They told us about this on the course I atttended about smart motorways. There is a 40 second buffer between the signs changing, so they’ve worked out the braking time if going at 70 to reduce down to say 50.
There is also equipment/ cameras monitoring the actual camera that prove the speed...
That’s where I got caught beginning of the year.
58 in a 50 zone 9pm at night , hardly any traffic got told at the course speed limit in place to allow pollution to settle.