Civil Engineering Reference
In-Depth Information
Special events
increase traf
c demand beyond capacity
—
especially when they
occur during peak hour traf
c
fl
ow
Driver Behavior
such as the presence of slow vehicles in the fast lane that block
ability to change lanes, or failure to maintain speed in an upgrade, reduces
speeds
9.4 Reliability Metrics
The recommended metrics shown in Table
9.1
are useful in evaluating both traf
c
congestion and travel time reliability.
9.4.1 The Buffer Time Index
The Buffer Time Index (BTI) is a measure of trip time reliability that expresses the
amount of extra time to be added to the average trip time in the peak hour if one
aims to arrive on time, 95 % of the time (e.g., being late for work 1 day out of every
20 work days). The BTI can be calculated using the Eq.
9.1
([
4
], p 16):
BTI
ð
%
Þ
¼½
ð
95th Percentile Travel Time
Avereage Travel Time
Þ
ð
9
:
1
Þ
=ð
Avereage Travel Time
Þ
100
%
Tabel 9.1 Recommended reliability metrics
Reliability
performance metric
De
nition
Units
Buffer index
Difference between 95th percentile TTI and average travel time,
normalized by average travel time
Difference between 95th percentile TTI and median travel time
(MTT), normalized by MTT
%
Failure and on-time
measures
Percentage of trips with travel times <1.1 MTT and <1.25 MTT
Percentage of trips with space mean speed less than 50, 45, and
30 mph
%
Planning time index
95th percentile TTI
None
80th percentile TTI
Self-explanatory
None
Skew statistic
(90th percentile TTI
-
median)/(median
-
10th percentile TTI) None
Misery index
(modi
ed)
Average of highest 5 % of travel times divided by free
fl
ow travel
None
time
Source Reference [
3
], p 6, Table ES.4
