Vinny Eastwood had something to say on this.

The study that was actually performed to see if cannabis has an effect on traffic accidents. And the results are that it lowers accidents.

Road rage? Nah man, i am feeling mellow.
Speeding? Nah man, i got all the time in the world.

And if they get really out of it, they stop their car.
Often on the side of the road.

So, if the po-po were actually after safer roads, a joint before driving would be mandatory. But, this isn't about safety, this is about getting more people in jail and confiscating cars and cash.