Why?
1) Stupid people will drive cars while on hallucinogens, which famously distort perception and judgement.
2) No sobriety field test is available for hallucinogens. Probably none will be for years.
3) Patrol officers will get poor and inconsistent training for misbehavior-based sobriety field tests for hallucinogens.
What could go wrong?
You can be impaired by all sorts of other, legal drugs today. Is there an epidemic of say, ambien impaired drivers? Oxy? Etc.
Regardless, even if there is a slight uptick (which I doubt will happen) in the number of mushroom-impaired drivers, I think we're still much better off as a society than imprisoning people who use them responsibly.
Driving tired and drunk is probably way worse a problem than being high.
I'm more worried about giving police the right to take blood out of your body than I am about tripping drivers.
It wouldn't surprise me if increasing shroom popularity decreased overall alcohol consumption and also increased people's concern for the safety of others, both contributing to reduced DWI in general.