WA's unique method of calculating blood alcohol levels favours drink-drivers and should be scrapped, research concludes.
The back-calculation process potentially reduces a drink-driver's blood alcohol level and can mean a lesser penalty.
WA is the only Australian State that back-calculates blood alcohol levels from evidentiary tests.
The policy is based on the premise that a driver's alcohol content will be on the rise when stopped for a roadside breath test and the evidentiary test either on a booze bus or at a police station will be higher than the actual alcohol level when driving.
To account for this presumed increase, evidentiary blood alcohol levels are back-calculated to the driver's last "time of drink".
Typically blood alcohol levels rise 0.016/100ml an hour for two hours then fall at the same rate.
A recent Curtin-Monash Accident Research Centre report found that of 114 drivers who recorded an illegal alcohol level at a roadside RBT, 26 were not charged with drink-driving.
Among the rest, 65 drivers faced a reduced drink-driving charge because of the back- calculation process.
Further analysis of 46 drivers who blew over the limit found 11 escaped a compulsory immediate disqualification because their initial blood alcohol levels of 0.11, 0.09 and 0.08 were all back- calculated to 0.07.
Lead author Monash University research fellow Belinda Clark said back-calculation did not achieve its desired aim and should be axed.
She found the logic behind the calculation was only applicable for 28 per cent of those who blew over the limit.
Ms Clark said though few people would actively try to exploit back-calculation, it needed reviewing.
"For most people, back-calculation doesn't warrant its application," she said.
Police Minister Liza Harvey said the Government would consider changing back-calculating alcohol levels if it received advice from WA Police and the Road Safety Council.
"At this stage I have not received any recommendations from the council on this matter," she said.
Road Safety Council chairman Murray Lampard said findings in the report that identified a reduction in the proportion of drivers detected with illegal blood alcohol levels were encouraging.
He declined to comment on back calculations.