Lots of research has aimed to explore marijuana’s effect on driving performance and psychomotor skills, and results have been fairly consistent across the board: Marijuana’s effects on driving are mild and not significant enough to play a role in crashes, especially in comparison to alcohol. These studies were varied and involved simulated driving, on-road studies, reviews of existing research and culpability in automobile accidents.
According to NORML, in simulated driving tests, marijuana impairment was normally characterized by subjects slowing down their driving speed and a decreased response time to emergency situations. However, these impairments don’t appear to play a significant role in traffic accidents.
Some laws have aimed to enact a THC limit that’s allowed in your system when driving. However, most experienced cannabis consumers know that this way of measuring marijuana intoxication makes no sense, as each person has their own individual tolerance.
Generally, it’s not good practice to drive after consuming cannabis, since marijuana’s effects on driving aren’t 100% benign.
If you’d like to learn more about marijuana’s effects on driving, NORML is a great resource for this: https://norml.org/library/item/marijuana-and-driving-a-review-of-the-scientific-evidence#Crash
Our answer’s page is also another way to learn about marijuana’s effect on driving, as well as marijuana driving laws: https://www.hellomd.com/search?type=question&term=marijuana%20driving
Cannabis is known to cause psychomotor retardation (slowing reflexes, etc). I have personally been involved in a rear-end collision where the driver behind me was obviously intoxicated on cannabis so admitedly I have my own bias. I have seen multiple studies in the Scientific American and also reported on the CBSnews website that might appear to give conflicting data: https://seattle.cbslocal.com/2014/02/04/study-fatal-car-crashes-involving-marijuana-have-tripled/ https://www.washingtonpost.com/news/wonk/wp/2017/06/26/what-marijuana-legalization-did-to-car-accident-rates/?utm_term=.32e7a4bbdf8e To me, it looks like one of the take-home messages would be: if you use cannabis AND alcohol together, you definitely shouldn’t drive. Like handling other medications that may impair driving reflexes/ judgement (i.e. benzodiazepines, opiates, muscle relaxants, anti-convulsants, etc) I tell patients to use their best judgement. Thanks for letting me express some alternative points of view!