Alcohol has one of the worst rebound effects of any drug, so the criminalization of drunk driving provides an inherent impetus for those who live in suburbia and don't want to spend the next day in a do-nothing hangover state to DD, limit one's consumption to a couple of beers, drive everyone home safely, and be productive the next day.
Driving under the influence of marijuana should be a more nuanced law. I would implement the bullsh**t .08 equivalent WA has in major cities as well as suburban municipalities during morning and evening rush hour (say 7-9 AM and 4-6 PM). This protects both the driver and others - it's not a pleasant feeling to be driving excessively high in environments with a lot of traffic and pedestrians, but there's really no discernible difference between driving high or sober on empty suburban or rural streets.
I think in general, intoxication due to Marijuana use should be judged with field sobriety tests. Someone with a high tolerance can certainly drive much better at a much higher blood concentration level than someone who smoked for the first time. Still, from what I have read the Washington law isn't too bad. I think it only checks for THC and not even its active metabolite. And THC undergoes rapid metabolism and in any case quickly partitions into fat tissue as opposed to hanging out in your aqueous blood.
