Esta librería se usa para poner multiples marcas (generalmente flechas indicando la dirección) en un trayecto y mostrar sólo las que no colapsan entre si; dejando una distancia en pixeles entre dos marcas.

TrajectoryMarkers.js

var TrajectoryMarkers = function(map, arrayMarkers, separation){
	this.set('markers', arrayMarkers || []);
	this.set('separation', separation || 50);//pixeles de separación
	this.setMap(map);
}
TrajectoryMarkers.prototype = new google.maps.OverlayView();
TrajectoryMarkers.prototype.onAdd = function(){
    var self = this;
    google.maps.event.addListener(this.getMap(), 'zoom_changed', function(){
        self.draw();
    })
}
TrajectoryMarkers.prototype.draw = function () {
	var n, pixelPositionA, pixelPositionB, sizeA, x, y, 
		abs = Math.abs,
		projection = this.getProjection(),
		separation = this.separation;
	n = this.markers.length;
    pixelPositionA = projection.fromLatLngToContainerPixel( this.markers[n-1].getPosition() );
    while(n--){
        this.markers[n].setMap(null);
        if(n>1){
            pixelPositionB = projection.fromLatLngToContainerPixel( this.markers[n-1].getPosition() );
            x = (pixelPositionA.x - pixelPositionB.x);
            y = (pixelPositionA.y - pixelPositionB.y);
            sizeA = this.getMarkerSize(this.markers[n]);
            
            if( this.isValid(pixelPositionA, sizeA, pixelPositionB) ){
                pixelPositionA = projection.fromLatLngToContainerPixel( this.markers[n].getPosition() );
                this.markers[n].setMap( this.getMap() );
            }
        }
    }
    this.markers[0] && this.markers[0].setMap(this.getMap());
}
TrajectoryMarkers.prototype.getMarkerSize = function (marker) {
	var icon,  size = marker.get('__iconsize');
	if(!size){
		icon = marker.getIcon();
		if(icon instanceof google.maps.MarkerImage){
			size = icon.size;
			marker.set('__iconsize', size);
		}
	}
	return size;
}
TrajectoryMarkers.prototype.isValid = function (pixelPositionA, sizeA, pixelPositionB) {
	var out = false, separationX, separationY;
	sizeA = sizeA || {width:0,height:0};

	separationX = this.abs(pixelPositionA.x)-this.abs(pixelPositionB.x);
	if( separationX > (this.separation + sizeA.width) ){
		return true;
	}
	separationY = this.abs(pixelPositionA.y) - this.abs(pixelPositionB.y);
	if( separationY > (this.separation + sizeA.height) ){
		return true;
	}
	return out;
}
TrajectoryMarkers.prototype.abs = Math.abs;