Return a list of start time for each headway-based run.

    Returns:
      a sorted list of seconds since midnight, the start time of each run. If
      this trip doesn't have headways returns an empty list.

def GetFrequencyStartTimes(self):
    """Return a list of start time for each headway-based run.

    Returns:
      a sorted list of seconds since midnight, the start time of each run. If
      this trip doesn't have headways returns an empty list."""
    start_times = []
    # for each headway period of the trip
    for freq_tuple in self.GetFrequencyTuples():
      (start_secs, end_secs, headway_secs) = freq_tuple[0:3]
      # reset run secs to the start of the timeframe
      run_secs = start_secs
      while run_secs < end_secs:
        start_times.append(run_secs)
        # increment current run secs by headway secs
        run_secs += headway_secs
    return start_times