Highway Capacity Manual Methodologies for Corridors Involving Freeways and Surface Streets (2020) / Chapter Skim
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Pages 238-250
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232 A P P E N D I X B Off-Ramp Queue Spillback Check The current methodology for Freeway Facilities analysis (HCM Chapter 10) evaluates the performance of each segment individually using standard 15-minute time periods.
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233 Figure B-1. Procedure for identifying spillback occurrence at an off-ramp/weaving segment Step 1 - Capacity Checks The first step in the methodology determines whether capacity is exceeded at any of the critical points along the diverge section: Case A β Ramp proper Demand at the study diverge ramp (vR, as defined in HCM Chapter 14)
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234 Source: HCM 6th Ed. Exhibit 14-12 Figure B-2.
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235 Step 2 β Queue Length Estimation In the second step, the procedure estimates the expected queue length for any conditions where demand exceeds capacity. Three cases may occur: Case A β Ramp proper In cases of demand exceeding capacity of the ramp proper, the bottleneck is the entry to the off-ramp, and the ramp proper would not necessarily have a queue present.
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236 Figure B-3. Examples of unbalanced ramp lane usage: (a)
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237 Unsignalized Intersections. Each unsignalized intersection type has its own methodology to estimate queue length.
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238 The individual ramp storage for each of the k lanes in the off-ramp, LR,k, can be estimated by assigning the intersection lane groups to ramp lanes, as previously described: πΏ , = β π , π₯ πΏ (Equation B-5) Where: Ni,k = Number of lanes in section i that are associated to ramp lane k Li = Section i length (ft)
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239 Example Problem 1 β Queue spillback from a downstream signalized intersection The exit ramp at I-95 SB to SW 25th Rd (Miami, FL) has a signalized intersection ramp terminal (Figure A-5)
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240 The signalized intersection performance was estimated using HCM methods (Chapter 19 β Signalized Intersections) , and the 95th percentile back-of-queue lengths were calculated as shown in Figure B-7.
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241 The 95th percentile queues to each ramp lane are: QL,1 = Q95%,LG1 = 1196.5 ft QL,2 = Q95%,LG2 + Q95%,LG3 = 1200.8 + 1532.3 = 2733.1 ft Available queue storage on each ramp lane are: LR,1 = = 400*
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242 Example Problem 2 β Queue Spillback from a Downstream Merge (Freeway-to-Freeway) A freeway-to-freeway two-lane ramp is evaluated for queue spillback (I-75 SB to SR-826 SB β Miami, FL)
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243 Figure B-10. Summary of analysis results for SR-826
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244 As shown, segments 1 and 3 (boundary basic segments) in SR-826 are operating at LOS C and D respectively for the entire analysis period.
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