Freeway ramps Capacity and Level of Service (HCM 2000) 1 Capacity and LOS - HCM 2000 There are three different types of freeway segments: Basic freeway segments traffic conditions are not affected by vehicles entering or leaving the freeway Ramps freeway segments with exit or entering ramps Weaving segments freeway segments where the crossing of two or more traffic streams traveling in the same direction occurs 2 1
Main Concepts: Figura 3.12 Rampas de Acesso a uma AE Ramps could be divided in 3 distinct components : 1.Ramp Freway junction : they are usually designed in a way that allows easy merging and divergence movements at high speeds; 2. Ramp roadway; and 3.Ramp-street junction, it could be another freeway or a highway. 3 Main Concepts: Figura 3.12 Rampas de Acesso a uma AE Capacidade de Secções Centrais de Rampas Free flow speed of ramp S FR (km/h) Capacity (vl/h) Single lane ramps Ramp roadways differ from the freeway mainline in that: They are roadways of limited length and width (often just one lane); Free-flow speed is frequently lower than that of the roadways connected, particularly the freeway; On single-lane ramps, where passing is not possible, the adverse impact of trucks and other slow-moving vehicles is more pronounced than on multilane roadways; and At ramp-street junctions, queuing may develop on the ramp, particularly if the rampstreet junction is signalized. two lane ramps > 80 2200 4400 < 65-80 2100 4100 < 50 65 2000 3800 30 50 1900 3500 < 30 1800 3200 NOTE: values for the ramp roadway capacity not that of the ramp freeway terminal 4 2
Main Concepts: Figura 3.12 Rampas de Acesso a uma AE The parameters that influence ramp juctions are the same that influence basic freway segments : lane width, lateral clearence, type of terrain, driver characteristics, the percentage of heavy vehicles, and: Total length of acceleration (LA) and deccelaration lanes (LD) Figura 3.13 - Convenção de numeração das pistas lane 3 lane 2 lane 1 Free flow speed of ramp (SFR) SFR is influenced by several factors like then number of lanes, ramp grade, visibility and curve radius Total freeway flow approaching merge area (VF) and its distribution among the lanes, and total total ramp flow (VR) 5 Main Concepts: 6 3
Main Concepts: 7 Methodology: The methodology is divided in 3 parts : 1. Compute demand flow rate immediately upstream of merge or diverge influence area (v 12 ) 2. Compute equivalent flows and capacities: i. Maximum total flow approaching a major diverge area on the freeway (v F ) ii. iii. iv. Maximum total flow departing from a merge or diverge area on the freeway (v FO ) Maximum total flow entering the ramp influence area (vr12 for merge areas and v12 for diverge areas) Maximum flow on a ramp (vr). 3. Compute density within the ramp inlfuence area (DR) and LOS NOTES: a) Flow rates are computed in the same way as for freeway basic segment, using - V, FPH, N, f HV e f p. b) In Freeways with 6 lanes (2*3) we should consider the effect of nearby ramps in the heavy vehciles distribution among lanes - aditional step c) The methodology has diferent calculations for merging and diverging ramps. 8 4
Methodology: Merging ramp Exit ramp 9 Methodology: 10 5
v R = on ramp demand flow rate[pc/h] v D = demand flow on adjacente downstream ramp [pc/h] L A = length of acceleration lane [m] Methodology: On ramps 1. Compute demand flow rate immediately upstream of merge influence area (v 12 ) em que v 12 = v F P FM v 12 = flow rate in lanes 1 and 2 [pc/h] v F = freeway demand flow immediately upstream of merge [pc/h] P FM = proportion of approaching freeway remaining in lanes 1 and 2 immediately upstream of merge S FR = free flow speed on ramp [km/h] L up = distance to adjacent upstream ramp [m] L down = distance to adjacent downstream ramp [m] 11 Methodology: On ramps NOTES: Adjacent on ramps do not affect subject ramp behavior, and the analysis proceeds using Equation 1 Where an adjacent upstream or downstream off-ramp (or both) exists, the decision to use Equation 2 or 3 versus 1 is made by determining the equilibrium separation distance (LEQ) between ramps 12 6
