BOSTON – Motorists cruising near reservoirs and streams may notice roadside signs that say: Low Salt Area.The three words inform winter snowplow drivers to shut off their salt spreaders in order to reduce water pollution and the destruction of wildlife habitat. But salt isn’t the only pollutant. Oil, grease and other substances routinely wash off the highway and into rivers, streams and bays.A federal court judge says the time has come to stop such sloppy environmental stewardship and, in a landmark decision, ruled that the Massachusetts Highway Department has been remiss in its public duties.Judge William G. Young ruled on May 30 that MassHighway violated federal clean water laws by failing to develop an adequate stormwater cleanup plan for its 2,500 miles of urban roads and bridges.The precedent-setting decision stemmed from a lawsuit filed by environmentalists, charging MassHighway was not properly implementing a provision of the federal Clean Water Act that requires them to control and clean up polluted water that flows off roadways into neighboring lakes and streams.”Rivers, streams and lakes throughout Massachusetts are suffering serious water quality problems due to untreated stormwater from MassHighway’s roads and bridges. The court’s ruling is a landmark victory for every resident concerned about the impact this major source of water pollution has on their rivers, lakes and streams,” said Chris Kilian, director of the Clean Water and Healthy Forests Program at the Conservation Law Foundation (CLF), a New England environmental group whose legal victories paved the way for the clean up of Boston Harbor and protection of Georges Bank, a critical ocean fishing ground.Cynthia Liebman, a CLF staff attorney, said MassHighway long ignored its legal responsibility to protect the state’s water resources. “The court has asked them to live up to that responsibility,” she said.After a six-day trial, the judge ordered MassHighway to commit to a two-year schedule for adopting a new, revised stormwater pollution cleanup plan. He also instructed the agency to quickly address pollution hot spots in Lancaster and the Charles River Watershed.Polluted stormwater is a significant factor in the ongoing failure of Massachusetts’s rivers and lakes to achieve basic water-quality standards.