforcement of road rules in Victoria. We deliberately do not quote specific km/h figures for the confidential states; nobody publishes them.
"But the research says 5 km/h doubles your risk"
The official case for near-zero tolerance rests almost entirely on one body of work — the Kloeden Adelaide studies. Take it seriously, because it's real. Then read the fine print.
In a 60 km/h zone, the risk of involvement in a casualty crash roughly doubles for each 5 km/h above 60 — a car travelling at 65 in a 60 zone has about double the crash risk. — Kloeden et al., CR 207 (BITRE)
The fine print that rarely makes the press release:
- It's a relative-risk multiplier, not a count of how many crashes are actually caused by 3–5 km/h over on a good road.
- It's a case-control study of urban 60 km/h zones in Adelaide (151 crash cases vs 604 controls) — not a universal law of the road.
- Doubling a small risk still leaves a small risk. A relative multiplier says nothing on its own about absolute road-toll benefit.
The honest position Speed matters. The defensible criticism isn't "enforcement does nothing" — it's that the safety benefit of chasing very small margins rests on a narrow evidence base, while the revenue benefit is large, immediate and budgeted. When the evidence is thin and the money is thick, ask which one is driving policy.
Slow can be dangerous too — which cuts both ways
The classic Solomon curve (1964) found crash risk is U-shaped against traffic flow: vehicles going much slower or much faster than the surrounding traffic both face elevated risk, with the lowest risk near the median speed. If that's true, then speed variance — not raw speed — is a key danger, and a camera that pushes a cluster of drivers to brake below the natural flow can increase variance.
In fairness: the low-speed limb of the Solomon curve is the most disputed part of that research (later work by West & Dunn, Kloeden, and Australian data from Fildes et al. challenged it). We flag it as supported for the high-speed side, contested for the low-speed side — because a site that only quotes the convenient half of the evidence is doing exactly what we're criticising.
Overview and critiques: Solomon curve (with primary citations). Full list on Sources.