HCM 6th Edition: Ramp Terminals and Alternative Intersections

HCM 6th Edition: Ramp Terminals and Alternative Intersections
Today’s web briefing is focused on the ramp terminals and alternative intersections evaluation methods that are in Chapter 23 of the newly released HCM 6th Edition.

Instructor [Note: Instructor should show their name, title, affiliation, and HCM-related background. If the presentation is via webinar, then add a photo of instructor] My name is: _______________. I will be the instructor for today’s presentation. I am a _______________ with ____________.

Overview and Background
Session 1 Overview and Background Briefing series overview Objectives and scope of briefing Note: “HCM” → Highway Capacity Manual Content and Structure of HCM Methodology Basics New Capabilities This briefing is organized into four sessions. The first session is intended to provide some context for our examination of the Ramp Terminals and Alternative Intersections chapter. In this session, I plan to review the schedule for this briefing series and then outline the objectives of this presentation. The second session will identify the content and structure of the HCM 6th Edition, as it relates to the Ramp Terminals and Alternative Intersections chapter. The third session will provide an overview of the methodologies in this chapter and highlight some of the new terms and concepts that are used. The last session will highlight some of the new evaluation capabilities of the methodologies.

Briefing Series Overview
What’s New – HCM 6th Edition New Features in Freeway Analysis Chapter Freeway Reliability and Strategy Assessment Urban Street Segments Urban Street Facilities Signalized Intersections Signalized Intersection Planning Application Roundabouts Ramp Terminals and Alternative Intersections Planning and Preliminary Engineering Guide Today’s briefing is being offered as one in a series of ten briefings on the HCM 6th Edition. This briefing on ramp terminals and alternative intersections is the ninth in this series.

Objectives and Scope Learning Objectives Scope of Presentation Focus
Learn about new capabilities of the ramp terminals and alternative intersections chapter Understand how the chapter can be used to evaluate terminal or intersection operation Scope of Presentation HCM 6th Edition Ramp Terminals and Alternative Int. chapters Focus Changes since publication of HCM 2010 There are two objectives for today’s discussion. One objective is to help you learn about the new capabilities of the Ramp Terminals and Alternative Intersections chapter. A second objective is to help you understand how the chapter can be used to evaluate ramp terminal and alternative intersection operation. Our discussion centers on the methodologies in the Ramp Terminals and Alternative Intersections chapter of the HCM 6th Edition. These methodologies are used to evaluate ramp terminal or alternative intersection operation. Given the limited time that we have for this presentation, we will need to focus our discussion on a few key topics. To this end, I am planning to focus on the changes made to the methodologies since their publication in the HCM 2010.

Presentation Overview
Session 1 – Overview and Background Session 2 – Content and Structure Session 3 – Methodology Basics Session 4 – New Capabilities We are now ready to transition to Session 2. In this session, we will discuss the content and structure of the HCM 6th Edition, as it relates to ramp terminals and alternative intersections.

Session 2 Content and Structure HCM organization Chapter titles
Chapter outlines For this session, I will show you the organization of the HCM and describe how it will be changed for the soon-to-be-published HCM 6th Edition. My approach will be to list the HCM chapters related to ramp terminals and alternative intersections, describe the changes to the chapter titles, and summarize the changes being made to chapter contents. Let’s get started...

Volume 2 – Uninterrupted Flow Volume 3 – Interrupted Flow
HCM Organization Volume 1 - Concepts Volume 2 – Uninterrupted Flow Volume 3 – Interrupted Flow Volume 4 – Applications Guide The HCM consists of four volumes. They are shown here. No changes to volume titles will be made for the HCM 6th Edition. The first three volumes are included in the printed copy of the manual. The fourth volume is available from the Internet at the address shown. The chapters we will be discussing today are located in Volumes 3 and 4. Let’s take a closer look at them...

Chapters HCM 2010 HCM 6th Edition Volume 3 Volume 4 Volume 3 Volume 4
22: Interchange Ramp Terminals (IRT) Volume 4 34: IRT Supplemental HCM 6th Edition Volume 3 23: Ramp Terminals and Alternative Intersections Volume 4 34: IRT Supplemental The interchange ramp terminal information is currently located in Chapters 22 and 34. Chapter 22 is located in Volume 3, and Chapter 34 is located in Volume 4. For the HCM 6th Edition, the title and scope of Chapter 22 has changed. The new chapter is numbered 23 and its title is Ramp Terminals and Alternative Intersections. It now includes a methodology for evaluating several types of alternative intersection configurations. No change has been made to the number of Chapter 34. Ok, now let’s take a look at the section titles in each chapter...

