Capacity Analysis of Interchange Ramp Terminals Final Report (1997) / Chapter Skim
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APPENDIX A State-Of-The-Art
APPENDIX A State-Of-The-Art
Pages 125-176
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From page 125... ...
More specifically, the issues related to the signal-controlled ramp terminals and traffic flow along the cross street through the interchange. Consideration was also given to the relationship between the interchange terminals and any adjacent, closely-spaced signalized intersections.
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From page 126... ...
A common theme In these problems was a lack of effective signal coordination between the ramp terminals (or between the ramp terminal and adjacent signalized intersections. Question 4 inquired about He types of analysis methods used to evaluate Interchange traffic operations.
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From page 127... ...
Number of Responses Alabama Alaska Arizona Arkansas California Colorado Connecticut Delaware noridla Georgia Hawaii Idaho Illinois Indiana Iowa Kansas Kentucky Louisiana Maine Maryland Massachusetts Michigan Minnesota Mississippi Missouri State Montana Nebraska Nevada New Hampshire New Jersey New Mexico New York Norm Carolina Nor h Dakota Ohio Oklahoma Oregon Pennsylvania Rhode Island South Carolina South Dakota Tennessee Texas Vermont .
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From page 128... ...
| Average Rank Inadequate Capacity 3.7 3.7 W~: ~ 2.2 2.3 t1~ Kid 1~:k 3.5 3.5 uestion 4. What analysis techniques do you use to evaluate traffic operations at the interchange ramp terminals?
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From page 129... ...
readily available. These Innitations hinder an engineers ability to analyze interchange traffic operations.
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From page 130... ...
Specifically, TRANSYT-7F was cited by nearly half of all the respondents. This may be due to the fact that TRANSYT-7F is sensitive to the proximity of adjacent ramp terminals or signalized intersections in its signal timing optimization routine.
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From page 131... ...
A.~.2 Second-Stage Survey The findings from the f~rst-stage survey provided important information regarding the extent of operational problems at urban interchanges and the general thoughts of the practicing engineering community regarding techniques for evaluation of these problems. These findings were used to develop the format and content of the second-stage survey.
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From page 132... ...
However, they are representative of urban interchanges that tend to experience traffic operational problems because of short ramp separation distances 3. Wheat is the distance between the ramp terminal and the nearest downstream signalized intersection (as measured along the cross street from stop line to stop lined ?
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From page 133... ...
As diamond interchanges were the most common interchange type cited In Question I, it is somewhat surprising that so many sites had two controllers at the interchange. One common controller for both diamond interchange ramp terminals is generally best able to maintain the type of two-way traffic progression necessary to eliminate queues on the street segment Internal to the ramp terminals.
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From page 134... ...
Based on the percentages listed above, it appears that "queue spiliback between ramp terminals" is the most frequently found problem at interchanges in narrow-rights-of-way. When combined with "left-turn bay overflow, " it would appear that traffic flow problems et interchanges are most frequently found between the ramp terminals, where the volume of left-turns is highest.
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From page 135... ...
Another response from a consultant in New York identified problems associated with turning movements. The respondent reported that the tight urban diamond exhibited left-turn bay queue overflow at one of the ramp terminals and severe turbulence associated with high-volume weaving on the cross street between the ramp and adjacent signalized Intersection.
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From page 136... ...
Of these problems, queue spillback tends to degrade He smooth flow of many interchange traffic movements and thereby, aggravate mild inefficiencies into significant capacity constraints. Thus, the indirect solution to many ~nterchange-related problems appears to be related to devising analysis techniques that are sensitive to the proximity to downstream queues, the propensity of these queues to spillback, and the relationship between queue-clearance-lime end the signalization of He interchange ramp terminals and adjacent intersection.
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From page 137... ...
Many older interchanges noted above have a predominant number of s~ngle-lane left turn lanes within the interchange and/or have single lanes at ramp terminals assigned to serve heavy left and/or right turning movements. Deficient turning capacity exists.
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From page 138... ...
Most interchange forms have two signalized intersections per interchange. The primary focus of previous research has been on diamond interchanges because they are the predominant signalized interchange form Ail.
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From page 139... ...
Common Two-Level Signalized Interchanges.
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From page 140... ...
Phase capacity is usually expressed in equivalent flow rate units of vehicles per hour consistent with traditional volume counting practice. Assuming there is only one phase of interest per cycle and noting that the numbers of cycles per hour are k(C)
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From page 141... ...
