Cars drive past a billboard advertising the belief that marijuana is safer than alcohol or football alongside a highway in Newark, New Jersey, Jan. 29, 2014. Several states have voted to legalize recreational marijuana use, though the drug remains illegal under federal law. Experts have debated whether driving high is more or less dangerous than driving drunk. Reuters

Hitting a bong is less harmful than hitting the bottle, advocates of marijuana legalization have claimed. As states increasingly move toward legalizing recreational pot, concerns over what constitutes responsible marijuana consumption has grown. Legal weed has left law enforcement, policymakers and the public alike to figure out which behaviors, like driving, are safe to do while high, and which should be avoided.

Whether driving stoned is more or less dangerous than driving drunk has long been debated. Research on the subject is hazy, at best, and there’s little consensus among psychologists or law enforcement on the issue. For every study suggesting marijuana use impairs driving ability, there's another study showing a high driver is no more likely to be involved in a crash than a sober driver.

One thing research has shown for sure is this: People who are stoned drive differently than people who are drunk. Drunk drivers speed. High drivers favor a less hurried approach.

A 1993 report from the U.S. Department of Transportation found that high drivers are only moderately impaired. “The impairment manifests itself mainly in the ability to maintain a steady lateral position on the road, but its magnitude is not exceptional in comparison with changes produced by many medicinal drugs and alcohol,” the report concluded. In short, marijuana’s “adverse effects on driving performance appear relatively small … All subjects were willing and able to finish the driving tests without great difficulty.”

Several states this year have made possessing and consuming pot legal for people 21 or older. Colorado and Washington were the first to legalize recreational pot in 2012, and the states opened their first pot retail stores this year.

Experts say the contrast between a drunk driver and a stoned driver comes down to awareness. A drunk driver tends to overestimate his or her ability, whereas a stoned driver usually knows that he or she is impaired. “The joke with that is Cheech and Chong being arrested for doing 20 on the freeway,” Mark Kleiman, a professor of public policy at the UCLA School of Public Affairs, told the New York Times in February.

Despite claims that high driving is safer than drunk driving, law enforcement has seen a spike in recent years in fatal car crashes involving drivers who were stoned. The number of deadly accidents involving marijuana has tripled over the last 10 years, according to a report published in February in the American Journal of Epidemiology. Today, one in nine drivers involved in fatal accidents tested positive for pot.

Here are four ways marijuana can affect driving ability:

Marijuana impairs a person’s ability to shift focus. Tetrahydrocannabinol, commonly known as THC, is the main active ingredient in marijuana that produces the high effect most consumers are after. THC has been known to impair focus, according to the National Institute of Drug Abuse.

Marijuana affects judgment, attention and other cognitive functions. These things “are impaired among heavy marijuana users, even after users discontinued its use for at least 24 hours,” according to PBS Frontline.

Marijuana makes people more relaxed. People who are high have more difficulty concentrating and can sometimes go into dreamlike states, studies have shown.

Marijuana can impair short-term memory and perception. “Memory impairment from marijuana use occurs because THC alters how information is processed in the hippocampus, a brain area responsible for memory formation,” the National Institute of Drug Abuse reported.