def correct_ttime_values(df, update_val=999):
    """Update travel times between TAZs so that when travel is impossible, ttime is very large. 
    This is necessary because in some of the original files, ttime=0 when travel between TAZs is impossible
    Use for when the original matrix is not missing any TAZ pairs. 
    Args: 
        df: matrix with ttimes between TAZs, where 0 indicate
        update_val (num): val to replace 0
    Returns: 
        DataFrame: matrix with updated ttimes between TAZs
    """
    return df.replace(to_replace=0, value=999)