Most drivers reported that cannabis use was very likely to impair driving

Use of Alcohol and Cannabis Among Adults Driving Children in Washington State...



Synthetic marijuana compounds are also associated with impaired driving...
"A 45-year-old male was witnessed to drive his vehicle into oncoming traffic and swerved the vehicle into a roadside guard rail. When the responding law enforcement approached the vehicle, it remained running and engaged in reverse gear but was lodged against the railing. The driver was asleep, held a lighter in one hand and two blunt cigarettes were on the car seat. The driver responded to verbal stimuli, indicated that he was “high” and that the cigarettes “possibly” contained marijuana."

Another recent study showed that the decrease in vigilance and driving performance were maximal around 5 hours after administration of cannabis. So people who think they aren't too high to drive may become impaired further while driving (from a delayed reaction to the drug)... Effect of Smoked Cannabis on Vigilance and Accident Risk

 

Pears can also cause impeared driving...