The Impacts of Congestion on Trip Time - Road Traffic Congestion - page 107

9.4.2 Planning Time Index

The Planning Time Index represents the 95th percentile travel time index (or
travel time). It can be estimated once the average travel time index (or travel time) is
known. Figure 9.5 shows the relationship between average TTI and the 95th per-
centile TTI developed by R. Margiotta of Cambridge Systematics [ 6 ].
The equation for the above curve is as follows:

95 percentile TTI ¼ 1 þ 3 : 67 Ln average TTI ð Þ ð 9 : 2 Þ

The above nonlinear relationship can also be expressed by a linear equation with
an r square of 0.78:

Planning Time Index PTI ð Þ ¼ 1 : 7 ð Average TTI Þ 0 : 39 ð 9 : 3 Þ

Example
If the average TTI is 1.2, the PTI = 1.7 (1.2) (0.39) = 1.65

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ow travel time is 20 min, it would take an average of
24 min to travel in the peak hour. But if it is important not to arrive late (95 % of the
time) at the destination, then one should schedule the peak hour trip to last 33 min
[(20) x (1.65)].

This means that if the free- fl

5.5

5

4.5

4

3.5

3

2.5

2

1.5

1

1 1.5 2 2.5 3 3.5

Average TTI

Fig. 9.5 Planning time index (95th percentile TTI) as a function of average TTI. Source
Reference [ 6 ], slide 23