Do you really believe nobody is driving under the influence of cannabis just because it is currently illegal? That is ridiculous. 
People have been driving under the influence of all kinds of drugs (including alcohol) ever since cars were invented. The ONLY reason there is not a reliable road side test for cannabis is because cannabis is illegal, so nobody is working on a test unit because those in charge share your unrealistic belief that nobody is driving under the influence of cannabis.
Road side tests for alcohol impairment have been around for decades, and it is high time (no pun intended) to have reliable tests for other drugs.