Civil Engineering Reference
In-Depth Information
Thus if one is willing to allocate 20 min to a shopping trip, the trip mobility of to
a shopping destination would be: 1.0 mile if by walking, 1.5 miles if by bus, 2.5
miles if by bicycle, and 4.8 miles if by auto.
This example shows that the mode
is operating speed is not always an indicator
of mobility. From the viewpoint of travelers what is important is distance traveled
within an acceptable door-to-door travel time. For example, the mobility advantage
of the bicycle over the bus for trips less than 30 min points to the need for
implementing bicycle networks in urban areas.
The above comparisons do not include rail rapid transit lines operating on grade-
separated or private guide
'
ways, whose line haul speeds are usually at least
double those of surface transit. Long established US rapid transit lines in New York
City, Chicago, Philadelphia, and Boston have typical line-haul speeds of 20
—
25
miles/h; newer regional lines have speeds in excess of 30 miles/h. Commuter rail
have line haul average speeds that can reach 40 miles/h. Rail modes were excluded
in Table
10.3
, however, to simplify the discussion.
Actual modal speeds will vary both within and among urban areas. Communities
interested in measuring modal mobility could develop curves similar to Fig.
10.3
to
re
-
ect local conditions (e.g., area size, travel barriers, availability of freeways, or
public transportation operating on exclusive right-of-way).
fl
10.4 Congestion Impacts on Mobility
Because congestion adds trip time to cover the same distance, travelers will reduce
their mobility as congestion increases.
10.4.1 An Illustrative Example: Measuring Freeway
Congestion in a Large Urban Area and Its Impact
on Trip Time and Trip Mobility
The following example considers an urban freeway used for a trip 10 miles long.,
and assumes that 70 % of the trip distance uses the freeway, and 30 % uses roads
connecting to the freeway.
Typically the rest of the road network connecting drivers
'
origin to their desti-
nations is less congested than the freeway segment of the trip
especially in sub-
urban areas. In addition, each trip experiences out-of-vehicle time involved with
walking and waiting for the vehicle. Therefore, these factors greatly shape how
freeway congestion impacts on trip time.
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