(Blue Diamond Road) Corridor Study - Regional Transportation ...

2. Current Conditions of Study Area RTC SR-160 Corridor Study
2.8.2 Congestion Index
The noted transportation engineer, Morris J. Rothenberg, defines urban highway congestion
as "a condition in which the number of vehicles attempting to use a roadway at any given
time exceeds the ability of the roadway to carry the load at generally acceptable service
levels.”5
There are two types of congestion: recurring and non-recurring. Typically, recurring
congestion occurs during the morning and afternoon rush hours as commuters travel to and
from work. Nonrecurring congestion is caused by random incidents, most often disabled
vehicles and accidents.
Different methods are used to determine a service level for each component of a
transportation network. In general, the capacity of a street is a measure of its ability to
accommodate a certain volume of moving vehicles. Typically, street capacity refers to the
maximum number of vehicles that a street element (e.g. an intersection) can be expected
to accommodate in a given time period under the prevailing roadway and traffic conditions.
On a typical county road like SR-160, lane capacities are normally in the range of 1,400 to
1,600 vehicles per hour.
One important deficiency related to congestion and the overall level of service provided to
roadway users is the Volume to Capacity Ratio (V/C). As the volume of traffic increases,
the density of vehicles increases as well as throughput, reaching a maximum flow rate when
volume equals capacity (V/C ratio equal to one). Past that point conditions break down
rapidly and delays increase (V/C ratio of over one).
The methodology for determining congested corridors for this study was to compare the
corridor's V/C ratio to a V/C ratio threshold. The impact of volume is different for different
roadway types (minor arterials, major arterials, highways, freeways, etc.), so thresholds
were established to account for these differences. The thresholds were determined by a
technical working group of the Congestion Management System project and differ by
functional class and area type types.
2.8.3 Level of Service (LOS)
The concept of levels of service (LOS) is well established in highway capacity analysis. LOS
is a quantitative measure that compares the vehicle flow of traffic with the vehicle capacity
available. The resulting volume to capacity ratio (VCR) is then classified in one of six levels
of service. A LOS is an “A, B, C D, E, or F” grading system that rates the quality of
5 Rothenberg, Morris J., "Urban Congestion in the United States: What Does the Future Hold", ITE Journal,
Volume 55, Number 7, July 1985, pp. 22-39.
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