You can always tell who is driving high. They are usually going about fifteen miles per hour with both hands on the wheel. 5mph if they see a cop.
This is true, don't drive on anything not even if you think "I can hang"
Marijuana impacts your ability to drive, not quite as bad as alcohol and one of the differences is the person on marijuana is AWARE that they are impaired and seek to compensate by driving much slower.
I saw a test case after colorado legalized it where they got people stoned and had them drive on a closed course with police officers watching them. They said they could tell the drivers were stoned because of how slow and intentionally careful they were. They said that even though they could tell, they wouldn't pull them over though because they weren't driving dangerously. But they could still tell.
Meanwhile a drunk driver is overconfident in their ability and less aware of how impaired they are, they tend to drive more recklessly and start falling asleep behind the wheel. Alcohol has FAR more dramatic effects on the ability of one to control coordination of their body as well. Marijuana, as a comedian once put it.. "you can still function.... you ain't crisp, but you'll function". I've never woke up laying on an unfamiliar floor (or ditch...) from marijuana or gotten into a fight or borderline fornication from marijuana... alcohol.. less inhibitions across the board, people more prone to violence, and alcohol impacts a person's ability to control their own body, to the point of passing out suddenly in some cases, with memory blackout, and winding up in said ditches and strange floors, people wind up with fornication and unwanted pregnancies, adultery, fights over said adultery, injuries they can't explain because they lost control and fell off something.. the list goes on as Proverbs 23 attests:
29 Who hath woe? who hath sorrow? who hath contentions? who hath babbling? who hath wounds without cause? who hath redness of eyes?
30 They that tarry long at the wine; they that go to seek mixed wine.
31 Look not thou upon the wine when it is red, when it giveth his colour in the cup, when it moveth itself aright.
32 At the last it biteth like a serpent, and stingeth like an adder.
33 Thine eyes shall behold strange women, and thine heart shall utter perverse things.
34 Yea, thou shalt be as he that lieth down in the midst of the sea, or as he that lieth upon the top of a mast.
35 They have stricken me, shalt thou say, and I was not sick; they have beaten me, and I felt it not: when shall I awake? I will seek it yet again.
Not that marijuana is harmless, THC is still an intoxicant, and it can lower inhibitions to other sins, particularly gluttony, laziness.. but you're always in enough control not to wind up fornicating with someone else's girlfriend and getting into a fight, and in enough control to not cause yourself injuries.. and you'll always at least find a couch to crash on rather than faceplanting the floor. Between the two? Alcohol is a more severe intoxicant. But really you shouldn't be intoxicated by anything, that's bringing you under the mastery of something that's not God, which is sin.
Using for medicine, or drinking a glass of wine or a beer with food? you aren't under the mastery of anything. Use to excess... and you cross a line.