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204 Big Data Analytics for Connected Vehicles and Smart Cities Practical Applications and Concepts for Transportation Data Analytics 205

Figure 10.4 Bottleneck speed profile template.

The rightmost black dot on Figure 10.4 indicates the beginning of the
bottleneck. This is the point at which the speed of the traffic drops below 60%
of the reference speed. The leftmost black dot in Figure 10.4 represents the end,
or the point at which traffic climbs through 60% of reference speed.
The gray shaded area in the center of the diagram represents bottleneck
conditions where the traffic speed has dropped below 60% and is not yet recov-
ered back to 60% of reference speed. The area to the right of the box represents
pre-bottleneck traffic speed, and the area to the left of the box represents post-
bottleneck traffic speed. The bottleneck is defined as the width of the gray box.
Along the bottom of the graph in Figure 10.4 a series of boxes is arrayed
to represent the TMC segments of the freeway. The two rightmost white boxes
are labeled TMC PRB (pre-bottleneck). The small gray boxes under the large
gray box are labeled TMCB (bottleneck).
Note that from a data perspective the bottleneck can be defined as a se-
quence of two pre-bottleneck TMC segments followed by one or more bottle-
neck segments.
This template provides us with a mechanism for analyzing speed variation
to identify and characterize bottlenecks. It also provides the definitions and
labels required to explain the analysis.
Making use of the template, an nPath analysis was conducted on the data
set to identify bottlenecks, and to characterize bottlenecks by the length of the
queue. Figures 10.5–10.7 show several different ways to visualize the results of
the analysis.
In Figure 10.5, the size of the circles is proportionate to the duration of
the bottleneck measured in minutes below reference speed.