Capacity Analysis of Interchange Ramp Terminals Final Report (1997) / Chapter Skim
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CHAPTER 3 Interpretation, Appraisal, Applications
CHAPTER 3 Interpretation, Appraisal, Applications
Pages 77-118
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From page 77... ...
Models for estimating these characteristics (i.e., saturation flow rate, start-up lost time, and clearance lost time) for all traffic movements at interchange ramp terminals and closelyspaced intersections are described in the following sections.
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From page 78... ...
The last two factors were developed for this research and are specifically applicable to interchanges and closely-spaced signalized intersections. A third adjustment factory was developed for this research that quantifies the effect of turn radius on the saturation flow rate of a left or r~ght-turn movement.
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From page 79... ...
In general, the saturation flow rate is low for movements that have a downstream queue relatively near at the start ofthe phase; it is high for movements that are not faced with a downstream queue at the start of the phase. Thus, the distance-to-queue adjustment factor is based on the distance to the back of the downstream queue at the start of the subject phase.
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From page 80... ...
Spillbackis characterized by the backward propagation of a downstream queue into the upstream intersection such that one or more of the upstream intersection movements are electively blocked from discharging during some or all of their respective signal phase. If spillback occurs dunug the phase, We saturation flow rate prior to the occurrence of the spillback is much ~ .~ ~ .
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From page 81... ...
. Distance to Back of Spillback Condition Queue at Start of Subject Phase, mNo SpillbackWith Spillback 150.6490.408 300.7870.579 600.8810.734 1200.9370.846 1800.9570 892 2400.9670.917 3000.9740.932 3600.9780.943 Turn Radius Adjustment Factor.
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From page 82... ...
Traffic PressureAdjustmentFactor. Saturation flow rates are generally found to be higher during peak traffic demand periods than dunng off-peak periods.
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From page 83... ...
The recommended start-up lost times corresponding to a range of saturation flow rates are provided in Table 24. These values Carl also be computed using the following equation: 1 = -4.54 + 0.00368 s.
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From page 84... ...
Clearance lost time at the end of the phase can be computed as: le = Y ~ RC - gY (46) where: le = clearance lost hme at end of phase, see; Y= yellowinterval,sec; RC = red clearance interval, see; and gy= effective green extension into the yellow interval, sec.
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From page 85... ...
Table 25. Lane utilization factors for lane groups with random lane choice (Ur)
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From page 86... ...
, the lane utilization factor can be computed as: Max (v' , v' ~ JO U = 1.05 dl dr `49y P v' where: Up = lane utilization factor for propositioning; I'm= number of vehicles in the subject lane group that watt be thing left at the downstream intersection, vpc; V 'fir = number of vehicles in the subject lane group Mat watt be turning right at the downstream intersection, vpc; and Marfv dab V dart = larger of v 'a'' and v 'or Lane Utilization Factor. The possibility of preposition~ng must be evaluated to determine whether to use Equation 48 or 49 to estimate the lane utilization factor U
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From page 87... ...
This phase notation was adopted by the widely used diamond interchange computer program, PASSER Ill, developed by Texas Transportation Institute in ~ 977 (139. Almost all signal timing plans used at two-level interchangestoday can be described by using the a:b:c phase sequence in four combinations together truth a related signal offset between the two Various phase overlap intersections.
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From page 89... ...
Interchange Signal Phase Sequences.
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From page 90... ...
(53) As long as the two signals operate independently, cycle times can be different to accommodate variable traffic demands, green splits can be provided without constraints to better satisfy those demands, and capacities are at a maximum.
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From page 91... ...
ib ~ SX) ic ~ Letting Y = v/s be the flow ratio of demand flow to saturation flow, Men ~ X)
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From page 92... ...
(65) For planning and design purposes, it is also convenient to work with locally adjusted average saturation flow values, and per lane volumes, to estimate the resulting flow ratios as (shim s,, nj (fj)
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From page 93... ...
to keep Me initial flow ratios the same, then the sum of the "equivalent through vehicle" critical lane service volumes for a given X' would be m =3 CV.
