The number of fatal and serious crashes of cars, trucks and buses without warrants or Certificates of Fitness is rising.
Transport researchers say it's concerning, but also puzzling as there is no clear link to the growing road toll.
From four such crashesin 2014, where the truck or bus did not have a six-monthly Certificate of Fitness (COF), the heavy vehicle number has shot up to about 60 crashes per year.
Traffic researcher Glen Koorey said the jump on historical averages is "concerning".
"It's a bit chicken and egg - are there more non-WOF/COF crashes because inspection processes have been slack, resulting in more non-compliant vehicles out there?" Dr Koorey of consultancy ViaStrada said.
"Or are crash inspectors picking up more non-WOF/COF situations, for example, because of the recent public focus on inspection processes?
"Either way, the recent numbers are a concerning proportion."
John de Pont, a specialist in truck safety research, is puzzled.
"I don't quite understand why there appears to be such a big increase. It may well reflect an increase in behaviour across the fleet - I mean, the proportion of vehicles running around without COFs," Dr de Pont said.
The Transport Agency and the police shed little light on the rise, other than to downplay any link between a lack of vehicle safety inspections and crashes.
Dr de Pont said he wants to know more: "They should at least try and find out why it's happening."
Recent research has shown up worrying practices in the trucking industry including around lack of maintenance.
"Concerns were expressed around how thoroughly the trucks were being maintained," Clare Tedestedt George said, based on trucker interviews for her PhD research. "Commonly cited reasons for poor maintenance was a lack of resources such as time and money."
As for the numbers of death and serious injury crashes involving cars and other light vehicles without a warrant of fitness, these have also risen, though less dramatically.
From a low in 2014 of 14 per cent and just 289 crashes (though a rate of 18 per cent has been more typical since 2010), this has risen to almost a quarter, numbering more than 500 serious crashes.
"I guess we're always sort of playing a waiting game to just see whether the trends continue to go up or are just a blip," Dr Koorey said.
A lot of crash research pays little attention to mechanical defect factors, and the research itself can be dated.
All the experts are keen to stress the lack of a WOF or COF does not mean this was a factor in any crash; and that having a WOF or COF does not mean a vehicle does not have defects that could cause a crash.
Dr de Pont said trucks travelled on average 100,000km a year, so it was common for them to develop defects before their six-monthly COF checks were due.
The Transport Agency said it was not aware of any evidence linking an increase in expired warrants with how many of these vehicles were involved in serious crashes; it has also said the percentage of unwarranted vehicles in the overall fleet was steady at around 12 percent.
There was no evidence of any link between the increase in serious crashes and a change in 2014 that led to many cars needing a warrant of fitness inspection every year instead of every six months, the agency said.
The rate at which vehicle defects are identified by crash investigators as a contributing cause has stayed steady, at around 9 percent of fatal crashes and 3 percent of serious injury ones.
The AA's Mark Stockdale said the latter was probably under-reported, given that injury-crash investigations were more limited.
"It's just not possible to do the same level of detailed investigation as with a fatal crash," Mr Stockdale said.
"But in saying that there are some things that can be easily determined at the roadside, like the condition of the tyre. Tyres is the number one vehicle defect."
Mark Stockdale said the AA would like police to do more checks on WOFs. Photo / RNZ (Phil Pennington)
The Transport Agency in a statement suggested it was difficult to pinpoint vehicle defects as a crash cause.
"It would require a high level of available evidence and a full police investigation. This information is not normally contained within a traffic crash report that the Transport Agency receives."
But the police said identifying defects was a priority for them, though whether a vehicle had a warrant of fitness, was not.
"Police traffic crash investigations investigate and report on any vehicle faults that may have contributed to the crash, and will assess the vehicle safety standards," the police said in a statement.
"Whether the vehicle's WOF/COF was current is not in itself a contributing factor."
Overseas, many US and Australian states did not require warrant of fitness checks yet had lower road tolls, Dr Du Pont said.
"However, because we have a long history of periodic inspection requirements, there will be a proportion of motorists who rely on the WOF inspections to identify safety defects on their vehicle," he said.
The AA said what was needed was more checks on WOFs by police.
Defects rank far behind other serious crash factors such as losing control, not staying left or riding a motorcycle.
The government's new Road to Zero proposal to rein in the road toll aims to review the warrant and certificate of fitness systems.
