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CHAPTER 1
INTRODUCTION AND RESEARCH APPROACH
1.1 OPERATIONAL PROBLEM
Tmff~c congestion is a growing problem on all forms of traffic facilities, particularly at grade-
separated, signalized interchanges in urban areas. Congestion reduces the efficiency and safety of
these major traffic facilities, primarily due to problems associated with queue spilIback and overall
capacity deficiencies. Queue spilIback cart be caused by poor signal timing or by traffic demand
exceeding the basic capacity ofthe facility to serve it. Queue spilIback may surprise motorists who
are not familiar with the area, causing serious merging and weaving problems from the on-ramp
terminals to the cross sheet arterial f1,29. In addition, spilIback can also reduce the output capacity
of the signal when signalized intersections are closely spaced, say less than 200 meters apart.
The signalized service interchange, typically either a diamond or partial cloverleaf, serves
a critical function in the urban highway system. Within freeway/arsenal and arterial/arterial
interchanges, signalized ramp terminals connecting to the arterial cross street are often the key
operational geometric element. Unfortunately, the interchange facility, which is a very costly link
in the highway network, often performs poorly, having numerous conflict points, high traffic
demands, large changes in speeds required by motorists, and high-volume horning movements often
exceeding four times the normal average observed at conventional intersections.
Traffic conflicts between the major through movements at interchanges are typically grade-
separated, while the other traffic movements are served at signalized intersections. To improve
major road operations, many maneuvers that tend to generate conflicts and delays (such as stopping,
mining, and weaving) are designed to occur on the minor cross street. However, the cross street may
also have large traffic volumes and the close spacing of the ramp terminals combined with the high
volume of interchanging traffic often cause significant operational problems. These problems
include long delays, poor minor-street signal progression, long queues and, as noted above, queue
spilIback between adjacent ramp intersections in some restncted cases.
Resolution ofthe operationalproblems nosed above is criticalto the safety end efficiency of
the traffic corridor. Moreover, Intelligent Transportation Systems (ITS) technologies are dependent
On the ability to divert traffic around congested freeway areas, open through congested signalized
serviceinterchanges. If the corndorinterchanges are operating inefficiently, the potential diversion
is diminished, and the benefits of ITS may be significantly reduced.
The Highway Capacity Manual (HCM) of 1994 f3J is the national guide (if not defacto
standard) for conducting highway capacity analysis and level of service evaluations. Chapters 9 and
~ ~ of the HCM cover signalized intersections and arterial streets, respectively. Neither of these
chapters, nor others in the HCM, address two important considerations: signal coordination needs
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and progression characteristics of signalized ramp terminals (at interchanges) and other closely
spaced signalized intersections, and (2) queue spillback on the cross street and onto the ramps.
Moreover, operational problems resulting from oversaturation, when the existing traffic demand
exceeds signal capacity, are only minimally addressed in the HCM.
Thus, a methodology is needed to analyze signalized ramp terminals and adjacent
intersections, simultaneously. The methodology should provide the procedures, guidelines and
analytic tools needed to efficiently conduct pertinent capacity analyses. This methodology should
be capable of addressing both coordinated pretimed and coordinated actuated signal systems through
signalized interchanges. Undersaturated as well as oversaturated capacity conditions should be
rationally addressed.
1.2 RESEARCH OBJECTIVE
The objective of this research was to develop and validate an appropriate methodology for
determining capacity and level of service (LOS) at signalized ramp terminals of two-level grade-
separated interchanges, including any adjacent closely spaced arterial intersections. The
methodology should describe and quantify appropriate measures of traffic performance (measures
of effectiveness, MOEs) at the signalized interchanges.
A wide range of service interchange configurations that include one or more traffic signals
at the ramp terminals should be specifically addressed. This statement of scope includes
conventional diamonds, compressed diamonds, tight urban diamonds, single-point interchanges, and
three partial cloverleafs (parclos): the traditional names are the parclo A, parclo B. and the parclo
AB. The parclo A is noted herein as a parclo AA because the maj or road' s approach traffic sees bow
loop ramps in advance of the crossing structure, likewise, the parclo B is noted as the parclo BB
because both loop ramps are seen beyond the cross street grade-separationstructure. The parclo AB
has one loop rainy in advance and one beyond the cross street bridge structure. Two basic varieties
of parclos exists: the two-quad and four-quad designs. The four-quad separates two conflicting right
turning movements from the signalized intersection area by placing each of them on direct
connecting ramps. Thus, the four-quad parclo is more expensive than the two-quad design, but
provides more capacity. Figure 1 illustrates the types of signalized service interchanges that were
considered in this pro ject. Unsignalized at-grade movements, such as right turns Smartly from the
_ _ _
off-ramps onto the cross street and resulting arterial weaving between the intersections, were also
observed in some cases.
