You're right Tim. Actually marginal means to be at the margin or at the fringe. That road is called the marginal, because it follows the margin of the river from Lisboa to Cascais, and that gives it the "Ocean Boulevard" aura.
When you questioned "Marginais" I thought you mistook marginais for the other sense of the word - marginais can also mean people who live at the margin of society. It is not a nice complement to call someone a marginal.
