How Substances Impact Motor Impairment

How Substances Impact Motor Impairment

Driving impaired by any substance, including marijuana, alcohol and opioids, is illegal in all 50 states and the District of Columbia. Though marijuana is becoming legalized in specific city and state jurisdictions, substance levels are still examined and determined by law enforcement once a driver is in a traffic stop. Law enforcement officers are trained to observe drivers’ behaviors and to identify impairment for all types of drug usage.

According to a 2020 study completed by the National Transportation Library, 56% of drivers involved in serious injury and fatal crashes tested positive for at least one drug(1). It is important to understand and know how specific substances impact motor impairment and know the ways to stay safe when consuming alcohol, marijuana, or opioids.


Some of the most common physical, psychological, and behavioral signs and symptoms of alcohol can begin immediately after usage. Among drivers with BAC levels of 0.08% or higher involved in fatal crashes in 2016, nearly three in 10 were between 25 and 34 years of age. Common symptoms of alcohol impairment include:

  • Slurred speech
  • Memory impairment
  • Impaired thinking


While becoming legalized across the United States, marijuana is known to impair motor skills, lane tracking and cognitive functions. A 2015 study in the Journal of Analytical Toxicology explored how driving after smoking cannabis with THC delays a driver’s ability to multitask; a critical skill needed behind the wheel(2). A few symptoms of marijuana impairment include:

  • Impaired judgment
  • Slower motor coordination
  • Delayed reaction time


According to a 2017 study on the use of prescription opioids and motor vehicle crashes, being under the influence of opioids while driving can nearly double the risk of being in a motor vehicle accident. Experts agree that driving or operating heavy machinery is unsafe and should be avoided until a stable dose has been reached. Symptoms of opioid impairment include:

  • Slower reaction time
  • Extreme drowsiness and dizziness
  • Clouded judgment

Gauging Impairment

EyeGage is actively researching and updating its mobile application to include features to improve and assist in individual and community safety to decrease accidents. New application features, including Should I Drive? and FriendGage, promotes easy ways for accountability and accessibility to understand impairment levels. Alcohol, marijuana, and prescription opioids should be consumed safely and understanding the symptoms of motor impairment will lead to a safer community.

(1) National Highway Traffic Safety Administration

(2) Journal of Analytical Toxicology, Volume 39, Issue 4, May 2015, Pages 251–261,