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From page 94... ...
Pretimed systems are fixed to prescribed durations regardless of current traffic demands and local capacity provided. Presumably forecasted traffic demands arid estimated roadway capacity were consideredin the initial selection of cycle times, but traffic conditions may have changed and operations deteriorated.
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From page 95... ...
Conditions which result in minimal queuing at green onset, such as low volume levels per lane and the presence of good signal coordination along the crossing arterial, may reduce the effective overlap. A simple kinematic equation is given in the PASSER Ill user's manual for estimating total overlap, assuming platoon acceleration from the stop line after a 0.5 second perception-reaction time, is: = 2 [0.50 + ~0.137 L ~ 3- 19 (76)
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From page 96... ...
Clearly, capacity analysis for signalized two-level interchanges requires knowledge of the same traffic, geometric and signal operations variables as signalized intersections. Estimation of effective green time and saturation flow are more complex, but the same concepts apply.
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From page 97... ...
VO LU M E ADJ USTM ENT M O DU LE ~ 3. SATU RATI O N F LOW RATE MO DU L E Peak Hour Factor Establish lane groups Ideal saturation flow rate Assign volumes to lane groups Adjustment factors (/NTERCHANGE Mode/)
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From page 98... ...
saturation flow rate for lane group under prevailing conditions, vphg, saturation flow rate per lane under ideal conditions (2,000) , pcphgpl; number of lanes for the lane group; adjustment factor for average lane width; adjustment factor for heavy vehicles; adjustment factor for approach grade, adjustment factor for parking, adjustment factor for bus blockage; adjustment factor for volume (traffic pressure)
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From page 99... ...
my, Q ~ ~ ' T~Voirnes~ ( P2BA ~( P2AA Figure 36. Database architecture of INTERCHANGE sofhrare.
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From page 100... ...
g = effective green time serving lane group, see; G = signal green interval of phase serving lane group, see, Y = signal yellow walking interval of phase serving lane group, see; Rc = all-red clearance of phase serving lane group, see; 15 = platoon start-up lost time as related to saturation flow, see; le = platoon clearance lost time, Y + RC - gy' see, gy = effective green extension into yellow, see, and CP = clear period during cycle/phase when subject flow is unblocked, sec. The platoon start-up lost time includes lost times due to perception-reaction time to signal onset arid to incremental accelerationhmes needed to reach the speed at saturation flow.
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From page 101... ...
The green extension into the yellow warning interval is determined from: gy = 1.48 ~ 0.0144 SL + 6.40 (X - 0.88) 1x where: gy = extension of saturation flow into yellow interval, see; SL = approach speed limit, km/h; X = phase volume-to-capacity ratio; and fx = indicator variable (]
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From page 102... ...
, vph, saturation flow rate for the lane group, vphg; effective green time for the serving phase, see; and average cycle length for the subject intersection, sec. Level-of-service for interchanges is based on a combination of degree of saturation, delay and overall travel speed on the link.
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From page 103... ...
If X< ~ and non-zero queue exists at the end of the analysis period, then (85)
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From page 104... ...
Table 25 provides the progression adjustment factor (PF) for the first term of the delay equation based on arrival type (AT)
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From page 105... ...
Table 25. Uniform Delay Adjustment Factors Progression Adjustment Factor (PF)
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From page 106... ...
Extremely large delays are theoreticallypossible if adequate queue storage is available. A practical maximum X value of ~ .2 is employed by the HCS software to limit the delay calculated, but no maximum delay value has been selected.
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From page 107... ...
Much of the operational procedure shown in Figure 39 would now be evaluated internally. The volume adjustment module, saturation flow rate module, effective green module, capacity analysis module, and level of service would all be computed u~thinINTERCHANGE.
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From page 108... ...
