These county, state and country boundaries are extracted from a single TopoJSON file. Counties are stroked in thin gray, states in thin black, and the country in thick black.
<!DOCTYPE html>
<meta charset="utf-8">
<style>
path {
fill: none;
stroke: #000;
stroke-width: .5px;
}
.land-boundary {
stroke-width: 1px;
}
.county-boundary {
stroke: #aaa;
}
</style>
<body>
<script src="https://cdnjs.cloudflare.com/ajax/libs/d3/3.5.5/d3.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/topojson/1.6.19/topojson.min.js"></script>
<script>
var width = 960,
height = 500;
var path = d3.geo.path();
var svg = d3.select("body").append("svg")
.attr("width", width)
.attr("height", height);
d3.json("/mbostock/raw/4090846/us.json", function(error, topology) {
if (error) throw error;
svg.append("path")
.datum(topojson.feature(topology, topology.objects.land))
.attr("d", path)
.attr("class", "land-boundary");
svg.append("path")
.datum(topojson.mesh(topology, topology.objects.counties, function(a, b) { return a !== b && (a.id / 1000 | 0) === (b.id / 1000 | 0); }))
.attr("d", path)
.attr("class", "county-boundary");
svg.append("path")
.datum(topojson.mesh(topology, topology.objects.states, function(a, b) { return a !== b; }))
.attr("d", path)
.attr("class", "state-boundary");
});
</script>