I feel that it actually is very easy to just creep over the speed limit, for eg, moving at 110 in a 100kmh zone, this is normal, one doesn't look at the speedo all the time.
The +3km allowance does nothing but make people paranoid and keep looking at the speedo, i find myself glancing at the speedo more often than i should because i do not want to get fined and have points deducted. Seriously, 64kmh in a 60kmh zone was their detected speed, and alleged speed was 61kmh, really, oh wow, thats so dangerous!
That happened 1year back, and i've copped it on the chin and promptly paid the fine, but that doesn't mean i don't think its ridiculous.