Joshua Gasper, the driver police say hit and killed Ohio State Trooper Kenneth Velez, is accused of being high on drugs at the time of the crash.
A recent study has found that more than 10 million Americans reported being under the influence of illicit drugs while behind the wheel.
Police do not test for drugs if alcohol is a bigger contributing factor. Many drivers who cause crashes have both drugs and alcohol, or more than one drug in their system. This makes it difficult to determine which substance had the greater effect.
But here's what we do know: Men are more likely than women to drive under the influence of drugs or alcohol, and it's more common among those 18 to 25. After alcohol, marijuana is the most common drug found in the blood of drivers involved in crashes. But the role marijuana plays in crashes is unclear.
Studies have shown marijuana caused an increase in lane weaving, poor reaction time, and altered attention to the road.
The next most common drug are prescription medications. A 2010 study found that 47 percent of drivers who tested positive for drugs had a prescription drug in their system, compared to 37 percent for marijuana, and 10 percent for cocaine. Many prescription drugs can lead to dizziness and drowsy driving.
According to the National Institute on Drug Abuse, it's almost impossible to measure how many care accidents are caused by drugged drivers because currently there is no road test to determine drug levels in the body.
Michigan just approved a one year pilot program to allow state police to administer a saliva test to drivers suspected of being on heroin, marijuana, and cocaine. The program will determine the accuracy and reliability of the test.