Guides
Written to answer one question properly. Every number here — a tolerance, a fine, a regulation — links to the source it came from, and anything we could not source is not published. See the editorial policy.
How camera enforcement works, where it is legal, and what the thresholds really are.
1 guideWhy indicated speed and true speed differ, and what the regulations actually require.
0 guidesTrip telemetry, G-force and what the numbers from a drive actually mean.
The 10% + 2 mph threshold is real, it is confirmed in police disclosures, and it is not a legal entitlement. Here is what the guidance says, who sets it, and why it can change without notice.
Nineteen states and DC permit automated speed enforcement, ten prohibit it outright, and twenty-one have no law at all. What that patchwork means for a driver crossing state lines.
Car speedometers are legally required to over-read. UN Regulation No 39 sets the exact allowance — up to 10% plus 4 km/h — and explains the gap between your dashboard and a GPS speed app.
Average speed cameras use ANPR to time vehicles between two points, calculating speed over a distance to deter continuous speeding.
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