People who smoke marijuana have been driving safely for decades without much of a problem as they are more relaxed. I would think that they cause less accidents than a person driving who is not under the influence of anything. Stress, distraction and the use of alcohol have been proven to cause more accidents than any other factor.

The complexity of a marijuana breathalyzer test is well illustrated here.
If anything, you can sometimes spot a driver who has smoked a joint because he is driving s l o w e r than the other cars :)

"People who smoke marijuana have been driving safely..." hahaha. What are you smoking? There are plenty of accidents from people high on pot over the years.
Making up an excuse for this shows that you are in denial about something. The issue is that they haven't figured out what level is safe, so right now, as long as it's detectable, it's a potential DUI and it's up to the cop/judge to determine if it is.
I know someone that got a DUI from being high and he got retested 30 days later and it was STILL in his system and that was a probation violation. The other problem with pot is that depending on how much and how frequent you smoke/ingest, it can stay in your system and still be detectable for up to 90 days or so. So, it's a really bad drug to get in the habit of doing if you don't want to get a DUI, especially if they start carrying around breathalyzers for pot.
The reality is is that it's illegal to drive while under the influence, it can stay present in your system for as long as 90 days, and they are developing breathalyzers to carry around. That's the reality, so now it's up to you on whether or not you are going to take the risk and continue to use it or not. If you use pot, then the statistical probability that you will eventually get a DUI and there is no way they are going to make it legal to drive under the influence. About the ONLY thing they might do, eventually, is determine the minimum levels of being unsafe, but that hasn't been determined, so until it has, best advice is to take Uber, Lyft, taxi or have someone else drive you around if you don't want to get a DUI.

With marijuana being legalized in so many states, a device to test for drivers under the influence. I think it could help make the roads safer.

Science being used to criminalize otherwise benign behavior. Good job, Police State! Keep it up. We're already seeing people retaliate against the police with deadly force.

There's a reason they call it dope - for the substance, for the user and for dickheads who justify it's unfettered use. Driving whilst under the influence of a controlled substance basically sends a two finger salute to everyone else in your life - family, friends, the law and especially the people who are on the sharp end of your arrant stupidity
When you have killed someone because you have been stupid enough to drive drunk, or stoned, perhaps your rose tinted view will change. Roads are more dangerous now, drivers less capable now, so many driving without proper instruction . . . . .
Personally i'd tattoo the heads of anyone convicted DUI - it might send out a message to the stupid people who think it won't matter

There is the better non drug test, seeing if people can do certain tests tells whether they are impaired or not. If they can't, they shouldn't be behind the wheel no matter why. On the pot testing, THC is only one of many factors in pot. And unless you can put a number on it reliably, it is useless in court. Since pot effects everyone differently as does alcohol, the physical tests are more accurate on both in real life. Pot works because all these compounds work together in small amounts each. Until they can quantify that, drug testing isn't good enough for legal work. Like lie detecting which isn't that accurate, it has to work extremely well to be evidence. We have good testing, Let them touch their nose, walk a straight line, etc instead of these testers that can't be accurate because pot is too complex.

Unlike alcohol - marijuana has NEVER been scientifically proven to cause any driver impairment at all.

This is undoubtedly the stupidest thing I've read lately. This is either a case of genuine ignorance by everyone concerned (which alone staggers the imagination) or knowledge suppressed for any of a number of reasons, all of them dishonest at best. Here's a news flash for those truly ignorant of the facts. If smoking marijuana really impaired driving, the population of the U.S. would be as much as 50% less than it is today. The reason being, by the late 70's until the late 80's, as much as 80% of the American workforce smoked pot.Those that didn't on the way to work (damn few) did so on the way home. For the blue collar part of the work force I know for a fact these numbers are accurate enough. Therefore,it would be only logical to extrapolate, if marijuana necessitated the restrictions that alcohol does, as those who ASSume the need for such testing ASSume, then people driving under the influence of marijuana should have eliminated the better part of at least a generation to a generation and a half of Americans in that time period. This of course is ridiculous and is the basis for my questioning the honesty of whatever panel that conceived the notion that any such device is mandated. Does everybody in the 50+ crowed suffer from amnesia? Or is it simply easier to follow the rest of the sheeple, like the inhabitants of the village in the old tale of the "Emperors New Clothes"?

The only drug test we ever need is an outcome determinative test: Did someone hurt someone else? Was it intentional or grossly negligent? If so it doesn't matter what state of mind some fancy test says you're in or not in. But don't confuse the intent to use the drugs with the intent to commit harm. And don't condemn the former if no harm was committed. Your rights as an American expand until they come up against someone else's rights. If they never do, you should be free to go about your merry way.