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Lec 20, Ch.18, pp.466-485: Analysis of signalized intersections, HCM (Objectives)

Lec 20, Ch.18, pp.466-485: Analysis of signalized intersections, HCM (Objectives). Understand the conceptual framework for the HCM 2000 method Know the modules in Chapter 10 & 16 of the Highway Capacity Manual: Input module, Volume adjustment module, and Saturation flow rate module

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Lec 20, Ch.18, pp.466-485: Analysis of signalized intersections, HCM (Objectives)

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  1. Lec 20, Ch.18, pp.466-485: Analysis of signalized intersections, HCM (Objectives) • Understand the conceptual framework for the HCM 2000 method • Know the modules in Chapter 10 & 16 of the Highway Capacity Manual: Input module, Volume adjustment module, and Saturation flow rate module • Learn how to use the Highway Capacity Software

  2. What we discuss in class today… • Why are the materials presented in Chapter 18 still relevant? • Conceptual framework of the HCM 2000 • Modules of the HCM 2000 • Introduction to the Highway Capacity Software

  3. Why are the materials in Chapter 18 still relevant? • HCM 2000 is now available and the models for signalized intersection analysis are similar to those in chapter 18 of the textbook except for a few new factors and additional analysis capabilities • The signalized intersection analysis in HCM 2000 has new delay models but mostly it is an improvement (approach delay, or “control delay,” is used instead of stopped delay and multiple time period analysis was added) to the 1994 HCM. Hence, if you can understand the models in the 1994 HCM, you can understand the models in HCM 2000. • The textbook has a lot of insights into the models (which you don’t get from the HCM documents). • HCM method does not take into account the potential impact of downstream congestion on intersection operation, nor does the methodology detect and adjust for the impacts of turn-pocket overflows on through traffic and intersection operation.

  4. Tools for analysis of signalized intersections • Highway Capacity Manual 2000, Chapter 10 & 16, Consisting primarily of deterministic analytic algorithms developed from theoretical considerations and/or empirical data and regression analysis and its companion Highway Capacity Software (HCS) • SOAP (“simulation” and optimization) • Synchro 5.0 (“simulation” and optimization, HCS analysis) • Transyt 7F (“simulation” and optimization, network of signalized intersections) • PASSER II (Signalized arterial analysis) • HCS Cinema & SigCinema (HCS + simulation based on NETSIM) • SimTraffic (simulation) • NETSIM (simulation)

  5. Conceptual framework for HCM 2000 (This conceptual framework is basically identical in HCM 1994) A. Critical lane group concept Critical lane analysis vs. Critical lane group analysis V S v s Critical lane analysis compares actual flow (v) with the saturation flow rate (s) in a single lane. Critical lane group analysis compares actual flow (V) with the saturation flow rate (S) in a group of lanes operating in equilibrium. In either case, the ratio of V to S is the same. This applies to shared lanes, also. Exclusive right- or left-turn lanes must be separately analyzed.

  6. Conceptual framework for HCM 2000 (cont) B. The v/s ratio: Normalized traffic volumes/flow rates Simple method (as a comparison): HCM 2000 considers 11 specific conditions affecting intersection operations: (1) Lane width, (2) Heavy vehicle presence, (3) Grade, (4) Parking conditions, (5) Local bus blockage, (6) Location within the urban area, (7) Lane utilization, (8) Left turns, (9) Right turns, (10) Pedestrian-bike adjustment for LT movements, and (11) Pedestrian-bike adjustment for RT movements. All adjustments are used to modify an ideal saturation flow rate to one that represents prevailing conditions for the lane group. (7), (10), and (11) are new additions to the 1994 model. We may not be able to compare directly lane groups because their conditions are different. So HCM use the flow ratio, v/s. This process is called “normalization.”

  7. Conceptual framework for HCM 2000 (cont) C. Capacity • In the simple timing method, the capacity of the intersection as a whole was considered. • HCM 2000 as well as 1994 HCM gives the capacity of each lane group. • Demand does not necessarily peak at all approaches at the same time. • Capacity may change for each approach during the day. (like the effect of curb side parking, bus blocking, etc.) • Capacity is provided to movements to satisfy movement demands. (Note: the critical capacity ratio v/c is still calculated in HCM 2000 as well as HCM 1994.)

