According to Massachusetts' White Cane Law, if you see a pedestrian using a guide dog or other service animal or carrying a white cane at an intersection, you must

If you see a blind or visually impaired pedestrian in the roadway, stop until the person is off the roadway. (Note: The laws on this are slightly different in some other states. In some states, you must stop at least 10 feet from a blind or visually impaired pedestrian. When traveling out of state, always check the local traffic laws.) Do not use your horn; it could startle the blind pedestrian.