People who support the legalization of cannabis in Texas often point to the fact that drunk drivers kill more people every year than those under the influence of marijuana. Even so, driving under the influence of weed can be dangerous. Like any other drug that affects emotions or thought processes, this may impair driving and cause accidents. Yet, police officers do not always have the tools at the ready to test for this.
NPR confirms that documenting drugged driving is difficult, especially in comparison to drunk driving. While marijuana typically shows up in a pee test if a person has indulged, it may not show up in breathalyzer or blood tests. This is due to the fat-soluble makeup of THC. Alcohol spreads throughout the body more evenly, but THC is absorbed in different ways and may remain in the brain for up to 28 days.
There is also the trouble of determining how high is too high even in states where cannabis is legal. While alcohol has a set 0.08 blood alcohol limit, it is difficult to set one for THC. Marijuana moves unevenly through the body from liquids into fatty tissues, like the brain, and then leeches into the blood again. Thus, measuring intoxication using biological or chemical means is difficult, especially during a traffic stop.
NBC News goes a step further to say that pharmacologists and brain scientists do not know how to measure THC intoxication and the extent of the impairment effects it may have on drivers. Sure, blood tests and urine tests may pick up THC, but the person may have indulged weeks prior. Some of the potential ways THC impairs drivers are slowing reflexes and making it more difficult to focus.
While there is a general consensus that driving high is dangerous, it may take a lot of further research and test developments before states have a solid threshold to work with. Note that in Texas, the recreational use of cannabis is illegal.