If you surf over to the NIDA website Drugabuse.com they have this to say regarding weed and operating heavy equipment.

Does Cannabis Impair Driving Ability?

Marijuana significantly impairs judgment, motor coordination, and reaction time, and studies have found a direct relationship between blood THC concentration and impaired driving ability (Accid Anal Prev. 2010;42:859-866; Clin Chem. 2013;59:478-492). Marijuana is the illicit drug most frequently found in the blood of drivers who have been involved in accidents, including fatal ones (Am J Epidemiol. 2014;179(6):692-699; although it is important to note that marijuana can remain detectable in body fluids for days or even weeks after acute intoxication). A meta-analysis of multiple studies found that the risk of being involved in an accident roughly doubles after marijuana use (Drug Alcohol Depend. 2004;73:109-119).

Accident-involved drivers with THC in their blood, particularly higher levels, are three to seven times more likely to be responsible for the accident than drivers who had not used drugs or alcohol. The risk associated with marijuana in combination with alcohol appears to be greater than that for either drug by itself (Clin Chem. 2013;59:478-492).

Cannabis is unique among intoxicants because of its very long half-life. A half-life is the amount of time needed for half of the serum level of a drug to be removed from the body. The longer the half-life the longer it remains in your system.

Cannabis has a long half-life in humans (67 days). In chronic cannabis users, it is particularly difficult to determine whether a positive result for cannabis represents a new episode of drug use or continued excretion of residual drug. Algorithmic models have been devised to determine whether THC levels represent new use or the carry-over from previous use. However, these models are not very accurate in discriminating new use and carry-over in chronic users.[ref]Iran J Psychiatry. 2012 Fall; 7(4): 149–156.[/ref]

The above quote reminds us that when chronic cannabis users are tested for THC, a positive result can be from last week’s ingestion or from the same day. Therefore, even though marijuana is the most commonly found drug in motor vehicle accidents (MVA) it may be just along for the ride (pardon the pun). It may have nothing to do with the intoxicated state the driver was in at the time of the accident.

And from the 2014 NIDA study by Dr Volkow:

Immediate and long-term exposure to the substance [marijuana] has been found to substantially impair driving ability…

Some of the clarifications on the benefits and harms of cannabis come from the website[ref](http://clinicalcannabisincontext.tumblr.com/) 07/12/2015[/ref] of Ian Mitchell, MD, FRCP (Emergency Medicine staff Physician – Royal Inland Hospital Emergency Department and King Street Clinic, Kamloops, BC).

Lastly, although several recent studies have not been able to show any evidence of increased risk of traffic accidents due to the use of cannabis, the literature in totality suggests an overall doubling of risk. This is roughly equivalent to a BAC of less than 0.05. Driving with a BAC of 0.07, legal in many jurisdictions, is far more dangerous BAC of 0.07, especially for the younger driver.  

The last sentence does not read well. His point is that a BAC of 0.07 is far worse for teenagers than the equivalent BAC of 0.05 that one would conceivably develop after smoking marijuana. In other words alcohol is still by far the main ingredient in your motor vehicle accident recipe.

Meanwhile, there does appear to be a synergy between pot and alcohol which makes it a bad idea should you elect to drive drunk and stoned. This is an important point for those who feel that cannabis has no untoward effects.

One can’t really argue these statistics. Pot is an intoxicant and you should never be driving while intoxicated. You just have to assign a drug-free friend or acquaintance to drive you to the nearest market for that gallon of ice cream you’re craving at 2 AM.

There clearly is a doubling of risk when driving after ingesting cannabis. Therefore, be careful and refrain from driving a car, or operating that front-end loader you recently purchased, if at all possible when high.