Google maps api - snap marker to nearest road

Use the directions service. Get driving directions from the point of interest to the point of interest. Should be on the road.

var directionsService = new google.maps.DirectionsService();

directionsService.route({origin:homeLatlng, destination:homeLatlng, travelMode: google.maps.DirectionsTravelMode.DRIVING}, function(response, status) {
  if (status == google.maps.DirectionsStatus.OK)
  {
    var homeMarker = new google.maps.Marker({
      position: response.routes[0].legs[0].start_location, 
      map: map,
      title: "Check this cool location",
      icon: image,
      shadow: shadow
    });
  } else {
    var homeMarker = new google.maps.Marker({
      position: homeLatlng,
      map: map,
      title: "Check this cool location",
      icon: image,
      shadow: shadow
    });
  }
});

example