The use of cannabis and prescription and other drugs are increasingly prominent on our roadways, where 16.2 percent of the nation’s 37,461 fatalities in 2016 were related to drug-involved driving. In the United States, several states have legalized the use of medical and/or recreational cannabis, increasing concerns about traffic safety. Aside from alcohol, cannabis is the most frequently detected drug in drivers who are involved in collisions. The impact of drugs on the brain and behavior varies considerably depending on the type of drug and how it is metabolized. There are also large variations across jurisdictions in the frequency of testing suspected impaired drivers for drugs, the consistency of laboratory drug testing practices, and the capacity of law enforcement. In the United States, 6,058 people were killed in drug-involved collisions in 2016, a 9.5 percent decrease from 6,696 in 2015, and a 7.6 percent increase from 5,630 in 2012. In 2016, among fatally injured drivers with known drug tests, 42.7 percent were positive for legal or illegal drugs.