Civil Engineering Reference
In-Depth Information
Table 10.4 Trip components for a trip distance of 10 mile
Trip segment
Distance
(miles)
Speed
(mph)
Time
(min)
% Trip
time
% Trip
distance
32.6
a
Other roads
2.9
20
8.7
29
27.6
b
44.9
a
Uncongested freeway
7.0
35
12.0
70
53.3
b
Congested freeway
7.0
25
16.8
70
22.5
a
Out of vehicle travel time
(walking, waiting)
0.1
3.0
6.0
1
19.1
b
100
a
Total
—
(uncongested freeway)
10.0
22.5
26.7
100
100
b
Total
—
(congested freeway)
10.0
19.0
31.5
100
a
No congestion on freeway
b
Freeway congested
Other assumptions are:
Congested freeway speed = 25 mph.
Average speed of the roads connecting to the freeway = 20 mph.
Out-of-vehicle time for the trip = 6 min (includes walking to and from the
vehicle and waiting for the vehicle).
The questions are:
I. What is the impact of peak period freeway congestion on trip time?
II. What is the impact of peak period freeway congestion on mobility?
Table
10.4
lays out the assumed values of the trip components under congested
and uncongested trip conditions for the 10 mile trip.
10.4.1.1 Findings
Key
findings are as follows:
Impact of Freeway Congestion on Trip Time
In this example, an increase in freeway congestion of 40 %, (4.8/12) increases trip
time by only 18 % (4.8/26.7). This means that one cannot estimate the trip time lost
to congestion by only measuring the freeway delay rate because the time spent on
the congested freeway segment comprises only a fraction of the total trip time [
7
].
Considering that over 65 % of urban trips is shorter than 5 miles [
8
], and that
trips shorter than 5 miles are unlikely to be freeway users [
9
], measuring the impact
of traf
c congestion on trip time using data only from freeways and other principal
arterials, is likely to overestimate average trip congestion delay.
