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On a Saturday night on a lonely stretch of country road, two cars approach each other. Silently, for no apparent reason, one car drifts into the oncoming lane. In a tornado of metal and glass, the Grim Reaper enters the scene. One driver is dead, the other is injured severely. When the medical personnel arrive, the waste of life hurts even the most hardened EMTs.

Later, the toxicology group reports that one driver had been using both alcohol and marijuana.

If you think this horrible event is rare, consider this. From 1993 to 2014, there were 14,742 incidents in which two cars collided and at least one person died. One car drifting into the other's lane caused 43% of all fatal two-car accidents.

The United State Department of Transportation keeps track of these kinds of collisions though the National Highway Traffic Safety Administration. Data on these crashes is carefully organized in the Fatal Analysis Reporting System (FARS).

FARS was a goldmine for public health researchers Guohua Li, Stanford Chihuri, and Joanne E. Brady. They combed the data to answer the question, what role did alcohol and marijuana play in these 14,742 accidents.

Working out of Columbia University's Mailman School of Public Health, they arrived at a frightening conclusion. The person who initiated the crash was more likely to have recently used alcohol, marijuana, or both at the same time.

In the collision above, the person who drifted into the coming lane was more likely to have recently used alcohol (28% of the cases) than the person who was not responsible for the accident (10%).

The person who drifted into the wrong lane was more likely to have used marijuana (10%) than the innocent driver (6%).

The person who caused the accident was more likely to have used both alcohol and marijuana (4%) than the driver in the other car (1%).

These data indicate that the use of either or both substances were not detected in most fatal accidents. But when they were present, in any combination, they were more likely to be present in the driver who caused the collision to occur.

This research from the Mailman School team raises some interesting public health questions. With an increasing number of states legalizing the use of marijuana, what safeguards, if any, do we want when we know that there is a significantly greater chance you will cause an accident if you are under the influence of or more of these drugs.

Our popular understanding is that driving and alcohol are a deadly combination, and the data from two-car fatalities bear that out. But marijuana use, and combination use, also increases the risk of collision.