Methodology: On ramps For UPSTREAM EXIT RAMPS: equation 1 ou equation 2 if L up L EQ equation 1 is used. L EQ = 0,0675 (v F + v R ) + 0,46 L A + 10,24 S FR 757 [m] For DOWNSTREAM EXIT RAMPS : equation 1 or equation 3 v D L EQ = 0,3596 + 0,001149 L A [m] If L down L EQ equation 1 is used. A special case exists when both a downstream and an upstream adjacent off-ramp exist. In such cases, two solutions for PFM may arise, depending on whether the analysis considers the upstream or the downstream adjacent ramp, because they cannot be considered simultaneously. In such cases, the analysis resulting in the largest value of PFM is used. 13 Methodology: On ramps 2) Determining Capacity (v R12 e v FO ) The total flow entering ramp influence area (v R12 ) is the sum of flow rate in lanes 1 and 2 upstream of merge (v 12 ) and of on ramp demand flow rate (v R ) : v R12 = v 12 + v R The freeway demand flow rate immediately upstream of merge (v FO ) is composed by the demand flow rate immediately upstream of merge (v F ) and on ramp demand flow rate (v R ): v FO = v F + v R In order for this methodology could be used, all the flows in the ramps influence area and on ramp demand flow should not exceed capacity. 14 7
Methodology: On ramps 2) Determining Capacity (v R12 e v FO ) When the total flow entering the ramp influence area (vr12) exceeds its maximum desirable level but the total freeway flow (v) does not exceed the capacity of the downstream freeway segment. In this case, locally high densities are expected, but no queuing is expected on the freeway. The actual lane distribution of entering vehicles is likely to consist of more vehicles in the outer lanes than is indicated by the models herein. Overall, operation will remain stable, and LOS F is not expected to occur. 15 Methodology: On ramps 3) Determining LOS Density of merge influence area is computed through the following expression: D R = 3,402 + 0,00456 v R + 0,0048 v 12 0,01278 L A Density (pc/km/lane) LOS 6 A > 6-12 B > 12 17 C > 17-22 D > 22 E Demand > Capacity F 16 8
Methodology: Off ramps 1) Estimation of flow rate in lanes 1 and 2 of freeway immediately upstream of diverge where The flow rate in lanes 1 and 2 of freeway immediately upstream of diverge (v 12 ) includes off-ramp demand flow (v R ) and it is estimated for the immediatly upstream section of the deceleration lane. v 12 includes the vehicles leaving the freeway (v R ) and a percentage of vehicles circulating on the freeways: v12 = flow rate in lanes 1 and 2 [pc/h] vr = off ramp flow rate[pc/h] vf = freeway demand flow rate immediately upstream [pc/h] PFD = proportion of through freeway flow remaining in lanes 1 and 2 immediately upstream of diverge v 12 = v R + (v F v R ) P FD 17 Methodology: Off ramps 1) Estimation of flow rate in lanes 1 and 2 of freeway immediately upstream of diverge where: P FD = proportion of through freeway flow remaining in lanes 1 and 2 immediately upstream of diverge v F = freeway demand flow rate immediately upstream v R = off ramp flow rate[pc/h] v U = demand flow rate on adjacent upstream ramp [pc/h] v D = demand flow rate on adjacent downstream ramp [pc/h] L up = distance to adjacent upstream ramp [m] L down = distance to adjacent downstream ramp [m] [pc/h] 18 9
Methodology: Off ramps 1) Estimation of flow rate in lanes 1 and 2 of freeway immediately upstream of diverge In 6 lane freeways the equation for P FD depend on the effect of adjcent ramps. Adjacent upstream off-ramps and adjacent downstream on-ramps do not affect subject ramp behavior, and analysis proceeds using Equation 5. Where an adjacent upstream on-ramp or downstream off-ramp exists, or where both exist, the decision to use Equation 6 or 7 versus 5 is made by determining the equilibrium separation distance (L EQ ) between ramps. 