So, if you know people are already driving while intoxicated on marijuana, when it is illegal, there is no new source of intoxicated drivers...
People would just be driving while intoxicated on a legal substance, rather than driving while intoxicated on an illegal substance. Your argument doesn’t make any sense.
More than that, the illegality of marijuana may actually incentivize its users to use it before they drive home, so as to minimize the length of time they're in possession.