title: Collision/overlap detection of circles in a d3 transition
body: <p>I'm using d3 to animate a route (path) on a map. When the route reaches a point along the route I'd like to popup some information. </p>
  
  <p>Most of my code is based on the following example. <a href="http://bl.ocks.org/mbostock/1705868" rel="nofollow">http://bl.ocks.org/mbostock/1705868</a>.  I'm really just trying to determine if there is a way to detect when the transitioning circle collides or overlaps any of the stationary circles in this example. </p>