19 Methodology: Off ramps 1) Estimation of flow rate in lanes 1 and 2 of freeway immediately upstream of diverge If the distance between ramps is greater than or equal to LEQ, Equation 5 is always used. If the distance between ramps is less than LEQ, Equation 6 or 7 is used as appropriate. LEQ is that distance for which Equation 5 and Equation 6 or 7, as appropriate, yields the same value for PFD. Thus, where an adjacent upstream on-ramp exists, Equation 6 must be considered. If Equation 6 is set equal to Equation 5, the following relationship is derived. vu Equation 5 = equation 6 L EQ = 0,2337 + 0,000076 v 0,00025 v if L up L EQ equation 5 is used Similar for downstream off ramps : equation 5 = equation 7 L EQ = if L down L EQ equation 5 is used. vd 3,79 0,00011 vf 0,00121 vr A special case exists when both a downstream adjacent off-ramp and an upstream adjacent on-ramp exist. In such cases, two solutions for PFD may arise, depending on whether the analysis considers the upstream or the downstream adjacent ramp since both cannot be considered simultaneously. In such cases, the analysis resulting in the largest value of PFD is applied. F R [m] [m] 20 10
2) Determining Capacity (v 12 e v F ) Methodology: Off ramps The three limiting values that should be checked in a diverge area are: The capacity of freeways approaching the diverge (VF) The capacities of the departing freeway leg or legs (VFO), or ramp (VR), or both, The maximum flow that can enter on Lanes 1 and 2 just prior to the deceleration lane (V12) 21 Methodology: Off ramps 3) Determining LOS Density in the influence area is determined by: D R = 2,642 + 0,0053 v 12 0,0183 L D Level Of Service Density (pc/km/lane) LOS 6 A > 6-12 B > 12 17 C > 17-22 D > 22 E Demand > Capacity F 22 11
Methodology: Off ramps and On ramps Determining speed at ramp influence areas Speed in lanes 3 and 4 in the in the influence area of one ramp (+- 450 m) is usually lower then the one in one freeway section. It is possible to estimate speed in lanes 1 and 2 (table 1) and in lanes 3 and 4 (table 2) Quadro 3.21 1 speed at ramp influence areas Type of ramp On-ramp Intermediate parameter vr12 1000 L A S m 0,321 0,0039 e 0,004 FR s = + 1000 Average speed in lanes 1 and 2 S R [km/h] S R = S FF (S FF 67) m S Off-ramp d S = 0,883 + 0,00009 v R 0,008 S FR S R = S FF (S FF 67) d S 2 - speed at ramp influence areas Type of ramp On ramp Average flow in lanes 3 and 4 Average speed in lanes 3 and 4 v OA [pc/h/lane] S O [km/h] < 500 S O = S FF 500 2300 S O = S FF 0,0058 (v OA 500) > 2300 S O = S FF 10,52 0,0058 (v OA 500) Off ramp < 1000 S O = 1,06 S FF 1000 S O = 1,06 S FF 0,0062 (v OA 1000) 23 Methodology: Off ramps and On ramps Determining speed at ramp influence areas Average per lane flow rate in outer lanes is calculated by : v OA vf v = N S R = space mean speed of vehicles within ramp influence area (km/h); for merge areas, this includes all vehicles in v R12 ; for diverge areas, this includes all vehicles in v12; S O = space mean speed of vehicles traveling in outer lanes (Lanes 3 and 4, where they exist) within 450- m length range of ramp influence area (km/h); S FF = free-flow speed of freeway approaching merge or diverge area (km/h); S FR = free-flow speed of ramp (km/h); L A = length of acceleration lane (m); v R = flow rate on ramp (pc/h); v R12 = sum of flow rates for ramp (vr) and vehicles entering ramp influence area in Lanes 1 and 2 (v12) at a merge area (pc/h); v OA = average per-lane flow rate in outer lanes (Lanes 3 and 4, where they exist) at beginning of ramp influence area (pc/h/ln); m s = intermediate speed determination variable for merge area; d s = intermediate speed determination variable for diverge area. N O = number of outside lanes in one direction (not including acceleration or deceleration lanes or Lanes 1 and 2), v F = total approaching freeway flow rate (pc/h), and v 12 = demand flow rate approaching ramp influence area (pc/h). For merge areas [km/h]: v S = v S R12 R12 R + v OA N O + v OA N SO O O 12 For diverge areas [km/h]: v12 + voa NO S = v12 voa NO + SR SO 24 12