The nationwide push to legalize marijuana has gained some traction in Virginia. Medical marijuana has been legal for several years, and a current bill in the state legislature would decriminalize possession of certain amounts of the substance. However, even in states where recreational use of marijuana is legal, it is still against the law to drive while under its influence, just as it is illegal to drive after drinking alcohol, another legal substance. The reason is the same: Alcohol and marijuana each have the ability to impair driving ability.

According to the National Highway Traffic Safety Administration, marijuana and its active ingredient, THC, impair several functions necessary for safely handling a motor vehicle:

  • Multitasking
  • Lane tracking
  • Cognitive functioning
  • Motor skills

Research conducted over the course of several decades has demonstrated marijuana’s ability to negatively affect these functions.

Nevertheless, misconceptions and rumors persist that marijuana does not impair driving. The truth is a bit more complicated. According to the Centers for Disease Control, it is difficult to firmly establish that marijuana increases the risk of car crashes because there are so many variables that are difficult, if not impossible, to control.

For example, there may be multiple drugs and/or alcohol in the driver’s system, and it can be difficult to determine which specific substance bears the most responsibility for the impairment. Depending on how often a person uses marijuana, and how much at a time, it may be days or weeks after the last use that the substance still shows up in the person’s system with drug testing. As of yet, there is no accurate roadside test for drug levels. If a person demonstrates an elevated blood alcohol level, authorities may not even conduct drug testing because the BAC level is sufficient evidence with which to charge a driver.

Marijuana is one of the substances most associated with drugged driving, after alcohol. There appears to be a greater risk of impairment from using both together than either one or the other.