This could, conceivably, be the hold up at federal level tbh. We have no current means by which we can objectively test for active impairment caused by THC...
Testing for alcohol is rare, and incident specific, because it's a measure of actual impairment. You aren't tested for alcohol to see if you've had a beer in the last 30 days, you're being tested to see if it's dangerous for you to be operating a vehicle, provide healthcare, carry a gun...
The basic principle behind alcohol testing is to determine actual impairment. The premise is that an agency is protecting others from dangers inherent in your being impaired.
The basic principle of drug testing is that the same danger from impairment is prevented by preventing impairment, but the premise is that any use is illegal. It's a "just in case" premise vs an actual matter of being presently impaired.
That fundamental difference is hugely notable in the case of DUI. How exactly do you mitigate the risk of DUI with THC? The current arguments in favor of legalization trend towards "it doesn't impair people as much", but that's a total cop out that doesn't address the issue, exactly the same way prohibition is.
We seriously need a solution to that, and I suspect it could very well be the "mystery cause" of federal legislators on the liberal side dragging their feet. They don't want to open the floodgates and make it unprosecutable to get in your car impaired, because even with an easy means to prosecute that if alcohol is the cause, it's still a huge issue... How bad will it be if it's effectively undetectable?
You want cops deciding based on how well you perform the little monkey dance? I fucking don't. I can't dance for shit perfectly sober...