I remember in high school there was a rumor that someone had invented a roadside test for detecting marijuana in someone’s system. This caused alarm for many people who had enjoyed relative security in their stoned driving (as opposed to the more risky drunk driving).
However, the public is much more likely to support legalization if we find an effective method of testing drivers. Presumably the police would be much more supportive, also. So perhaps investing some of the money we spend shooting at smugglers on the border would be better spent finding an effective roadside test for the police to regulate stoned driving.
Some worry that legalization would lead to an industry that is dependent upon people using marijuana consistently and spending their money on it, much like tobacco or alcohol. But we regulate alcohol and tobacco marketing specifically for this reason, and I see no reason that the same can’t be done for marijuana. The “grow your own” policy of allowing growing, possession, and consumption but maintaining the illegality of selling marijuana maintains the black market for people who don’t have a garden to grow it in, and so we would just continue wasting our police and throwing people in jail for a drug that is relatively harmless.