This is cheaper to store than a MovementID. It simply indexes into the list of movements.
A Movement groups all turns from one road to another, letting traffic signals and pathfinding operate at a higher level of abstraction.
A movement is like a turn, but with less detail – it identifies a movement from one directed road to another. One road usually has 4 crosswalks, each a singleton Movement. We need all of the information here to keep each crosswalk separate.