The mayor signaled a possible compromise Monday on a proposed ban on plastic grocery bags. View Full Caption Flickr/life_chasing

CITY HALL — The number of plastic and paper bags Chicagoans used to haul home their groceries dropped by 45 percent after city officials imposed a 7-cents-per-bag tax in an effort to keep the disposable sacks out of area landfills, city budget documents reveal.

Because of the drop in use, the bag tax is now expected to generate $5 million for the city this year. When the tax went into effect in February, city officials had initially expected it to add $9.2 million to the city's coffers in that time, according to Mayor Rahm Emanuel's 2018 spending plan.

The city had already dropped its estimate of how much the tax would bring in once before. In July, bag use had fallen so much that officials believed it would bring in $7.7 million from the tax for the year.

City officials also expect the bag tax to generate $5 million in 2018, according to the spending plan.

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A study paid for by the city — and conducted by ideas42, a behavior design lab, as well as researchers from New York University and the University of Chicago Energy and Environment Lab — found the number of plastic and paper bags Chicagoans used to haul home their groceries dropped 42 percent in the first month after the tax was imposed in February 2017.

The study measured plastic and paper bag use at large grocery stores in Chicago one month before and one month after the tax was imposed.

Mayor Rahm Emanuel said he was thrilled by the study's findings.

"By decreasing our paper and plastic bag use, Chicago is making important progress in reducing our carbon footprint as well as reducing street litter and improving recycling operations," Emanuel said in a statement in April.

Before the tax went into effect Feb. 1, shoppers took home an average of 2.3 disposable bags every time they shopped at a big grocery store. After the tax went into effect, shoppers took home one fewer bag, according to the study.

The tax was prompted after the city's ban on single-use, thin plastic bags approved by the City Council in 2014 was largely considered a failure after many retailers switched to thicker bags that were considered worse for the environment.

Under the current tax, the city gets a nickel from the sale of each bag, with the store owner getting the other two cents.