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OCR for page 119
CHAPTER 4
CONCLUSIONS AND RECOMMENDATIONS
4.1 CONCLUSIONS
Mobility at a signalized (service) interchange is dependent on many factors that influence
capacity. The importance of the prevailing factors may change depending on the general volume
level and degree of existing congestion. Traditionalprevailing factors affecting capacity include the
interchange geometry, traffic mix, and signal green splits of the serving phases. During periods of
oversaturation where the storage links are filled, additional non-traditional factors come into play.
During oversaturation,the upstream input capacity becomes highly dependent on downstream signal
timings and capacity to keep the output link clear. Upstream input capacity cannot exceed the total
downstream service capacity available, and may be less than this capacity if demand starvation on
short links occurs. In addition to phase capacity, signal offset during oversaturation is the most
important factor in the allocation of downstream capacity to upstream phases. Also, this of Eset is not
the offset that provides optimal progression during uncongested flow periods.
Signal timing and coordination together with signal spacing (link length) are important
variables in detennin~ng the ability of the cross arsenal to move traffic. Moreover, in the
mesosaturation (near capacity~range of traffic conditions,the ultimate determination of the question
of "oversaturation?" depends on whether potential flow Impediments due to queue spillback can
be sufficiently mitigated to the extent that oversaturation can be avoided.
Capacity analysis methods of signalized interchanges should be able to determine whether
oversaturation wall occur. However, this determination is a complex task in the mesosaturation
range near capacity. These conditions not only depend on nominal volumes arid phase capacities,
but conditions also depend on He interactions of the variables with queue storage, link storage
capacity and signal timing. Critena have been provided that indicate the likelihood of queue
spillback effecting nominal output capacity. These criteria can address probably 80 percent ofthe
operational problems. However, close-calls can only be solved now using computer simulation
models, like NETSIM, or possibly using models like the PDX Mode! developed in this research.
Phase capacity is dependent on the prevailing saturation flow rate and effective green time
when motion can occur at the stopline. Based on this research, it is concluded that the distance to
the downstream queue, the radius of the turn path, and traffic pressure have a significant elect on
the saturation flow rate of a traffic movement. Specifically,saturation flow rate decreases when the
distance to the downstream queue decreases and is relatively short. This effect is amplified when
the signal timing relationship between the two intersections allows queue spilIbackto occur. As the
distar~ce-to-queuevanable is bounded to a maximum value equating the length of the downstream
street segment, the effect of distance-to-queue also includes the effect of spacing between
interchange ramp terminals or between a ramp terminal arid a closely-spaced intersection.
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Turn radius has a significant effect on the saturation flow rate of a turn-related traffic
movement. Saturation flow rates are lower for turn movements with small radii than they are for
turn movements with large radii. In the context of junction type (e.g., single-point urban diamond,
diamond interchange, etc.), the saturation flow rates for the left turn movements at single-point
urban interchanges are more nearly equal to those of through movements because ofthe large turn
radii associated with this interchange type.
Traffic pressure, as quantified by traffic volume per cycle per lane, has a significant effect
on saturation flow rate. Traffic pressure relates to the presence of aggressive, commuter drivers in
the traffic stream. Traff~cvolumeis used as a surrogate measure of the number ofthese aggressive
drivers in the traffic stream. Saturation flow rates of low-volume movements are much lower than
those of high-volume movements because the low-volume movements have less traffic pressure.
Other factors were examined for their potential effect on saturation flow rate. These factors
include: g/C ratio, junction type, downstream signal indication at the start of the upstream phase, and
dual versus single left-turn lane. Of these factors, only g/C ratio was found to be correlated with
saturation flow rate in a statistically significant manner. Specifically, the saturation flow rate for
left-turn movements with low g/C ratios was found to be higher than the rates of similar movements
with larger g/C ratios. This effect was also found in the through movements studied, however, it was
much smaller in magnitude and not statistically significant. Therefore, it was determined that more
research is needed to verify the significance of this trend and its magnitude before an adjustment
factor for g/C effect can be recommended.
The definition of effective green time should be changed to be only the time when saturation
flow can occur at the stopline for existing conditions. This definition is more robust and can be used
in all operating conditions, including periods of oversaturation. Moreover, delay estimates wait be
improved using this new definition of effective green even during undersaturated conditions because
the queue profile wall be estimated more accurately than using the current HCM methodology.
The estimation of phase lost time should be improved. Start-up lost time is not a constant
value, rather, it is statistically dependent on the prevailing saturation flow rate. Specifically, start-up
lost time increases with increasing saturation flow rate. This paradoxical increase is due to the
increased acceleration time the discharging queue requires to attain the higher speed associated with
a higher saturation flow rate. The term "lost" is a bit of a misnomer in this case, as more lost time
occurs as saturation flow gets better. Start-up lost times typically range from 0.61 to 3. ~ ~ seconds
for prevailing saturation flow rates of 1,400 to 2, ~ 00 pcphgpI, respectively.
