Bluetooth Travel Time (updates every 2 minutes) DOT

Department of Transport dataset. 
The excess delay in minutes, calculated every 30 seconds. This is the difference between the thick blue and thick red lines in the previous chart. Unlike the delay parameter that has a fixed baseline travel time, excess delay has a baseline that changes depending on the time of day. In the previous figure at about 8:20, the expected travel time (red line) was 4 minutes. The actual travel time (blue line) was about 6 minutes. This equates to an excess delay of 2 minutes (6 minus 4) and a delay of 5 minutes (6 minus 1). Excess delays greater than zero indicates the link is operating worse than expected for this time of day. You can have negative values when the link is travelling better than expected.