As much as people like to kvetch about governments doing this for revenue- would you want your kid to be hit by a car travelling 20 or 27?

https://www.ntsb.gov/safety/safety-studies/Documents/SS1701.pdf

the ACPO guidance used to be 10%+2 as an allowable margin of error- largely due to differences in calibration of speedometers. I haven’t worked in the field for 10+ years though and believe this is now no longer