The anti-cannabis campaign has been significantly fueled by claims on fatalities due to cannabis-impaired driving. Some studies, such as an older one from the 1970s , have pointed out that the euphoria-inducing phytocannabinoid, tetrahydrocannabinol (THC), may cause prolonged human reaction time. This means that a driver under the influence of cannabis may not be able to act swiftly on the road to avert an accident, placing them and passengers at risk.
With all the positivism surrounding the legalization of cannabis, this has become a pertinent issue. That said, the evidence on cannabis-related motor vehicle crash fatalities, especially post-legalization, has been conflicting. 
To try to clear the waters, a group of researchers set out to investigate effects of THC & cannabidiol (CBD) on driving performance. The randomized clinical trial included 26 participants who were regular cannabis users. It was carried out by researchers from the University of Sydney over a 10-month duration. 
The study model took a double-blind approach. The study participants were offered different variations of cannabis: THC-dominant, CBD-dominant, a balance of THC/CBD-equivalent, and placebo.
The THC and CBD were offered in a dose of 13.75 mg, which represents an average dose. The cannabis products and placebo were vaporized by the participants.
The study participants were subjected to eight driving tests after vaporizing the cannabis products. The tests were timed from 40 to 240 minutes after consumption, and 85% of the study participants (N=22) completed all eight driving tests.
The researchers used a Standard Deviation of Lateral Position (SLDP) to quantify lane weaving, swerving, and overcorrecting—consequently, driving performance.
The following results were obtained with the SLDP criteria:
|40-100 minutes||240-300 minutes|
|THC-dominant cannabis||20.59 cm||19.88 cm|
|CBD-dominant cannabis||18.21 cm||19.03 cm|
|THC/CBD equivalent cannabis||21.09 cm||20.59 cm|
|Placebo||18.28 cm||19.37 cm|
Between 40-100 minutes, SDLP increased by approximately 2.3-2.8 cm following the consumption of THC-dominant cannabis as well as the THC/CBD-equivalent cannabis. The researchers compared this to a blood alcohol content (BAC) of 0.05%, currently a legal level in all 50 states (all states are 0.08% except Utah). The SDLP of those with a BAC of 0.05% was measured at 2.4 cm. There was no significant change in the CBD-dominant cannabis relative to placebo.
After 240 minutes, the SDLP was relatively constant for the different cannabinoid concentrations relative to placebo, demonstrating no real change.
From the results, it appears that CBD does not impair driving performance given that the results were comparable to those of the placebo. THC-dominant and THC/CBD-equivalent cannabis increased SDLP at 40-100 minutes to an extent akin to a legal BAC level but this effect fizzled out with time. 
The real question regarding the SDLP is what specifically does a value of +2.3-2.8 cm mean if legal levels of alcohol result in +2.4 cm of change compared to placebo or, in the case of states with a 0.08% BAC limit, an SDLP of +4.3 cm?  Can it be said that THC-dominant or THC/CBD-equivalent cannabis products impair driving performance when US law permits up to 0.08% BAC in all but one state, and that BAC results in a larger SDLP?
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