Use of the MoPac toll lanes has been steadily growing in the seven months since the lanes opened throughout their entire 11-mile lengths, but it is falling slightly short of a first-year estimate made before construction began. Revenue from the added lanes on each side of North MoPac Boulevard, due to variable toll rates that on average have been higher than expected, has at least met expectations this spring and could exceed them when the agency begins in earnest to pursue unpaid tolls.