Civil Engineering Reference
In-Depth Information
Recurring Congestion
Occurs every weekday (or weekend day) at the same
general location and time.
—
-
Non-Recurring Congestion
—
A random event (a road incident or inclement
-
weather) that restricts traf
c
fl
ow.
Unacceptable Congestion
is the travel time or delay that exceeds estab-
lished or agreed upon norm. This norm can vary by location in a geographic
area, by type of transportation facility, by travel mode, and time of day.
—
-
Mobility
Use of travel time contours (isochrones) to denote the distance
covered under congested conditions within a given travel time.
—
-
Accessibility
the achievement of travel objectives within time limits that
are regarded as acceptable.
—
-
Travel Time Reliability
—
The ability to predict
the arrival
time at
the
-
beginning of a trip.
Traf
c congestion re
fl
ects the
difference
between the travel time experienced
during busy traf
c periods and when the road is lightly traveled. It is also expressed
as the ratio of actual travel time and uncongested travel time or the ratio of actual
versus 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).
8.1.1 Congestion Thresholds
Traf
c congestion thresholds can be de
ned of one of two ways:
1. Using free-
ow speed as a congestion threshold.
2. Establishing acceptable minimum speed for various types of facilities and
operating environs.
fl
ow speed as the congestion threshold might be appropriate in rural
areas, or in the middle of the night, or on Sunday morning in large urbanized areas.
But it might not be realistic to use it as a congestion threshold value to quantify
peak periods traf
Using free-
fl
c congestion in large urban areas.
Establishing how much congestion delay travelers are willing to tolerate has
been a concern and a challenge to traf
c engineers and transportation planners 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 longer in larger cities;
In larger cities congestion is more tolerable than in smaller cities;
Travelers expect to travel faster on freeways than on city streets.
