heed pandas warning

backend/app/get_travel_time.py

@@ -97,7 +97,9 @@ def get_travel_time(start_node, end_node, start_time, end_time, start_date, end_
     # filter out hours with too much missing data
     observations = hr_sums[ hr_sums['length'] / total_corridor_length >= 0.8 ]
     # extrapolate over missing data within each hour
-    observations['tt_extrapolated'] = observations['tt'] * total_corridor_length / observations['length']
+    observations = observations.assign(
+        tt_extrapolated = lambda r: r.tt * total_corridor_length / r.length
+    )
     # convert to format that can be used by the same summary function
     sample = []
     for tup in observations.itertuples():