The crowd in the Theory Wellness parking lot illustrates the appeal of legal cannabis across a broad range of consumers. Great Barrington itself is demonstrating the wider economic impact that legalization can have, as some businesses in the area report a boost to their sales.

"Our business is drawing 1,000 people a day into town, about 90% of whom would otherwise not be here," said Brandon Pollock, chief executive of Theory Wellness, which is independently owned. "You see folks from New York City doing aggressive carpooling, with four or five people packed into a Zipcar."

That figure of a thousand people applies mainly to weekends and includes some visitors who came along for the ride and did not necessarily purchase weed. Even so, business for the recreational dispensary has so far been double what Pollock and his partners had projected, pushing the dispensary to add more than 40 employees, for a total of nearly 50, to deal with the demand. Several local shops have told him of a ripple effect on their business, with double-digit increases in sales since Theory added recreational offerings.

On a recent Saturday, the line that wound around the side of the parking lot included a handful of city residents as well as visitors from Albany; Saratoga Springs; Glens Falls; Newark, N.J.; Philadelphia; Hartford, Conn.; and Clemson, S.C. All had made the drive exclusively to shop at the dispensary—despite having connections to black market dealers at home. Though they knew they would pay more at a legal shop, they felt it was worth the added expense to be assured a safer product that they could buy in the open.

"I know a dealer, but I don't know where [his product] is coming from," said Steve, a middle-school math teacher in Brooklyn who was making his fourth monthly trip to Theory Wellness.

He planned to buy vape pens and cartridges costing between $200 and $300 that he said would help him relax and ease the pain in his leg from a motorcycle accident. He said he would combine the trip with a stop at the MGM Resorts casino, about an hour away in Springfield, Mass., but would have preferred buying his cannabis at home.

"Why do I have to drive 140 miles to come here?" Steve asked. "I'm just trying to get a little pain management."

For other New Yorkers, the excursion was more like a holiday. Bronx resident Priscilla, who works at a nonprofit for the homeless, had driven up with her husband and planned to stay at a nearby bed-and-breakfast.

"I said, 'Come on, it's legal!'" she recalled. "'We'll spend the whole weekend here, honey!'"

She had learned of Theory Wellness from her son-in-law, whom she said suffers from an anxiety disorder and prefers cannabis to his prescription medicine. Anxiety is not a qualifying condition for a medical-marijuana prescription in New York, so he has been driving up on a regular basis from the city.

Even those who were satisfied with their dealers were looking forward to making a legal purchase.

"We wanted to experience going into a store and being able to shop," said Anthony, an operations manager in Queens who had arranged the weekend trip as a birthday present for his girlfriend. "Usually, with a dealer, there's just one or two options."

The variety, he added, would make up for the higher prices. An eighth of an ounce of smokable weed—"flower" on the Theory Wellness menu—averages $50, plus $10 from the 20% sales tax, compared to black market prices in the city of $25 to $30. Anthony was prepared to spend as much as $350.

"My mother actually wants edibles," he added. "A few people put in orders with us."