The average yellow warning interval used by drivers clearing the intersection at the end of
the phase is termed "green extension, or end use." In the context of phase capacity, end use is
equivalent to an extension of the effective green period into the yellow. The study of end use
indicates that it is a relatively constant value for intersections and interchanges and that it averages
about 2.5 seconds for most undersaturated conditions. Thus, this quantity can be subtracted Tom
the signal change interval duration to estimate the lost time at the end of a signal phase.
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Lane use is almost always uneven (or unbalanced) in intersection lane groups. The degree
of this imbalance is expressed in terms of the lane utilization factor. The lane utilization factor
vanes depending on the nature of drivers' lane-choice decisions (i.e., to minimize travel time or to
preposition for a downstream turn). Lane utilization factors based on travel time minimization tend
to be subject to randomness In the lane-choice decision process. The factors stemming from this
process range from I.! to 2.0, depending on the number of lanes in the lane group and its
corresponding traffic volume. Lane utilization factors based on driver desire to preposition can vary
widely, depending on the volume of traffic that is propositioning in the subject lane group.
Neither the signal capacity of various interchange types nor their relative capacity per lane
was specifically determined within this research. Some examples of this form of analysis are
illustrated in a related NCHRP publication (29. However, examination of parclos as compared to
diamonds reveals obviously different traffic volume input patterns that may result in one design
being more efficient than another for a given case. The software INTERCHANGE described in
Appendix F can readily examine the patterns provided by each interchange type. Parclos versus
diamonds also have more right-turn capacity per input lane due to their normal signal overlaps, but
this feature may tend to overflow downstream closely-spacedlinks more than diamonds. Moreover,
single-point diamonds are known 629 to have more arterial right-turn capacity per input lane than on
its exit ramps because of unbalanced right-turn signal overlaps using three-phase signal operations.
4.2 RECOMMENDATIONS
The ideal saturation flow rate recommended for signalized (service) interchanges is 2,000
pcphgpl. In the context ofthe factors studied for this research, this ideal flow rate applies to through
traffic movements that have an infinite distance to the back of downstream queues, operate under
non-spiliback conditions, discharge along tangent (i.e., straight) and level alignments, and have
traffic volumes that are relatively high, reflecting those found during peak demand periods.
It is recommended/hat the equations providedin Chapters be used to estimate the saturation
flow rate, start-up lost time, green extension, end lost time, and lane utilization factor. In recognition
of the relationship between saturation flow rate and start-up lost time, it is recommended that He
equations provided in Chapter 3 be used to estimate ad necessary phase capacity characteristics. In
other words, selective use of only some of the equations in Chapter 3 for a capacity analysis is not
recommended.
The recommenced green extension value is 2.5 seconds for most undersaturated conditions.
Other values are possible if the approach speed is outside the range of 64 to 76 km/in or when the
volume-to-capacityratio for the analysis period is above 0.~. An equation is provided in Chapter
3 for these situations.
The definition of effective green should be changed slightly from that used in the ~ 994 HCM.
The new definition should be "effective green is that time dunng the subject phase when saturation
flow at the stopline can occur unfler prevailing conditions." All "lost" times should be removed
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Tom the phase, including: start-up, opposing queue blockage, output blockage due to spillback, and
phase clearance lost times. This definition is very robust and covers all operating conditions,
movements, arid phases, including protected-plug-permitted. The PDX Model should be considered
for estimating the output Clear Period and effective green time of the subject phase when
oversaturation is likely. High-volume links which are nominally oversaturated or less than 200
meters long should be analyzed for queue spillback blockages using the features provided in the
PDX Model.
Implementation of the PDX Model features into internationally recognized computer signal
timing optimization programs, such as the PASSER programs, TRANSYT and SIDRA, is highly
recommended. None currently handle oversaturated conditions very well, and the addition of the
PDX Model features would give the programs the capability to reliably estimate queue spillback
elects on saturation flow and elective green fume. Some work toward this objective is known to
be already underway (] 7, 1S, 19~.
Ramp weaving speeds and crossing capacity can be estimated using the methodology
presented in Chapter 3 and Appendix E. Adequate travel distance to the back of the downstream
receiving queue must be available for this capacity to be attained.
The Highway Capacity Manual should contain a chapter on Interchanges which emphasizes
the unique forms and features of interchanges together with the special challenges associated with
urban interchange traffic operations in general. Two-level signalizedinterchanges operating within
a crossing arterial system should be presented together with freeway system integration issues
associated with freeway traffic management. Special design and operational issues dealing with
continuous one-way frontage roads should be presented. Unsignalizedrota~y interchanges could be
identified as an alternative design concept. Moreover, He selective application of signalized
Interchanges to upgrade the capacity of a major urban arsenal corridor should be noted, as illustrated
In Appendix F of this report.
A major development of computer software should be funded in support of the Highway
Capacity Manual effort. However, this software should not be limited to just being a processor to
the HCM Interchange chapter that probably would focus just on "operational analysis of existing
conditions." This new software should promote "options analysis" as needed to expediently conduct
operational impact analyses for preliminary planning and design activities. The new program
INTERCHANGE described In Appendix F illustrates some of the analysis concepts and software
features recommended. However, significant funding is still needed to complete a professional-level
software package for interchanges.
4 - 4
Representative terms from entire chapter:
flow rate
