A report shows drivers who smoke cannabis within three hours of getting behind the wheel double the risk of a serious crash.
Attitudes to drug driving are changing in the wake of random roadside testing.
The tests were introduced across Australia over the past five years, and road safety expert Professor Jeremy Davey says even drug takers are now starting to realise that taking drugs affects their driving ability.
His comments come after the release of a report in the latest British Medical Journal that shows drivers who smoke cannabis within three hours of getting behind the wheel double the risk of a serious crash.
Prof Davey, from the Centre for Accident Research and Road Safety - Queensland, says early evidence in their research shows that the culture of drug-driving has changed.
"Before there were random roadside drug tests of saliva, people didn't think they were impaired," he says. "People were ignorant of the effects of drugs on their driving.
"With the raising of the profile of drug driving we're starting to see an early shift of behaviour where people are deciding not to drive after taking illicit substances because they feel unsafe.
"It's hard to say how many or what proportion just yet." He says the impact has been mainly on occasional drug users, not habitual offenders. "It's people who don't classify themselves as criminals."
"It's a driving decision, not necessarily a drug-taking decision which is a positive outcome for road safety." He says there has been "very good broad comunity acceptance" of random drug tests, even among drug users.
"They are the people who live next door to you; regular people," he says. "Half the community have used cannabis at some stage in their life." Prof Davey agrees with the British research about the effects of drugs on driving ability.
"What people tend to forget is that cannabis, extacy or methamphetamines are psycho-active drugs just like alcohol," he says. "They change your mood, perceptions, congitions and behaviours, which is why people use them for an altered state.
"While different drugs have different effects they all impair driving ability. "Driving is a complex task that involves congnitive processing or thinkiing, and coordinating it into driving behaviour."
Prof Davey says drug taking has similar effects to alcohol, pointing out that a blood alcohol level of 0.05 made drivers twice as likely to be invovled in a road accident, 0.08 increased that incidence to seven times and at 0.10, drivers are 25 times more likely to be involved.
"People under-estimate the effect of alcohol and drugs on driving," he says. "Australia is a world leader on drink-driving prevention and with the introduction of random roadside drug testing we are the first country in the world to introduce it on a comprehensive basis.
"Now we have to start moving into the education area but there is a cost."