The court needs to determine who actually was the driver though, especially if your car can be proven to be driven by a number of people. If they don't send you the fine for 6 weeks how can you remember if you were speeding at a certain time/place when you have no notification at the time of the event?
If someone said to me 'Where were you 27 days ago?' I wouldn't have a clue. Hell, I can't even remember what I did last week.
Hence in WA they fine you regardless for 'failing to take reasonable measures to identify the driver of your vehicle'. Too many people with short memories over here!
