score:9

Accepted answer

i would create two separate g elements for the layers in the beginning. this way you can control their order:

var countourg = svg.append("g"),
    streetg = svg.append("g");

d3.json("streets.json", function(error, topology) {
  streetg.selectall("path")...
});

d3.json("contour.json", function(error, topology) {
  contourg.selectall("path")...
});

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