MDT testing is unlike Alcohol testing, it can't test how much cannabis you've consumed, just that you've consumed it or it's in your system. Unlike drinking alcohol, cannabis could be consumed secondhand at a party by just being in a confined space next to someone who is smoking cannabis, this use to be called "a Dutch oven". Through eating hemp seeds, consuming hemp oil. I'm talking about industrial hemp, it still contains small amounts of THC.
Whether it's by "a Dutch oven" eating seeds or whatever, you may still gve a positive result for cannabis.

Several cannabis users I've interviewed have told me " I just finished a joint, got in the car and was pulled over by the MDT unit, they tested me and I thought I was gone. Came back with a negative result and I was told to go"
I've heard this anecdotal tale several times which makes me thing, how long does it take for THC to be present in saliva on your tongue? because if it's not detected withing the the first 30 minutes, the MDT unit is useless.
If there is any danger driving under the influence of cannabis (and there is no proof that I'm aware of) the danger would be in the immediate 30  minutes after consuming.

Firstly what needs to be established; is there any danger driving on cannabis and if so, why?

This interview with Irvin Rosenfeld dispels much of the propaganda propagated by governments Irvin Rosenfeld interview

For the past 30 years Irvin has been legally using cannabis for his serous medical condition, he has also worked as a stockbroker for all that time, driven to work each day and never had an accident.

Further studies contracted by the dept of transport in the USA have reported cannabis is much safer than alcohol and just slightly more dangerous than normal driving. So why the sudden MDT awakening?
Does the government have data proving driving on cannabis is dangerous? Lets have a gander at it, if there is an "it"

I'm all for making the roads a safer place to be, because t is getting pretty crazy, but let's be honest about it, show us the data that proves that it's dangerous to drive after consuming cannabis.

NT Police say no leniency for medical marijuana users when new drug-driving laws start

TERRITORY stoners and medicinal marijuana smokers are becoming increasingly paranoid they will be caught out by tougher drug-driving laws.

But Police Minister Peter Chandler said there would be no leniency for medicinal marijuana smokers when the new laws come into effect on February 1.

Drivers face lengthy driving bans and thousands of dollars in fines for returning a positive test. The NT News has spoken to cannabis smokers worried they will be penalised despite not being under the influence of the drug.

One casual drug user said there was a subculture of Territory cannabis smokers who used cannabis responsibly.

He said there was a difference between casually smoking cannabis and driving after smoking meth.

“We are organised people. We have got a family and kids. When the kids go to bed at night we’ll indulge in a chuff,” he said.

“There’s quite a few people that indulge in marijuana frequently and no other drug. They enjoy that and they are no harm to the community.

“We’re made to be instant criminals straight away.”

A medicinal cannabis user said he used the drug to ease the pain of his illness.

But Mr Chandler made no apology for the tough stance.

“There will be a zero tolerance approach to drivers who drive under the influence of drugs. Any trace of prohibited drugs in your system will result in a penalty – cannabis, ice or ecstasy, no drug is excluded,” he said.

“All drugs are illegal and if you’re caught risking your own life and the lives of others you will be prosecuted. There is a simple solution. Don’t take drugs.

“Cannabis – even medicinal cannabis – isn’t legal in the Northern Territory. If you’re driving under the influence of cannabis then you’re both a fool and committing a crime.”

Drug drivers face two categories of penalties if they return a positive test and a lab analysis confirms the presence of illicit substances.

A driver caught with traces of drugs in their system is issued with a $400 on-the-spot fine.

For a second offence, drivers face a minimum three-month disqualification, a $1200 fine or six months in prison.

However, drivers face a six-month driving ban, a $1500 fine or 12 months in prison if caught driving under the influence of illicit substances.

The penalty doubles for a second offence.

reference: http://www.ntnews.com.au/news/northern-territory/nt-police-say-no-leniency-for-medical-marijuana-users-when-new-drugdriving-laws-start-february-1/news-story/5078fb0709f18188a13d5450374104ac