lines are objects with an optional label field, and a vertices field which contains a list of objects with x and y attributes, similar to polygons. Additionally, line objects have an optional spline flag, which determines whether the curve used is linear (if the value is false or the param is not passed) or cardinal (if the value is true).
{
...
"lines": [
{
"vertices": [
{
"y": 323,
"x": 414
},
{
"y": 164,
"x": 616
},
{
"y": 334,
"x": 776
}
],
"label" : "crosswalk",
"spline": true
}
]
}