ROCKVILLE — A five-cent tax on plastic bags isn’t changing the behavior of Montgomery County shoppers as much as officials had hoped.

A year after the fee went into effect, it’s generating twice the revenue officials expected. County data shows the bag tax is on pace to exceed $2 million in revenue. The county had estimated it would generate $1 million.

The tax was meant to discourage use of plastic, disposable bags to make up for litter found in streams and ponds. It’s meant to encourage use of reusable bags.

The District of Columbia created the region’s first bag fee in 2010. The city claimed an 80 percent drop in bag usage the first year.

But officials now acknowledge they’re not sure how many bags were used before the taxes.