protected static function processBatchTimesChunk($chunk, $timeout) {
    $pid = pcntl_fork();
    if ($pid>0) {
      // parent
      return;
    }
    // child or could not fork
    foreach ($chunk as $NaPTAN) {
      self::fetchData($NaPTAN, $timeout);
    }
    if ($pid != -1) exit;
  }

  public static function preCacheTimes(array $NaPTANs, $threads=5, $timeout=15) {
    if (!count($NaPTANs)) return array();
    $chunks = array_chunk($NaPTANs, ceil(count($NaPTANs) / $threads));
    // spawn threads
    foreach ($chunks as $chunk) {
      self::processBatchTimesChunk($chunk, $timeout);
    }
    // wait for completion
    $status=0;
    while (pcntl_wait($status, WNOHANG)>=0) { /* do nothing */ }
  }