Only one cannabis-impaired driving charge has been laid in Calgary since legalization last October, according to police.

The Calgary Police Service (CPS) believes that number will go up with more officer training and as roadside screening devices advance in technology, but one of the city's top impaired driving defence lawyers says he's not so sure that will be the case.

"There's no charges, there's no wave of cannabis impaired drivers, there's just a big nothing so far," says Tim Foster, whose firm gets about 30 impaired driving cases every month with almost all being alcohol-related.

It's always been illegal to drive while high, so Foster says he wasn't expecting an influx of charges after legalization.

"We have hardly seen any at all. We may have seen one since cannabis became legal," he said. "Despite all the alarmism with the police and the government saying this was going to be a huge problem, it has turned out to be exactly the opposite."

The Alberta government says that, provincewide, eight impaired-by-drug charges have been laid in the past year. 

But the Criminal Code doesn't distinguish between marijuana and other drugs.

Calgary police say they've arrested several drivers suspected of marijuana impairment but laying a charge takes leg work and time.

"Unfortunately, we can't lay those charges immediately because we have to send a body substance sample to RCMP crime lab and wait for those results," says Const. Andrew Fairman with CPS's traffic section.

"We've got a number of those that are currently in process."

CPS does not have a roadside screening device.

There are two federally approved devices but many police services, like Calgary's, haven't purchased any because they're waiting for the technology to advance. At the moment, the "scientific support behind that isn't there," says Fairman.

CPS is working to train more officers to do road-side sobriety tests for detecting marijuana impairment, but right now the service is "a little behind the eight ball" in getting those front-line officers trained, according to Fairman.

"The numbers may be a little bit low because of the fact that we're still trying to get the police officers on the street trained in that sobriety test."