There's always been potheads behind the wheel, however once you legalize it either medically or recreationally, the availability will mean more use and therefore more impaired drivers.
That will depend entirely on the effectiveness of the
public education program, which should include saturation publicizing of DUI pot arrests and punishments.
Public education is the way cigarette smoking was decreased by more than 50% -- in spite of the fact that cigarette addiction is more tenaciously addictive than heroin! I know this from personal experience because I smoked cigarettes for thirty-five years and
public education convinced me to quit, which I did in 1985.
It should be some consolation for the
Reefer Madness group to know there is a significant difference in the effects of DUI marijuana and DUI alcohol -- as any experienced highway cop will attest. Drunk drivers are aggressive, they drive erratically, and they speed. Stoned drivers are not aggressive, they are able to control their vehicle, but they tend to drive below the speed limit -- which is how the cops pick them out. (They drive slow and are smiling.)