Junction Finding by mbostock

index.html

<!DOCTYPE html>
<meta charset="utf-8">
<style>

.mesh {
  fill: none;
  stroke: #333;
  stroke-width: .5px;
  stroke-linejoin: round;
}

.junction {
  fill: black;
  stroke: white;
}

</style>
<body>
<script src="//d3js.org/d3.v3.min.js"></script>
<script src="//d3js.org/topojson.v1.min.js"></script>
<script>

var width = 960,
    height = 500;

var path = d3.geo.path()
    .projection(null);

var svg = d3.select("body").append("svg")
    .attr("width", width)
    .attr("height", height);

d3.json("us.json", function(error, us) {
  if (error) throw error;

  svg.append("path")
      .datum(topojson.mesh(us))
      .attr("class", "mesh")
      .attr("d", path);

  var junctionByPoint = d3.map(),
      arcCountsByPoint = d3.map();

  us.arcs.forEach(function(arc) {
    var start = arc[0],
        startCount = arcCountsByPoint.get(start) || 0,
        end = arc.reduce(function(p, v) { return [p[0] + v[0], p[1] + v[1]]; }),
        endCount = arcCountsByPoint.get(end) || 0;
    if (arcCountsByPoint.set(start, startCount + 1) > 1) junctionByPoint.set(start, start);
    if (arcCountsByPoint.set(end, endCount + 1) > 1) junctionByPoint.set(end, end);
  });

  svg.selectAll(".junction")
      .data(junctionByPoint.values())
    .enter().append("circle")
      .attr("class", "junction")
      .attr("transform", function(d) { return "translate(" + transform(d) + ")"; })
      .attr("r", 2.5);

  function transform(point) {
    return [
      point[0] * us.transform.scale[0] + us.transform.translate[0],
      point[1] * us.transform.scale[1] + us.transform.translate[1]
    ];
  }
});

</script>