As the Northstar commuter rail line approaches its first birthday, ridership is about 5 percent below projections. The 40-mile line from Big Lake to downtown Minneapolis began carrying commuters Nov. 16, 2009, amid much fanfare. Its trains carried thousands of Twins fans to the new ballpark this season. But Metro Transit spokesman Bob Gibbons says Northstar’s popularity with commuters has been hampered by high unemployment, relatively low gas prices and mild fall weather. Plus, he says, the state recently improved U.S. Highway 10. And parking rates in downtown Minneapolis have dropped along with the economy. Lagging ridership is one reason officials decided last week against applying for federal funding to extend the line from Big Lake to St. Cloud.