Threshold = {
  change: function() {
    var i = this.selectedIndex;
    this.value = this.options[i].value
    $$('#nav li.threshold a').each(function(a){
      a.href = a.href.toString().sub(/\/threshold\/[\d{1,3}\/]*/, '/threshold/'+ this.value+ '/')
    },this)
  }
}

$('threshold')['threshold'].onchange = Threshold.change