Civil Engineering Reference
In-Depth Information
Time Index (TTI) in that it is sensitive to the basic assumption of what constitutes
congested speed.
8.2.1.7 Illustrative Application of the TTI (I-93 Southbound:
Medford to Boston)
Recurrent congestion was common on southbound I-93 before Boston
s Central
Artery Tunnel and the new cable stay bridge over the Charles River was completed.
Figure
8.2
shows the difference between peak (6 a.m.
'
10 a.m.) and off peak travel
times traveling from the Roosevelt Circle off-ramp to Storrow Drive off ramp, for
three time periods: Fall 1994, 1998; and Spring 1999. The maximum travel time
intensity was experienced at 8:00 a.m., and ranged from 18 min in 1994, to 23 min
in the Fall of 1998. The travel time without delays was about 7 min; and the
maximum time loss during the am peak period ranged from about 10 min in the Fall
1994, to 17 min in Fall 1998. The travel time index (TTI) ranged from 2.7 to 3.8.
-
8.2.2 Duration
The duration of congestion depends upon the types of congestion (recurring or non-
recurring). It also depends upon city size and type of roadway. Congestion generally
is of long duration on major roadways (e.g., freeways) in large urban areas
—
especially where roadways converge. Duration is less frequent in small urban areas.
Fig. 8.2
AM peak period travel times for I-93 Southbound: Roosevelt circle off-ramp to storrow
drive off-ramp.
Source
[
5
]
