Середина маршрута на картах Google

var directionDisplay;
var directionsService = new google.maps.DirectionsService();
var map;
var polyline = null;
var infowindow = new google.maps.InfoWindow();

function createMarker(latlng, label, html) {
  var contentString = '<b>' + label + '</b><br>' + html;
  var marker = new google.maps.Marker({
    position: latlng,
    map: map,
    title: label,
    zIndex: Math.round(latlng.lat() * -100000) << 5
  });
  marker.myname = label;

  google.maps.event.addListener(marker, 'click', function() {
    infowindow.setContent(contentString + "<br>" + marker.getPosition().toUrlValue(6));
    infowindow.open(map, marker);
  });
  return marker;
}

function initialize() {
  directionsDisplay = new google.maps.DirectionsRenderer({
    suppressMarkers: true
  });
  var chicago = new google.maps.LatLng(41.850033, -87.6500523);
  var myOptions = {
    zoom: 6,
    mapTypeId: google.maps.MapTypeId.ROADMAP,
    center: chicago
  }
  map = new google.maps.Map(document.getElementById("map_canvas"), myOptions);
  polyline = new google.maps.Polyline({
    path: [],
    strokeColor: '#FF0000',
    strokeWeight: 3
  });
  directionsDisplay.setMap(map);
  calcRoute();
}

function calcRoute() {
  var start = document.getElementById("start").value;
  var end = document.getElementById("end").value;
  var travelMode = google.maps.DirectionsTravelMode.DRIVING

  var request = {
    origin: start,
    destination: end,
    travelMode: travelMode
  };
  directionsService.route(request, function(response, status) {
    if (status == google.maps.DirectionsStatus.OK) {
      polyline.setPath([]);
      var bounds = new google.maps.LatLngBounds();
      startLocation = new Object();
      endLocation = new Object();
      directionsDisplay.setDirections(response);
      var route = response.routes[0];
      var summaryPanel = document.getElementById("directions_panel");
      summaryPanel.innerHTML = "";

      // For each route, display summary information.
      var path = response.routes[0].overview_path;
      var legs = response.routes[0].legs;
      for (i = 0; i < legs.length; i++) {
        if (i == 0) {
          startLocation.latlng = legs[i].start_location;
          startLocation.address = legs[i].start_address;
          marker = createMarker(legs[i].start_location, "midpoint", "", "green");
        }
        endLocation.latlng = legs[i].end_location;
        endLocation.address = legs[i].end_address;
        var steps = legs[i].steps;
        for (j = 0; j < steps.length; j++) {
          var nextSegment = steps[j].path;
          for (k = 0; k < nextSegment.length; k++) {
            polyline.getPath().push(nextSegment[k]);
            bounds.extend(nextSegment[k]);
          }
        }
      }

      polyline.setMap(map);

      computeTotalDistance(response);
    } else {
      alert("directions response " + status);
    }
  });
}

var totalDist = 0;
var totalTime = 0;

function computeTotalDistance(result) {
  totalDist = 0;
  totalTime = 0;
  var myroute = result.routes[0];
  for (i = 0; i < myroute.legs.length; i++) {
    totalDist += myroute.legs[i].distance.value;
    totalTime += myroute.legs[i].duration.value;
  }
  putMarkerOnRoute(50);

  totalDist = totalDist / 1000.
  document.getElementById("total").innerHTML = "total distance is: " + totalDist + " km<br>total time is: " + (totalTime / 60).toFixed(2) + " minutes";
}

function putMarkerOnRoute(percentage) {
  var distance = (percentage / 100) * totalDist;
  var time = ((percentage / 100) * totalTime / 60).toFixed(2);
  if (!marker) {
    marker = createMarker(polyline.GetPointAtDistance(distance), "time: " + time, "marker");
  } else {
    marker.setPosition(polyline.GetPointAtDistance(distance));
    marker.setTitle("time:" + time);
  }
}
google.maps.event.addDomListener(window, 'load', initialize);

// from http://www.geocodezip.com/scripts/v3_epoly.js, modified to use the geometry library
// === A method which returns a GLatLng of a point a given distance along the path ===
// === Returns null if the path is shorter than the specified distance ===
google.maps.Polyline.prototype.GetPointAtDistance = function(metres) {
  // some awkward special cases
  if (metres == 0) return this.getPath().getAt(0);
  if (metres < 0) return null;
  if (this.getPath().getLength() < 2) return null;
  var dist = 0;
  var olddist = 0;
  for (var i = 1;
    (i < this.getPath().getLength() && dist < metres); i++) {
    olddist = dist;
    dist += google.maps.geometry.spherical.computeDistanceBetween(this.getPath().getAt(i), this.getPath().getAt(i - 1));
  }
  if (dist < metres) {
    return null;
  }
  var p1 = this.getPath().getAt(i - 2);
  var p2 = this.getPath().getAt(i - 1);
  var m = (metres - olddist) / (dist - olddist);
  return new google.maps.LatLng(p1.lat() + (p2.lat() - p1.lat()) * m, p1.lng() + (p2.lng() - p1.lng()) * m);
}

html {
  height: 100%
}

body {
  height: 100%;
  margin: 0px;
  padding: 0px
}

<script type="text/javascript" src="http://maps.google.com/maps/api/js?key=AIzaSyCkUOdZ5y7hMm0yrcCQoCvLwzdM6M8s5qk&libraries=geometry"></script>
<div id="tools">
  start:
  <input type="text" name="start" id="start" value="Hyderabad" /> end:
  <input type="text" name="end" id="end" value="Bangalore" />
  <input type="submit" onclick="calcRoute();" /><br />
</div>
<div id="map_canvas" style="float:left;width:70%;height:80%;"></div>
<div id="control_panel" style="float:right;width:30%;text-align:left;padding-top:20px">
  <div id="directions_panel" style="margin:20px;background-color:#FFEE77;"></div>
  <div id="total"></div>
</div>