  8. Conceptual framework for HCM 2000 (cont) D. Level of service • The 1994 HCM uses “average individual stopped-time delay.” The 2000 HCM uses “average control delay.” See the difference in the delay models. d1 = uniform control delay. You must divide d1 obtained by HCM 2000 by 1.3 to compare with d1 by HCM 1994. d2 = adjustment for randomness d3 = adjustment for initial queue (left over vehicles caused by oversaturation)

  9. Conceptual framework for HCM 2000 (cont) D. Level of service (cont) • All the HCM delay models assume random arrivals. Hence, the delay model produce delays for approaches with random arrivals. Urban signals are coordinated; hence, many do not have random arrivals. This is corrected by the “quality of progression” factor called “Arrival Type” factor (DF in HCM 1994 and PF in HCM 2000) • For uninterrupted facilities, like freeways, v/c has a direct connection with the performance of the facility. So, if v/c = 1.0, the facility is at the capacity. • For signalized intersections (interrupted facilities), this is not necessarily true – especially when delay is used as the MOE. • You may get LOS=F even if v/c is well below 1.0. For instance LT vehicles may have a long stopped delay even if its v/c is low. • HCM 1994 delay model focuses on the first 15-min interval. So, even if it is over-saturated (v/c > 1.0), we get a relatively smaller delay. See pages 422 and 423. HCM 2000 has 3 study approaches: Single analysis period for 15 min and 1 hour, and multiple 15-min analysis periods (See Exhibit 16-6.)

  10. Conceptual framework for HCM 2000 (cont) E. Critical lane analysis concepts 1. Saturation flow rate (this model has changed) Saturation flow rate for a lane group Ideal saturation flow rate, 1900 pcphgpl 2. Capacity of a lane group (this concept does not change)

  11. Conceptual framework for HCM 2000 (cont) E. Critical lane analysis concepts (cont) 3. v/c ratios.  “degree of saturation” • Three issues: • Capacity is practically always estimated (because it is difficult to measure.) • In existing cases demand is often measured by “departure flows” although it should be “arrival flows.” • For future cases, predicted arrival volumes are given (by a planning model) instead of actually counted volumes. Case 1: v/c > 1.0 resulted in an analysis for an existing signalized intersection. If demand is measured by a departure flow (assuming it was correct), this cannot be accepted. If arrival flows are measured, v/c > 1.0 may occur – this becomes obvious because queue forms); Capacity must have been underestimated because HCM models are based on national average values.

  12. Conceptual framework for HCM 2000 (cont) E. Critical lane analysis concepts (cont) 3. v/c ratios.  “degree of saturation” (cont) Case 2: v/c > 1.0 resulted in an analysis for a planned signalized intersection. In a planning case, both demand and capacity are estimates. But, it may indicate that the forecast demand flow exceeds the estimated capacity of the lane group, and a problem will likely exist. Demand is an arrival flow for a predicted case because those values come from a planning model. Computation of a v/c ratio for a give lane group (this model does not change:

  13. Conceptual framework for HCM 2000 (cont) Computation of a v/c ratio for an intersection as a whole: The critical v/c ratio for the intersection  defined as the sum of the critical lane group flows divided by the sum of the lane group capacities available to serve them (compare this one with the Simple Method in Ch 17). If the Xc > 1.0, then the physical design, phase plan, and cycle length specified do not provide sufficient capacity for the anticipated or existing critical lane group flows.  Do something to increase capacity: (1) longer cycle lengths, (2) better phase plans, and (3) add critical lane group or groups (meaning change approach layouts)

  14. Conceptual framework for HCM 2000 (cont) Computation of a v/c ratio for an intersection as a whole (Additional comments): • If the critical v/c ratio is less than 1.00, the cycle length, phase plan, and physical design provided are sufficient to handle the demand and flows specified. • But, having a critical v/c ratio under 1.00 does not assure that every critical lane group has v/c ratios under 1.00. When the critical v/c ratio is less than 1.00, but one or more lane groups have v/c rations greater than 1.00, the green time has been misallocated.

