def assign_national_road_terrain(x):
    """Assign terrain as flat or mountain to national roads

    Parameters
        x - Pandas DataFrame of values
            - dia_hinh__ - String value of type of terrain

    Returns
        String value of terrain as flat or mountain
    """
    terrain_type = x.dia_hinh__

    if terrain_type is None:
        return 'flat'
    elif 'flat' in terrain_type.lower().strip():
        # Assume flat for all roads with no terrain
        return 'flat'
    else:
        # Anything else not included above
        return 'mountain'