I could care less if they legalize weed. I will not do it. That said, I am a firm believer that the issue holding the legalization of weed back nationwide is an ability to test if a person is current under the influence. We can test of a person smoked within x days but that does nothing to tell us if they are currently under the influence like we can with alcohol. A field sobriety test is subjective that can be successfully fought in court without the BAC.
ummm...not really hard to tell if somebody is high or not. Just like alcohol, cops can just look at your eyes, and know instantly.