The Impact of Traffic Congestion on Accessibility - Road Traffic Congestion - page 126

11.2.1.1 Spatial Accessibility

Spatial Accessibility is the area accessible within an acceptable travel time. It is
determined by the modal mobility within a travel time budget:
For example:
The time it takes to reach a location 10 miles away is:

30 min by car

￿

60 min by bus

￿

180 min by walking.

￿

Conclusion: If one ' is travel time budget is 30 min, this location is only accessible

to by car.
Two examples illustrate the application of this accessibility concept:

Example 1

This example determines the Destination Opportunity Area accessible in a 30 min
trip, by the given mode:
(a) For walking, biking, and motor vehicle, the area is approximated by a circle
with a radius equal to the distance traveled in 30 min trip time. However, often
the 30 min travel time contour is elliptical to re fl ect different speeds for each

direction of travel.
(b) For a bus trip, the accessible area is approximated by: (the distance traveled by
bus) × (0.5 miles service area — i.e., 0.25 miles based on each side of the bus

line).
(c) Distance traveled in 30 min = [speed of travel mode] × [30 (excess travel

−

time)]/60.
(d) Typical modal speeds and modal excess travel time are found in Chap. 10 ,
Table 10.3 .
(e) Excess travel times (from Table 11.3 ) are subtracted from total travel times to
calculate distance traveled.

1. Destination Opportunity Area Accessible by Walking (assuming a fully
interconnected street grid)

(a) Distance traveled at 3 mph for a 30 min trip = 1.5 miles (in all directions)
(b) Walking destination opportunity area = 7 square miles (1.5 × 1.5 × 3.14).

2. Destination Opportunity Area Accessible by Bicycle (assuming a fully
interconnected street grid)

Urban area
(a) Distance traveled at 8 mph for a 30 min trip = 4.0 miles
(b) Destination opportunity area reachable by bicycle = 50 square miles
(4

× 4 × 3.14).