Traffic Conditions. then the hourly phase capacity is the product of the capacity per phase times the number of phases (cycles)
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From page 142... ...
~... s = saturation flow rate for the subject lane group, expressed as a total tor all lanes in the lane group under prevailing conditions, vphg; so = ideal saturation flow rate per lane, usually 1,900 pcphgpl; N = number of lanes in the lane group; fW = adjustment factor for lane width (12-ft larches are standard)
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From page 143... ...
Each factor accosts for the impact of one or several prevailing conditions that are different from the ideal conditions for which the ideal saturation flow rate applies. The factor represents the average adjustment needed over the entire duration of the displayed effective green time.
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From page 144... ...
Adjustment Factor For Grade (f) Grade %G Type Percent Grade Factor (f:)
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From page 145... ...
Adjustment Factor For Area Type (fa) Type Of Area COD All other areas Area Type Factor, (fa)
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From page 146... ...
The bus blockage adjustment factor,fb',, accounts for the impacts of local transit buses that stop to discharge or pick up passengers at a near-side or far-side bus stop within 76 m (250 ft) of the stop line (upstream or downstream)
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From page 147... ...
Cases 1-6: Exclusive/Shared Lanes and Protected/Permided Phasing fRT= 1.0 - PRT [0.15 ~ (PEDS/2100)
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From page 148... ...
, Proportion of left-tu~ning vehicles using a shared lane group, and Opposing flow rate when permitted left tutus are made. The left-turn adjustment factor is ~ .0 if the lane group does not include any left turns.
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From page 149... ...
These aspects are considered first for all of the adjustment factors noted in Equation A-9 for saturation flow. fLT fD, fv Factor Recommendation fw = same lane w~dth factors should be used, although larger vehicles on the average may be found at interchanges; fHY = same heavy vehicle factor and E T ECUS should be used, although larger and heavier vehicles may use some interchanges, and if so, larger ET should be applied based on field observations; fg = adjustment factor for grades should be similar for interchanges, fp = parking wait not likely be permitted in/around interchanges, so this factor is not needed; fig = buses may be stopping on the cross street, so this factor should be retained; fa = area-type factor is not needed for interchange environments; fry = right-turn factor should be retained, but give additional consideration to · .- .
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From page 150... ...
gp gbq = = Yro vgO sgo (A-11) unblocked green time of the chase serving the lane grouts see; original effective green time of the phase, see; time opposing queue blocks the permitted phase from serving the ~...
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From page 151... ...
. The resulting relationships of saturation flow for permitted left turns versus opposing volume are estimated by Equation A-12 are shown in Figure A-3 for two types of led turn lane use.
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From page 152... ...
by opposing queues for permitted left turns, Sa is the adjusted saturation flow, end f,: is the product of all relevant adjustment factors to existing conditions during the phase. The recommended method for estimating phase capacity per cycle essentially sums component maximum allowable flows over the cycle according to ~gf Sf C (A-14)
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From page 153... ...
(A-21) equivalent permitted left turn saturation flow rate Trough opposed flow from Equation A-12; saturation flow rate for protected left turn operations for otherwise existing conditions; and base ideal saturation flow for signalized intersections, pcphgpl.
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From page 154... ...
Sa =-f Sf = asf, a =-f < ~(A-25) HCM Delay Estimation Method: Recommended Delay Estimation Method: C (1 - g/C)
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From page 155... ...
Output flow from the stop line may be blocked and otherwise impeded by several conditions. Permitted left turns are blocked by opposing queues from using a portion of the displayed green interval.
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From page 156... ...
A.2.7 HCM Vehicular Delay Methodology -- of- ~ -- -rid ~ Vehicular delay is recognized by the 1994 HCM as being a significant traffic performance measure and, consequently, is used as the sole cr~tenon for the level of service provided, for isolated intersections. Federal Highway Administrationhas sponsored a research project in coordination the Highway Capacity Committee of TRY to provide a recommended update for HCM chapters on isolated signalized intersections (Ch.9)
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From page 157... ...
If X ~ I, or oversaturation is present, then n . d3 = 3 600 ' ~ c, If X< ~ and zero queue exists at the stay ofthe analysis penod, then d3 = o where: C = average cycle length, see; g = average effective green time, see; X = degree of saturation for subject lane group; PF = progression adjustment factor; fpp = early/late arrival adjustment factor; T = analysis period in hours, in which the mode!
