Measuring Traffic Congestion - Road Traffic Congestion - page 90

TTI ¼ ð Actual Travel Time Rate Þ=ð Travel Time Rate during free

flow conditions Þ

ð 8 : 4 Þ

or

TTI ¼ ð Free flow Traffic Speed Þ=ð Actual Traffic Speed Þ

For example, a TTI of 1.30 indicates that a trip taken during the peak period will
take 30 % longer than if the same trip were made when traf flows freely. To

illustrate: a TTI of 1.30 indicates that a trip that takes 40 min at 3 a.m., will take
52 min if made in the peak period. Typically, a free-

c fl

fl ow freeway speed of 1 min/

mile has been used as a base.
Comment :
The TTI is a computationally correct and easily understood metric. But it is
sensitive to how the base

speed is applied. As mentioned earlier, in a
large city it is not realistic to travel at free

“ free fl ow ”

flow speed (or at the posted speed limit)
in the peak hour. It is not logical, therefore to compare actual peak hour travel times
to free-

fl

fl ow peak hour travel times when free- fl ow in the peak hour is a practical

impossibility in a large city.
While the TTI may be an appropriate metric in tracking congestion over time for
the same area , it should not be used to compare areas served by road networks with
different free-flow speeds. The example below shows what happens when one
evaluates the impact of congestion on city and suburban streets using the TTI (see
Table 8.3 ).
Table 8.3 compares free- ow and congested speeds for city and suburban streets.

The TTI of 1.33 is the same for city streets and suburban roads

fl

— suggesting that

congestion impacts city streets and suburban streets equally.
But it does not. The delay rate added to city streets is higher than that added to
suburban streets:

on city streets the delay rate increases by 0.65 min/mile (2.65 2.0), but

￿ -

on suburban streets the delay rate increases by 0.5 min/mile (2.0 1.5).

￿ -

Referring to Table 8.3 , city street congestion would reduce the distance that can
be traveled in 30 min by 3.7 mile (from 15 to 11.3 mile); while the distance traveled
for same 30 min trip on suburban streets is reduced by 7.5 mile (from 22.5 to
15 mile). This example shows that the same proportional change in the travel time

Table 8.3 Measuring traffic
congestion on city and
suburban streets using the
travel time index (TTI)

City streets Suburban streets

Free-flow speed 30 mph 40 mph

(2 min/mile) (1.5 min/mile)

Congested speed 22.6 mph 30 mph

(2.65 min/mile) (2.0 min/mile)

TTI 1.33 1.33