Civil Engineering Reference
In-Depth Information
24.3 Causes of Congestion
The major cause of traf
c demand and
roadway capacity. This imbalance can occur at an intersection or along a major
transportation corridor and its congestion impacts can propagate throughout an
entire area. It can result each day at the same place and time (recurring congestion),
or it can result at random at random places and times (non-recurring congestion).
Specific causes of this demand
—
capacity imbalance include (1) population,
employment, and motor vehicle growth, (2) concentrations of activities in space and
time, (3) VMT growth from increasing travel distances between decentralized
places or work locations and residence, (4) physical and operational de
c congestion is the imbalance between traf
ciencies of
streets and highways, (5) network capacity constrained by physical and topographic
barriers, (6) inability of investments in highway transportation to keep pace with
VMT growth, and (7) unexpected events (e.g., incidents, bad weather, work zones)
that reduce the throughput capacity of roadways.
The intensity, extent, and duration of congestion generally increase as urban
areas get larger and their economies expand. Therefore, larger cities generally are
more congested than smaller cities; and dynamic, growing cities are more congested
than cities facing an economic downturn.
24.4 Measuring Traffic Congestion Delay
Traf
ects the difference between the travel speed when a road is
lightly traveled, and the travel speed during busy traf
c congestion re
fl
c periods. It is also expressed
as the ratio of actual travel time to uncongested travel time or as the ratio of actual
vs. uncongested travel time rates (e.g., min/mile). The three basic components of
traf
c congestion include intensity (amount), extent (area or network coverage), and
duration (how long it lasts).
However, although the common practice in measuring congestion uses free-
fl
ow
speed as the congestion threshold (e.g., see TTI
s Annual Urban Mobility Reports),
this practice can overstate the magnitude of rush-hour congestion in large urban
areas.
Establishing how much congestion delay travelers are willing to tolerate has
been a concern and a challenge for many years. Key considerations include trip
length, city size and facility type.
'
Longer trips are impacted more by congestion than shorter trips;
Congestion is usually greater and lasts for longer periods in larger cities;
In larger cities congestion is more tolerable than in smaller cities;
Travelers expect to travel faster on freeways and suburban highways than on
city streets.