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From page 158... ...
The generalized delay model for interrupted flow facilities can be used for coordinated approaches by selecting appropriate values for coordinated conditions and type of traffic control. Table A-13 provides the progression adjustment factor (PF7 for Me first ted of Me delay equation teased on arrival type (AT7 together with the early/late arrival factor Ups, which also depends on the degree of saturation of the lane group.
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From page 159... ...
Table A-13. Uniform Delay Adjustment Factors Progression Adjustment Factor (PF)
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From page 160... ...
A.2.S HCM Capacity Analysis Recommendations The general HCM capacity, delay and level of service methodology used for isolated signalizedintersections end arterial street systems should be generally applicable for interchanges. However, the effects of signal coordination and limited queue storage available due to the closelyspaced signals having high turning traffic should be more precisely identified.
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From page 161... ...
The preferred definitions for g and s are as follows: g -- gf = effective green time during phase (cycle) when platoon flow can occur at rate Sf, see; and s -- Sf (zJ = maximum average platoon flow that can occur dunnggf, considering the distance z to the back of the downstream queue at start of green, vpsg/vphg.
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From page 162... ...
This phase notation was adopted by the widely used diamond interchange computer program PASSER Ill, developed by Texas Transportation Institute in 1977 (59. Almost all signal timing plans used at two-level interchangestoday can be described by using the a,b anc!
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From page 163... ...
~1 -it SINGLE - POINT a c b a /k ~ , 'I ~ = ~ rev PARCLO AA - 2Q '_ ' my :: _ ~ ., PARE) BB - 2Q I' tic b - b ~ a a _ ~ 1 b a a Figure A-S.
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From page 164... ...
The four-phase strategyis depleted for diamond interchanges. As can be seen, partial cloverleafs (parclos)
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From page 165... ...
Interchange Signal Phase Sequences.
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From page 166... ...
ib (g Chic Ci (A47) Letting [3 equal the sum of the three lost times per phase, the total effective green time per cycle is gia gib gic Ci [3 And since the v/c ratio for a phase or related lane group "m" is given by X
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From page 167... ...
First, a signal timing strategy is usually assumed that provides equal v/c ratios for all critical phases such that X
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From page 168... ...
to keep the flow ratios the same, then the sum of the "equivalent through vehicle" critical lane service volumes for a given Xj would be m =3 CV.
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From page 169... ...
Coordinated Intersections. Traffic signals at most interchanges are coordinated to improve overall traffic operations because the intersections are closely spaced and traffic volumes are often high.
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From page 170... ...
Actuated control is reactive to queues and tends to be unstable in congested operational environments and, therefore, may generate extremely long cycles and undesirable green splits based only on maximum green settings, which may further exacerbate the congestion. Another critical operational feature affecting interchange traffic operations is whether the green splits at the two intersections depend on one another.
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From page 171... ...
This fact may be a significant constraint on the optimal solution depending on the cycle time, minimum green times required, and traffic pattern being serviced 664. The computer signal timing program PASSER Ill, developed at Texas Transportation Institute, contains these strategies for developing optimal signal timing for all fonns of two-level signalized diamond interchanges.
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From page 172... ...
TABLE A-15. Critical Lane Capacity of Diamond Interchanges by CapeNe/Pinnell, Lee, and NCHRP 3-40 Cycle ~OriginalCritical ~Updated Critical Length Lane Capacity Lane Capacity(2)
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From page 173... ...
. The average phase capacity, P., can be estimated by dividing by the number of critical input phases, n, on the external approaches to yield Pa = s~atl/n + ~/nC ~ (Is ~ Ic)
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From page 174... ...
used in the ~ 985 HCM was ~ .5 for through traffic only, but Molina (169 showed that the PCE for typical through moving urban truck traffic averages about 2.7, web a range from I.7 for small trucks to 3.7 for five-axIe trucks. The 1994 HCM provided an small increase in the average PCE of heavy vehicles at signalized intersections to 2.0.
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From page 175... ...
"Optimization of Pretimed Signalized Diamond Interchanges." Transportation Research Record 644, Transportation Research Board, Washington, D.C.
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From page 176... ...
Berry. "Effects of Design Alternatives on Quality of Service at Signalized Diamond Interchanges." Transportation Research Recorc!
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