With legalization sweeping across the US and medical marijuana up to the half way mark, many are raising concerns about people driving after having used marijuana. Here are five things you need to know about marijuana and driving.

1. What we Know

While we know that people who use marijuana can do simple tasks well while they’re high, they do have to use more of their brain to concentrate on the task. Driving relies on reaction times, and most people who use cannabis will attest to the fact it can slow you sown somewhere. It also affects your peripheral vision, so for things like people stepping out in front of a car, they cannot respond as quickly. The other side of this, is that people who use marijuana and drive tend to understand they are impaired and drive to compensate for it.

2. What we Don’t Know

Studies vary as to how much driving under the influence of marijuana can effect the risk of being in a crash, some studies say it doubles the risk while others saying it can even make drivers safer.

3. How High is Too High?

THC can be traced in the blood of some daily users for weeks after they last used the drug, meaning that blood tests aren’t a good way to test how impaired people are. Most states haven’t set a threshold for impairment, but the UK sets it at 2ng, which is enough to get most people a day after they smoke.

4. Real-World Experience

A year after the legalization in the Washington, 25% more people tested positive for marijuana than before legalization. A University of Colorado Medical School study found that the share of drivers involved in fatal motor vehicle crashes in Colorado who tested positive for marijuana more than doubled between 1994 and 2011. A National Safety Council study looked at the prevalence of drivers involved in fatal crashes who tested positive for marijuana in 12 states from 1992 to 2009, before and after implementing medical marijuana laws. Only three states showed an increase — California, Hawaii and Washington — and those appeared to be a one-time increase possibly associated with differences in testing.

 5. What’s Next

The National Highway Traffic Safety Administration is conducting research to get a better idea of how pot affects driving. NHTSA and Washington state officials have also teamed up to assess change in marijuana use by drivers before and after the state allowed retail sale of the drug, with results due next year.