Civil Engineering Reference
In-Depth Information
rate (e.g., TTI = 1.33) in city and suburban streets produces a larger impact on the
mobility of suburban travelers than on that of city travelers.
Therefore, the TTI should not be used to compare the effects of congestion
between areas where the respective road networks have
signi
cantly different free-
fl
ow speeds.
8.2.1.5 Travel Time Tax
TTT
—
In its 2010 Annual Report, INRIX introduced the Travel Time Tax, or TTT [
11
].
The TTT is defined as:
TTT
¼
TTI
1
:
0
ð
8
:
5
Þ
The TTT is a surcharge to free-
fl
ow travel time. For example, a TTT of 1.
30
represents a tax of
30
% (1.
3
ow travel time. The TTT is
based on the same methodology as the TTI, but it communicates the results in a
different way.
1.0) added to the free-
fl
-
8.2.1.6 Comments on the Uses of the Travel Time Index (TTI)
and the Travel Time Tax (TTT)
The TTI and TTT can best be used for tracking congestion over time for the same
facility type within the same area. They should not be used to compare areas with
roadway networks that have different free-
fl
ow speeds, or that serve different
average trip lengths.
The travel time tax (TTT) represents the actual time loss incurred and it is
expressed in minutes per mile. The total time loss (delay) along a roadway can be
obtained by weighing the time loss for each section of road by its length and
volume. Table
8.4
provides an illustrative example.
The TTT provides a useful metric for measuring and aggregating delay over a
given roadway. It is based on real-time measurement of speeds and travel times, and
is easy to understand and use. However, it has the same limitations as the Travel
Table 8.4
Illustrative application of the travel time tax (TTT) compared to the TTI
Variables
Arterial street
Freeway
RS
(reference free-
40 mph
(1.5 min/mile)
60 mph
(1.00 min/mile)
fl
ow speed)
HS
(actual highway speed)
30.76 mph
(1.95 min/mile)
46.15 mph
(1.30 min/mile)
TTI = RS/HS
1.30
1.30
TTT = (TTI
−
1.0)
0.30
0.30
Additional delay rate @
TTI = 1.30 or TTT = 30 %
(1.95
1.5) =
+ 0.45 min/mile
−
(1.30
1.00) =
+ 0.30 min/mile
−
