Few industries, if any, are growing as quickly as legal marijuana. According to cannabis research firm ArcView, the North American legal-weed industry generated $6.9 billion in sales in 2016, which still pales in comparison with the $46.4 billion in sales estimated on the black market. However, by 2021 it's believed that the legal North American pot market will have vaulted in value to $21.6 billion, representing a compound annual growth rate of 26%.

If there's a single factor that can be pinpointed for this growth, it'd be the rapid shift in consumer opinion toward marijuana. Gallup, which has taken periodic polls on cannabis' favorability for nearly 50 years, found in 1995 that only a quarter of those surveyed wanted it to be legal across the United States. Of course, this poll was also conducted during the War on Drugs. As of 2016, Gallup's same poll found that an all-time record 60% of respondents want to see it legal nationally. The belief is that as this figure rises, pressure will mount on lawmakers in Congress to adjust their scheduling of the drug.

Paramount to the success of the U.S. legal-weed industry, aside from organic growth in already legal states, is the ability to expand into new states. The 2016 elections saw a nearly "green" sweep, with marijuana initiatives and amendments being passed in eight out of nine states (sorry, Arizonans). The chances are decent that the 2018 elections will bring another serious opportunity for adult-use marijuana to be legalized in a handful of states.

Which states are the likeliest to vote on recreational marijuana in 2018?

Of course, we have to remember that not all states have the same voting process. Whereas some states have a referendum and initiative process, whereby residents can gather signatures to introduce and vote on propositions, bills in other states have to be introduced and passed entirely at the legislative level. For instance, both houses of Vermont's legislature voted in favor of legalizing recreational cannabis earlier this year, however Gov. Phil Scott (R-Vt.) vetoed the measure. Despite presumed strong support for recreational cannabis in Vermont, its residents have no official say in the process, save for voting Scott out of office, should they choose to do so in a future election. Two dozen states in the U.S. lack this referendum and initiative process.

So which states are the likeliest to have their residents heading to the polls in 2018 to vote on a recreational marijuana bill? Once we exclude the aforementioned states without a referendum and initiative process, the following three rise clearly to the top of the list.

Arizona

One of the more interesting quirks of voting on marijuana bills is that the first time isn't always the charm -- but the second time seems to be. In the November 2016 elections, Arizona wound up voting down Prop 205 (52% opposed to 48% in favor), which would have legalized recreational marijuana in the state. However, the margin of loss was close enough that pro-legalization groups are expected to focus their efforts on the state for the 2018 election.

History would suggest that Arizona has a decent shot of approval in 2018. Oregon, which is one of the most pro-legalization states in the country, initially failed to legalize an adult-use measure itself when voters went to the polls in 2012. Oregon wound up officially legalizing through a 2014 vote by state residents.

The same can be said for Florida, which voted down a medical-marijuana amendment by just 2% in 2014. With a new medical-cannabis amendment on the ballot in 2016, and pro-legalization groups putting a lot of effort into Florida between 2015 and 2016, the measure passed with ease last November. The second time could very well be the charm for Arizonans.

Michigan

Residents in Michigan have been here before. It appeared that the Wolverine State would have no issues getting a recreational-weed initiative on the 2016 ballots, but they were thwarted by a number of invalid signatures. The upcoming election in 2018 could be redemption for pro-legalization groups.

According to MLive.com, as of mid-July the Coalition to Regulate Marijuana Like Alcohol (CRMLA) had collected more than 100,000 signatures, which is well on its way to the 252,523 valid signatures needed to get an initiative on the ballot come the 2018 elections. CRMLA spokesman Josh Hovey has suggested that the current momentum could see the pro-legalization group hitting its signature target in four months instead of the six months given to collect valid signatures. If approved, there would be a 10% excise tax and 6% sales tax on legal weed.

Polling data also suggests that Michigan presents a compelling case to legalize. A statewide poll of 600 Michigan voters from Marketing Resource Group this past May found that 58% support legalizing adult-use weed, compared to just 36% who opposed it. This is a stark reversal from the 55% who opposed it and 41% who supported the idea of recreational legalization in 2013.

Ohio

Lastly, Ohioans could head to the polls for a second time in three years in 2018 to vote on recreational marijuana. The last time they went to the polls in November 2015, they voted down Issue 3 with authority (35% to 65%).

However, we have to keep in mind that the reason Ohioans voted down Issue 3 was that it had two inherent flaws. To begin with, Issue 3 called for just 10 predetermined sites to grow and manufacture recreational weed. It would have created an oligopoly for some well-to-do backers and ensured that competition stayed out of their way for years. It would have been terrible for Ohio's consumers. The second problem with Issue 3 was that it was a dual medical and recreational marijuana legalization measure. That's never been done before, and it's exceptionally difficult to pull off without the medical infrastructure already in place.

The reason 2018 looks so promising is that Ohio's legislature wound up passing a medical-marijuana law in 2016, not too long after Issue 3 was defeated. By 2018, the state will have about two years of planning under its belt, and presumably around a year of experience with medical cannabis sales and patients. This, along with a focus from pro-legalization groups, could make Ohio a state likely to vote on recreational cannabis next year.