Civil Engineering Reference
In-Depth Information
Fig. 5.1 Average traf
c speed versus population density in the New York metropolitan area.
Source
Adapted from Reference [
5
], Table 10
5.4 Concentration of Travel Demand in Time
If all trips made by urban residents were spread equally among the hours of the day
traf
c congestion would be largely eliminated. But people engage in activities that
are scheduled at particular times and not others. Accordingly urban travel patterns
are determined by the schedules of human activities (work, school, shopping,
vacation, etc.) that vary by time of day, day of week, and month of the year. The
concentration of this travel during speci
c time periods gives rise to traf
c
and
transit congestion.
The time concentration of these trips in time produces peak traf
c
demands
in
excess of roadway
capacity
at various times of the day
typically the morning and
evening peak periods. To address this effect the travel tax was introduced by
INRIX. It describes congestion as a
—
ow travel time. For
example, a driver traveling at the rate of 1.20 min/mi would experience a tax of
20 % of the free-
“
surcharge
”
on free-
fl
ow travel rate of 1.0 min/mi.
As shown in the example of Fig.
5.2
, (Ref. [
6
], p 11), the travel time tax
1
peaks
at 17 % in morning peak hour and at 23 % in the evening peak hour.
The hourly distribution of the travel time tax indicates that the AM peak period
begins at 6 a.m. and ends at 10 a.m.; while the PM peak period begins at 3 p.m. and
fl
1
Travel time tax is term introduced by INRIX. It is meant to describe congestion as a surcharge
on free-flow travel time. For example, a driver traveling at a rate of 1.20 min/mi would experience
a tax of 20 % if the free-flow travel rate is 1.0 min/mi.
