Marijuana contains about three times much tar as does tobacco, BUT it contains somewhat fewer carcinogens. It is STILL smoke though, so it contains a LOT of carcinogens, but if you smoke pot, you're more likely to die of COPD complications than cancer. The negative effects can be almost entirely neutralized by making it into edibles. The only remaining negative effects are... It being illegal.
From what I remember, due to the way the THC is stored in the lipids as mentioned before, when taken by mouth (but not when smoked; I know not why) marijuana can give you hallucinations for 30 days after taking it, which is not a very positive effect; at least, it's an effect that should make driving illegal for some condition* after consumption of cannabis by mouth.
But this is all remembered from the time I debated whether marijuana should be legalised in my country, and that was a few years back.
*Blood concentration? Some other way? I have no idea.
This isn't exactly the case. You are technically still high for those thirty days, yes, but you don't usually hallucinate. I've hallucinated exactly twice from smoking marijuana, and the first time could have been because of cross-contamination from someone smoking psilocybin mushrooms from the pipe we used. Both hallucinations were rather mild, certainly not impairing to the extent people believe hallucinations to be, and notably neither were from edible marijuana, which has a different high than smoked marijuana.
The problem is that people have confused hallucinations from dissociative drugs with hallucinogenic drugs. A hallucination from a hallucinogen will generally make you go, "Wow, I'm seeing things. This is crazy." Whereas you ACTUALLY believe those things are there when you are hallucinating from a dissociative drug. Some hallucinogens sort of cross the boundary between the two, such as mescaline or PCP (PCP is actually a horse tranquilizer, btw) causing vivid, lifelike hallucinations that are incredibly easy to be misled by. Driving on hallucinogens of ANY sort is DEFINITELY NOT A GOOD IDEA.
I would rate marijuana as impairing about as much as a few beers, depending on your tolerance and amount consumed. It is entirely possible that you should definitely not drive after smoking, and is very most often the case, but some people can get away with driving after just a beer or two. It all depends on the person.
And the only way they'd be able to do it would be urine tests, and they'd just ALWAYS be positive. Either you don't smoke or you don't drive for 30 days.