  15. Conceptual framework for HCM 2000 (cont) F. Effective green times and the application of the lost times • HCM delay models use “effective green time” and “effective red time.” • HCM models assume that all lost times happen at the beginning of the phase. (These basic concepts in HCM 2000 are identical to those in HCM 1994.) gi ri tL

  16. HCM 2000 computational modules The 2000 HCM signalized intersection analysis consists of 5 modules:

  17. A. Input Module Input Module: Many parameters are considered. See Exhibit 16-3 (Table 18-2 in the text). Geometric, traffic, and signalization conditions are considered  Some of them are self-explanatory. • Area type: CBD intersections have lower saturation flow rates (in general). • Parking conditions and parking activity: Parking activity within 250 ft of the stop line is considered. Parking activities interfere traffic flow • Conflicting pedestrian flow: Pedestrian flow between 1700 to 2100 ped/hr completely blocks right-turn vehicles. HCM 2000 considers bicycles as well. • Local bus volume: Buses must stop to be considered in this parameter. If they pass through the intersection, not stopping for passengers, they are considered as heavy vehicles. • Arrival type: The single most important factor influencing delay predictions.

  18. A. Input Module (cont) More discussion on Arrival type (Exhibit 16-4, p.474 text):

  19. A. Input Module (cont) More discussion on Arrival type: Need to compute a platoon ratio: Rp = 1.00, when the proportion of vehicles arriving on green is equal to the g/C ratio. (Same as Exhibit 16-11) These Rp values are used to determine DF values. See Table 18-15 on page 508.

  20. B. Volume Adjustment Module 1. Conversion of hourly volumes to peak rates of flow The 2000 HCM model provides 3 analysis types. But typically an analysis of the peak 15-min period within the hour of interest is done. Since demand volumes are often given as full-hour volumes, each must be adjusted to reflect the peak 15-min interval using the peak hour factor. If peak 15-min volumes are provided, PHF = 1.0. If queue carryover occurs, a multiple-period analysis is best. In this case do not use PHF.

  21. B. Volume Adjustment Module (cont) 2. Establish lane groups for analysis Defacto LT lane: A shared TH-LT lane is functioning like a left-turn lane because there are so many LT vehicles. There is no way to know it from the beginning. So start the HCM procedure that there is no de-facto LT lane. In the procedure the proportion of LT vehicles in the left lane (PL) is estimated. If PL = 1.00 (or higher), the lane beccomes a Defacto LT lane. And you start the computation from the start! 3. Adjustment for RTOR: Reduce the RTOR volume from the RT volumes for analysis. Note: In the previous HCM, adjustment for lane utilization was included in the Volume Adjustment Module. In HCM 2000, lane utilization is considered in the Saturation Flow Rate Module

  22. C. Saturation Flow Rate Module The saturation flow rate module is the most important part of the 1994 HCM model (and HCM2000). The prevailing total saturation flow rate for each lane group is estimated. • This is a HCM2000 saturation flow model. Compare with the one in the text which comes from HCM1994. First 7 are easy; the last two factors are really involved.

  23. C. Saturation Flow Rate Module (cont)

  24. C. Saturation Flow Rate Module (cont)

  25. C. Saturation Flow Rate Module (cont) • Right-turn adjustment factor, fRT (modified): The pedestrian effect term was removed and two new factors for pedestrian-bicycle blockage effect were created)

  26. C. Saturation Flow Rate Module (cont) 8. Left-turn adjustment factor, fLT Exclusive LT lane with protected LT phasing Exclusive LT lane with permitted LT phasing Exclusive LT lane with compound LT phasing Shared LT lane with protected phasing Shared LT lane with permitted phasing Shared LT lane with compound phasing Case 4 is rare because it may waste green time. Case 6 may waste green time, also. See Table 18-12. Major sections D and E starting page 485 discuss these models. Appendix C of the 2000 HCM discusses the models.

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