We use 10km and 20km excess of speed limit as 2 different triggers.
Discretion is 5 seconds entering a lower speed limit. Reports are done monthly - one month = 1 record, records kept for 6 months.
10km over - reported to line manager who meets with staff member. 3 records is a referral to HR.
20km over - report to line manager and general manager, 2 records is referred to HR.
140km/hr is gross misconduct (we've never had this, but managers wanted a top number in).
We've been doing this for 12 months now and it has significantly changed driver behaviour.

Having been a council enforcement officer for Pool Fencing in a past life, there is zero provision for exemption from the Act. So maybe the issue is not the person with the clipboard but legislation that takes no account of the context of risk.