Civil Engineering Reference
In-Depth Information
For analysis purposes accessibility measures should be developed for each mode
of transportation available to travelers. And for each type of opportunity (e.g., work,
shop, medical), the same zone can have different accessibility measures: one for
those who walk or bike; another for those who ride transit, and yet another for
automobile users. Accessibility measures can also vary by time of day (i.e., peak
and off peak hours), and by type of opportunity.
11.3 Congestion Impacts on Modal Accessibility
Evaluating the impact of road traf
c congestion on accessibility requires measuring
the effects of road congestion on traveler mobility within the impacted area. For
travelers, increasing congestion reduces the number of destination choices, and for
freight carriers with a
xed
fl
fleet size, it reduces their market areas.
11.3.1 Examples
Several examples illustrate the likely congestion effects on accessibility.
11.3.1.1 Example 1
Assumptions:
1. trip time budget = 30 min
2. base-line average peak speed;
30 mph private vehicles
15 mph buses
3.
ten-year traf
c growth is projected to reduce peak traf
c speed:
20 mph private vehicles
10 mph buses
Base Line Condition
1. If Trip is by Auto:
Out-of-vehicle time = 3 min
Distance traveled in 27 min (30
3) at 30 mph = 13.5 miles
−
Accessible area = 572 square miles
2. If Trip is by Bus:
Out-of-vehicle time = 13 min
Distance traveled in 17 min (30
13) at 15 mph = 4.25 miles
−
Accessible area = (4.25)
×
2
×
0.5 = 4.25 square miles
