Retail vacancies have increased in recent years on the mile-long stretch in the Back Bay neighborhood, which contains eight blocks of shops, ranging from upscale Chanel to fast-fashion Forever 21, plus salons, galleries and cafés.

The iconic shopping street has been facing increased competition from the nearby Prudential Center and the Copley Place mall, and also from the downtown and Seaport districts.

Newbury Street, one of the city’s best-known upscale shopping boulevards, has seen little significant new development in years. And to a great extent, its retail inventory consists of 19th-century row houses that are charming but relatively small and inflexible. Moreover, high rents and strong opposition to new development from local residents have caused many growing retailers and trendy eateries to relocate to other parts of the city.

“Newbury’s not one single owner,” said Chris Talanian, director of business development at Boston-based C. Talanian Realty, which owns a large portfolio on the corridor, speaking to Urban Land. “It’s not a lifestyle center, where someone controls the ecosystem and can do Art Fridays or other events to drive the atmosphere.”

So the city has stepped in to help. Three years ago the city of Boston, together with the Back Bay Association and individual retailers, launched a continuing initiative that has reinvigorated Newbury Street. The program, called Open Newbury Street, actually closes the avenue to motor vehicles and turns it into a pedestrian zone for three Sundays every summer (this year’s program ends on Sept. 9).

Open Newbury Street is restricted to the existing merchants and food purveyors. The city provides streamlined permitting to enable the retailers to take advantage of the outdoor space with clothing racks or chairs and tables. For the past three years, the event has grown in duration and success. Some chains even reported a doubling of sales during the period.

New this year is a mobile parklet that occupies two parking spaces and gives pedestrians a place to sit and unwind among planters. The parklet is designed by landscape architect Kyle Zick and sponsored by developer Jamestown, which manages several properties on the street.