A man with an assault-style rifle fired randomly at passing cars while striding down a busy road outside Boston , wounding two people and sending drivers scrambling until he was wounded when a state trooper and a former Marine opened fire, authorities said. As bullets tore through at least a dozen cars, including a state police cruiser, panicked motorists abandoned their vehicles or hid under them for cover, according to prosecutors and state police. Authorities said the gunman fired more than 60 rounds during the Monday afternoon attack. The shooting happened on a heavily traveled road along the Charles River in Cambridge , home to Harvard University and the Massachusetts Institute of Technology. Sidewalks and riverside paths in the area are often crowded with pedestrians, joggers and cyclists.…