Most speeding offences require the use of a speed measuring device to detect and 'prove' an offence. However, a number of jurisdictions have a separate offence where 'speeding' can still be charged, including 'in motion', without lidar or radar.
For example an officer following or pursuing an offender can apply a 'negligent' or 'wreckless' driving charge based in context of the officer's observations and evidence gathered, such as following or pursuing an offender well above the speed limit, observing the calibrated speedometer in the patrol car, without the use of a speed measuring device.
It's been a while since I've looked at it though some Australian police forces have a calibrated speedometer installed on the dash that reads out the vehicle's speed based from the rear differential[1], separately to the vehicle's 'stock' speedometer. The reasoning, I understand, is that this is more precise, as legally the stock speedometer can display a speed up to 10 km/h lower than actual (but not above).
It's the other way around: the speedo can overestimate your speed but not underestimate it. If you follow the limits with an overestimating speedo, you drive under the limit. With an underestimating speedo, you end up over.
Anecdotally, when I pass those roadside speed alert signs, the speed they show and the speed on my speedometer is rarely more than +/- 1 mph. I think modern speedometers are pretty accurate, as long as the OE tire size is used.
That my experience too, the speedometer, speed my phone thinks I'm going, and static radar signs all more or less agree. Plus the only times I've been nailed for speeding, I was speeding, not "just kissing" the limit. Point being I don't think +/-1mph really matters in practice 99%+ of the time, it's usually getting tagged when being overconfident in the passing lane or something like that.
For example an officer following or pursuing an offender can apply a 'negligent' or 'wreckless' driving charge based in context of the officer's observations and evidence gathered, such as following or pursuing an offender well above the speed limit, observing the calibrated speedometer in the patrol car, without the use of a speed measuring device.
It's been a while since I've looked at it though some Australian police forces have a calibrated speedometer installed on the dash that reads out the vehicle's speed based from the rear differential[1], separately to the vehicle's 'stock' speedometer. The reasoning, I understand, is that this is more precise, as legally the stock speedometer can display a speed up to 10 km/h lower than actual (but not above).
[1] https://www.drive.com.au/news/inside-a-highway-patrol-car-th...