So how many plastic bags did Montgomery County shoppers use before they started to cost a nickel each?

Last week’s story about the county’s year-old bag tax noted that revenues from the fee have been higher than projected. That suggested consumers have not been as quick to abandon the environmentally harmful sacks as hoped.

While there is strong anecdotal evidence that waterways and stream banks are cleaner, the county acknowledged it had no firm estimate of pre-tax bag usage. Officials established 82.9 million as a baseline, but acknowledged that it had extrapolated the figure from the District, which had adapted its estimate from the city of Seattle when it passed its bag fee.

Now comes one possible answer: 300 million.

It’s from a 2009 report by the U.S. International Trade Commission, which said Americans used 102.1 billion plastic bags in 2008. That’s roughly 335 bags per person, or about 300 million bags annually in Montgomery, said Julie Lawson, director of the Trash Free Maryland Alliance, who cited the study in a statement Tuesday.

“If the pre-fee bag usage was in fact closer to 300 million, then the post-fee numbers actually indicate significant behavior change, in the neighborhood of a 60-70 percent reduction,” Lawson said. That is close to what the District has reported since putting its bag fee into effect.

It means, she said, that the bag tax revenue taken in by Montgomery ($2 million through November 2012) is actually not higher than desirable, but seems to be “right on track.”