ACC's new vehicle levy system - which will bring down the cost of licensing vehicles - has been criticised for determining owners of some new cars pay more than those with unsafe, older vehicles.
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But Nikki Kaye, the Minister for ACC which consulted with motoring organisations including the Automobile Association, said there was not enough crash data for newer vehicles to determine how safe they are.
Motorists will be left with a few more dollars in the pocket from July 1 as the ACC portion of licensing goes down by an average of 41 per cent, depending on the type of vehicle they own.
All cars are now given a banding - L1 to L4 - under the new Vehicle Risk Rating system. The lower the band, the higher risk the car presents and the higher licensing fee the owner will pay. The fees vary from as low as $68.46 up to $158.46.
Ms Kaye said she fully backed the new system which involved critiquing crash data from 85 per cent of the country's 2.8 million vehicle fleet. The remaining 400,000 of those vehicles were less than three years old which meant there hadn't been enough time to gauge enough raw road testing data. Instead, the vehicles had been tested in a laboratory under the ANCAP (Australasian New Car Assessment Programme) system.
"There will be no perfect system in the sense that when you are dealing with newer cars you don't have that real world crash data. You have to decide on a system that takes either the data that you've got, either from testing and we think that's a principled approach, or you just look at the way the past vehicles may have performed and we think it's much more principled to take actual crash data."
Ms Kaye said she expected there could be situations where people pointed out differences in the car's manufacture to the new levy.
"We can say that there is integrity in this system, that their vehicle levy is based on actual crash data."
Data used for the remainder of the country's 85 per cent of vehicles has been collected by police in Australia and New Zealand since 1987 involving 5.5 million police-reported crashes.
But road safety campaigner and Dog and Lemon Guide editor Clive Matthew-Wilson said there were "countless" mistakes in the system. "They appear to have produced some computer-based formula that works out all cars but whoever worked out this formula knows less about road safety than my car ... it's just a joke."
He pointed to the Nissan Caravan models between 2001 and 2011 which received the worst crash results in an Australian crash test, yet owners will pay the lowest fee.
Mark Stockdale, AA's principal advisor of regulations, admitted the levies still needed some "fine tuning".
He said it was important that motorists with newer cars with more modern safety features received lower levies as they presented less of a risk on the road.
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