Highway Capacity Manual Methodologies for Corridors Involving Freeways and Surface Streets (2020) / Chapter Skim
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Pages 299-309
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From page 299... ...
293 A P P E N D I X D On-Ramp Queue Spillback Check Queue spillback into an arterial intersection may occur when the freeway merge segment has insufficient capacity to process the ramp's demand. Spillback may also occur in cases of ramp metering.
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294 Figure D-1. Procedure for detecting spillback occurrence at an on-ramp
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295 Step 1 β Demand Estimation The first step in the methodology calculates the entering demand flow rate at the onramp (vR) , as a function of the upstream intersection configuration and operations.
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296 π , = π , , π , , (Equation D-3) Figure D-2.
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297 With very few adjustments, estimating the on-ramp throughput from this intersection type is a relatively straightforward task. The first step is to identify the movements that discharge to the on-ramp and their respective ranks (priority orders)
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298 Where: Ξ»LT = departure rate from major street left turn into the on-ramp (veh/h) vLT = demand flow rate for the major street left turn cp,j = potential capacity for the major street left turn (veh/h)
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299 π£ = , + , + , (Equation D-10) Where: vR = on-ramp flow rate (veh/h)
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300 Step 2 β Capacity Estimation As shown in Figure D-1, capacity at the on-ramp must be estimated in order to predict the occurrence of queue spillback. Three cases may occur: Case 1 β Ramp metering is active In this case, the metering rate is a required user input (veh/h)
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301 Example Problem β Signalized intersection with ramp metering This example is based on the configuration used in the HCM Chapter 34 (Ramp Terminals and Alternative Intersections Supplemental) , as shown in Figure D-5.
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302 There are two movements that discharge directly to the on-ramp: WBR (Ξ¦16 β Phase I) and EBL (Ξ¦5 β Phase III)
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303 Table D-3 β On-ramp throughput for WBR movement Movement WBR demand flow rate (veh/h) WBR arrival rate (veh/s)
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