Civil Engineering Reference
In-Depth Information
Congested Conditions
1. If Trip is by Auto:
Out-of-vehicle time = 3 min
Distance traveled in 27 min at 20 mph = 9.0 miles
Accessible area = 254 square miles
2. If Trip is by Bus:
Out-of-vehicle time = 13 min
Distance traveled in 17 min at 10 mph = 2.83 miles
Accessible area = (2.83)
×
2
×
0.5 = 2.83 square miles
These impacts are summarized in Table
11.2
. This table shows that while congested
roads reduce the mobility of bus riders and auto drivers by the same percentage,
congested roads reduce private vehicle drivers
'
accessibility by a greater amount
than they do for bus riders.
11.3.1.2 Example 2
Another way of identifying the accessibility impacts of traf
c congestion is to
compare peak and off-peak trip distances contours (by mode) from major focal
points such as the city center, major outlying residential areas, or commercial
centers.
Figure
11.6
shows the area boundaries within 30 min travel time contours by
automobile during the peak hour, when there is congestion and during the off-peak,
when there is no congestion. The shaded area between the two contours represents
the reduction in accessible area due to traf
c congestion.
Table 11.2 Traf
c congestion impacts on mobility and area accessibility for the case example of a
30-min trip
Mobility
Area Accessibility
Travel
mode
Base-line
speed
(miles)
Congested
speed
(miles)
% Change Base-line
speed
(square
miles)
Congested
speed
(square
miles)
% Change
Private
vehicle
13.5
9.0
33
572
254
56
−
−
Bus
4.25
2.83
−
33
4.25
2.83
−
33
