Answer: It depends.
A quick summary is that there is no definitive measure of impairment, and if a person hires a good attorney in a cannabis-only DUI case then the defendant will likely win. The following is a detailed description of the logic:
Mostly, impairment depends on how sensitive the driver is to cannabis. Some people can drive very well while super-stoned, and some people should not drive if they have just one toke. The overall number of people in and given legal cannabis state (let's say, Colorado) that show up with THC in their system when tested (during auto accidents and police stops) have indeed increased. However, presence of THC does not imply causality. Police don't say if other drugs (like alcohol) were present as well as cannabis in the statistics that they cite. In this case of when law enforcement does not identify this as a confounding variable, the conclusion must be negated by anyone who knows scientific method and proper analysis of data. In fact, studies have shown that fatalities and number of actual accidents have actually declined in states with legal cannabis laws (see this citationhttps://coloradopolitics.com/marijuana-legalization-has-not-increased-traffic-fatalities/ ). This finding is despite that the overall NUMBER of drivers with THC in their system has increased. Law enforcement does not make the distinction between these two very important sets of data....and if law enforcement wants to protect society then they would accept the science on the topic. To further this topic, the National District Attorney's Association (the body of lawyers that are assigned to prosecute cases on behalf of local government entities) even states that there is no scientific science that says cannabis per-se causes impairment. Here is the quote, and the citation: While marijuana use has been shown to impair cognitive or executive function, driving performance, and increase crash risk, scientific studies have not yet demonstrated support for marijuana “per se” levels similar to alcohol in impaired driving legislation. As described, marijuana contains tetrahydrocannabinol (THC), more specifically Delta 9 THC, which is the psychoactive component of marijuana that causes impairment. Delta 9 THC can only be detected in blood. 73-90 percent of this is eliminated in as little as 45 minutes to approximately an hour and a half.50 On the other hand, marijuana metabolites, the byproducts in the blood as a result of the body metabolizing the marijuana, remain in the blood for a much longer period of time. Detection of the metabolites may be the result of marijuana consumption several days or weeks prior to the sample collection and may not scientifically equate to impairment. Some of the issues surrounding the challenges to studies that would scientifically support a marijuana “per se” level include:
Varying concentrations of THC in marijuana. Generally, the concentrations used in studies are much lower than what is available in real-life settings. Additionally, concentrations vary depending on the form of marijuana ingested.
Differences between users of marijuana. A chronic, frequent user may develop tolerance to some effects of marijuana but not all effects, including the impairing effect. The effect of THC consumption on impairment of driving performance may be higher for occasional, recreational users than for frequent users.
Differences in ingestion of marijuana. Smoked marijuana leads to a different absorption rate and release rate of the psychoactive ingredient than does eating marijuana edibles.
Combined use of marijuana and alcohol or marijuana and other drugs. Various studies have demonstrated that the combined use is associated with significantly greater cognitive impairment and crash risk than the use of one alone.51
All states differ, however. Here is a site that has a list of the state laws: https://www.leafly.com/news/cannabis-101/cannabis-dui-laws-by-state
Essentially, there is no clear definition of impairment, and field sobriety tests are the only way that officers can truly determine level of alleged impairment. This assessment may or may not hold up in court, and is specific to each individual case.
I hope this helps you to understand this topic a little bit more.