INTERCHANGE Procedural Design Analysis performed using existing software packages Inferface · HCS PASSER tl, ID TRANSY7-7F Offer srare 1 A~atysis Operational analysis as perforrnedby ff,e specific program Output Capacity Delay LOS Input Turning movemerd volumes for one interchange forth Type of analysis requested Int~nge types to analyze Geometric and Simon Condemns 1 C_ Analysis 1 Database conversion algoritfuT Analysis to be performed using proposed new single software package , _ _ Analysis Volume A~J~nt Module Satua0 - Flew Rate Module Effective Green Module Capacity Analysis Module Level of Service Module Output Future or existing fuming volumes for chosen interchanges LOS and performance measles for chosen intone Ranldng based on o~a~al performance measures and LOS Figure 39. Fltow diagram of optional procedural dlesign for INTERCHANGE.
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From page 109... ...
The following models are provided as guidelines toward improving traffic signal control during these problematic situations. Signal timing, traffic pattern, and queue storage length are principal factors of flow control during such high-volume conditions.
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From page 110... ...
To minimize the chances of demand starvation, the link offset should not be less than L 3600 NL e..= " u s! u 3.5 ~ ~ where: u N s 1 0,j = relative offset between major phases on link id, see, L = length of link id, meters; ~ .~, , naming speed of arterial traffic flow, mps, number of lanes on link id; saturation flow of downstream phase, vphg; and queue storage length per vehicle, about 7 m/vein.
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From page 111... ...
| Input Data Geometr~cs Traffic | M a ne Over S peed to r Arteria I Th roug h Ve h icles, Um a I | Weave Maneuver Distance, Drew | 1 , Probability of Being Blocked, Pu r Maneuver Speed for Weaving Vehicles, Um,w Figure 37. Flow chart to determine speeds in an arterial weaving section.
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From page 112... ...
Traffic conditions include the average arterial speed entering the weaving section, volume of weaving vehicles, and available weaving maneuver distance. The primary traffic event is the probability ofthe weaving vehicle not teeing blocked from entering the weaving section or delayed during its maneuver.
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From page 113... ...
(99) Like the arterial maneuver speed, the weaving maneuver speed is defined as the average running speed of weaving vehicles through the weaving section.
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From page 114... ...
= 2 lanes Length of the weaving section, to = 200 meters Traffic Data: Average arsenal speed entering Me weaving section, Ua = 12.5 m/s (45 km/in) Average arsenal flow rate entering the weaving section, Va = 1,000 vph Average weaving flow rate, Vw = 170 vph Saturation flow rate per large, s' = 1,800 vphgpl Elective red time for the downstream intersection, r = 50 seconds Average lane length occupied by a queued vehicle, [v = 7.0 m/vein Analysis: 1 .
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From page 115... ...
Flow chart to determine ramp weaving capacity.
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From page 116... ...
3.7.2 Adjustment for Sneakers Additional ramp capacity may be obtained by ramp vehicles weaving across the arterial flow during the phase change intervals of the upstream signal. Simulation studies suggest that as many as three "sneakers"per phase change may cross during capacity conditions .
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From page 117... ...
In order to obtain the ramp weaving capacity for different progression factors, weaving adjustment factors for progression, fpF are determined and multiplied by the ramp capacity which has been adjusted for sneakers. The adjustment procedure is QPF QR fPF where: QPF Q R fPF ramp weaving capacity adjusted for progression, vph; ramp weaving capacity for random flow, vph; and ramp weaving adjustment factor for progression.
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From page 118... ...
Ramp capacity for random flow Substituting values in the above equation yields QR QR = 701 vph Accounting for ramp vehicles weaving during the phase change interval Q R = QR + Sneaker Volume = 701 + 216 = 917vph For progressed flow along the arsenal having a PF of 0.2 and a v/c of 0 6, IF can be determined from Tahie 20 as ~ 074 Therefore Ramp Capacity for a PF of 0.2 and v/c of 0.6: QPF = Q R *
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