The research objectives of this project were to specifically (1) evaluate the state-of-the-art
in operational analysis of signalized ramp terminals, including adjacent closely spaced intersections,
(2) develop and test a new methodology for analyzing the capacity and LOS for these interchanges,
and (3) validate the proposed methodology. The research results support the development of a new
chanter in the HCM that is compatible truth existing chanters. No operational computer software
~ - ~ - ~ ~ ~
packages were to be developed as end products of this research effort.
1 - 2
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Figure I. Examples of Signalized Service Interchanges in urbar~suburtc~n areas.
1.3 RESEARCH APPROACH
I.3.} Field Survey
To insure that the scope and nature of the research were well defined, a two-stage survey of
practicing traffic engineers was conducted during the initial task of this research. An objective of
the survey was to gain insight into the current practices and concerns of engineers who evaluate
interchange traffic operations. To achieve broad-based unbiased results, both public arid private
sectors were initially surveyed. The first-stage survey was mailed to more than 2,400 engineers
across the U.S.A. and abroad during February, 1994. A total of 350 returned questionnaires were
deemed completely responsive and valid. The first-stage survey results revealed that engineers
analyze traffic operations of interchanges quite often, and that most service interchanges are
diamonds (either compressed or tight urban). Most engineers responded that inadequate capacity
and queue spilIback are the most common operational problems experienced at the interchanges
under their responsibility. All respondents cited operational problems existing at interchanges, and
unsatisfactory methods of analysis. The findings indicated a need to further examine the reported
traffic problems and the way Hey are analyzed.
~ - 3
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The purpose of the second-stage survey was to further inquire about specific operational
problems at interchanges,the methods that are presently used to analyze these problems, actions that
are taken or recommended to resolve these problems, and identification of interchanges that could
potentially serve as study sites. The second-stage survey was mailed during the last week of March
~ 994. This survey was sent to ~ 90 individuals who indicated a willingness to respond to it from the
first survey. The results from the second-stage survey helped in guiding Me direction of research
described in the following chapters. The second-stage survey was then used to determine what
specific problems are encountered in interchange analysis. The results of the second-stage survey
Indicated that queue spillback, lane-changing, and arsenal weaving between the ramp terminal and
downstream intersection all deserved further investigation. Predicting the effect that queue spillback
has on the output (saturation flow) of the traffic signal appears to be the most significant need for
development. In addition, thirty-five interchanges located around the country were suggested as
candidate study sites for subsequent model development, testing and validation.
1.3.2 Traffic Models
Following a detailed literature review of intersection and interchange capacity models,
analysis methods and recent research, the development of a targeted set of traffic models was
initiated toward satisfying the specific operational needs identified in the field survey and project
objectives. Specified areasincluded: (~) discharge headways, (2) queue storage and spilIback, (3)
lane utilization, and (4) arterial weaving. The general effects of cross street (arterial) progression
on queuing and delay were also desired together with a formal casting of the interchange capacity
analysis methodology.
{.3.3 Field Studies
An extensive program of field studies was conducted of traffic operations at interchanges
identified by the field survey as experiencing operational problems of congestion and queue
spillback. A total of eighteen special studies were conducted at twelve interchanges located in five
states. Data collection combined inputs from video cameras, tape switches and remote sensing of
controller operat~ons~nto a computer-baseu~ata collection system. Subsequent data processing by
special video/traffic fusion software provided a chronographic record of observed traffic operations
and control. Further mode} building and statistical analysis used standard statistical packages 64J
and special software developed by the research team on a prior NCHRP project (29.
Traffic simulation was also used to develop and test analytic models of the targeted study
areas. TRAF-NETSIM (5) was calibrated with field study results and used to develop analytic
models of arterial weaving. Extensive testing and validation of other theoretical models of queue
spilIback and signal capacity were likewise conducted.
~ - 4
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1.3.4 Study Results
The following chapter provides an overview of the research results followed by an appraisal
of the significant findings, followed by our conclusions and recommendations. No software was
specifically developed as deliverables for this project, although development of spreadsheets and
some coding was necessary to test and evaluate some of the models and methods recommended. In
particular, the PDX Model required extensive software coding to get the related complex algrorithms
described to perform efficiently for a wide range of traffic conditions. A prototype software design
for service interchange database management' named INTERCHANGE' was also developed to
demonstrate an efficient database architecture for processing all types of two-level signalized
service interchanges for subsequent capacity and operational analyses.
The details of the user surveys' field studies' and traffic models are provided in subsequent
appendices. Some data bases may be available on request through NCHRP to the subcontractor-
Un~versity of Nebraska-Lincoln.
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Representative terms from entire chapter:
ramp terminals
