The effects of alcohol on coordination and motor skills are well pronounced and objectively evident. Even an experienced drunk is going show significant, well defined symptoms of alcohol intoxication past a certain dose or blood concentration, no matter how much tolerance they have built up. Marijuana, on the other hand, primarily affects less significant receptors in the brain and causes effects that are mostly subjective and far less consistent than those of alcohol. It is definitely dangerous to drive while truly "intoxicated" from marijuana, but there is an exceptionally wide range of what could be considered as "intoxication" compared to other drugs, from person to person. There are countless immeasurable variables that markedly influence its effect on driving performance.
The idea of a blood level limit for THC is absurd. The system used by most states works just fine; if there is observable evidence that you are operating your motor vehicle unsafely or you appear intoxicated for definable and valid reasons, you can be arrested for DUI. In the case of accidents where intoxication is suspected and a blood draw is taken, THC levels are still obviously taken into consideration, just without a defined "legal limit". As is the case with most drugs, if I'm not mistaken.
Trying to compare marijuana with alcohol and act like they cause the same kind of impairment is ridiculous. Millions have been driving under the influence of marijuana since the 1960s, and there has never been a documented scourge of stoned drivers causing mass traffic fatalities, at least not anywhere near the level that's witnessed with other drugs.