Volume 3 Chapter Outline
HCM 2010 Chapter 22 Introduction Types of interchanges LOS criteria Methodology Signalized interchanges Interchanges w/roundabouts Interchange type selection Applications Example Problems HCM 6th Edition Chapter 23, Part A Concepts LOS criteria Chapter 23, Part B Introduction Types of interchanges Methodology Signalized interchanges Extensions Interchange w/roundabouts Ped. and bicycle analysis Applications The new chapter 12 includes three parts. The first part (Part A) discusses concepts common to both ramp terminals and alternative intersections. The second part (Part B) presents the methodology for evaluating ramp terminals. It is taken from existing Chapter 22, although, as the slide indicates, the sections have been reorganized for the new Part B. The methodology specifically addresses motorized vehicle operations. However, there is some guidance in the section titled “Extensions” for evaluating the service provided to pedestrians and bicyclists. This guidance describes how the pedestrian and bicycle methodologies in other HCM chapters can be adapted for the evaluation of ramp terminals. We will discuss the new Part C on the next slide. However, before leaving this slide, I should note that the example problems have been moved from Chapter 22 to Chapter 34, which is in Volume 4.

HCM 2010 Did not exist HCM 6th Edition Chapter 23, Part C Introduction Concepts Restricted crossing U-turn intersection (RCUT) Median U-turn intersection (MUT) Displaced left-turn intersection (DLT) Methodology Extensions Ped. and bicycle analysis Applications Part C of Chapter 23 will include a new methodology for evaluating alternative intersections. Specifically, the methodology addresses the evaluation of three intersection types. They are: 1. Restricted crossing U-turn intersection (RCUT) 2. Median U-turn intersection (MUT) 3. Displaced left-turn intersection (DLT) The methodology specifically addresses motorized vehicle operations. However, there is some guidance in the section titled “Extensions” for evaluating the service provided to pedestrians and bicyclists.

HCM 2010 Chapter 34 Example Problems Diamond interchange Parclo A-2Q Diamond w/spillback Diamond w/starvation Single-point urban interchange (SPUI) Diamond w/adjacent intersection Diamond w/roundabout Alternative analysis tool HCM 6th Edition Chapter 34 Introduction Example Problems Same as HCM 2010, plus Diverging diamond interchange (DDI) Four-leg RCUT w/merge Three-leg RCUT w/stop Four-leg RCUT w/signal Four-leg MUT Partial DLT Full DLT Interchange Type Selection O-D and Turning Movements Chapter 34 continues to be a repository for the example problems. The existing chapter 34 has eight example problems that focus on ramp terminals at typical interchange forms. Chapter 34 in the HCM 6th Edition will retain the eight existing example problems, and it will add sample problems for the diverging diamond interchange, and several different types of alternative intersection.

Questions on Content or Structure?
Content and Structure HCM organization Chapter titles Chapter outlines Questions on Content or Structure? This concludes the session on Content and Structure of the HCM. Let’s take a few minutes to answer some questions.

Session 1 – Overview and Background Session 2 – Content and Structure Session 3 – Methodology Basics Session 4 – New Capabilities OK, let’s continue with Session 3 - Methodology Basics

Session 3 Methodology Basics Ramp Terminals Alternative Intersections
New interchange configuration Calculation framework New input data Alternative Intersections New intersection configurations For this session, we will separately discuss the ramp terminal methodology and the alternative intersection methodology. For each methodology, we will identify the intersection configurations addressed, review the sequence of calculations, and then we will discuss the new types of input data that are needed. Let’s start with the ramp terminal methodology...

New Interchange Configuration
Ramp Terminals New Interchange Configuration Diverging diamond interchange (DDI) Crossover at each terminal The ramp terminal methodology in Part B of Chapter 23 has been expanded to include the signalized terminals at a diverging diamond interchange. There are two terminals at this interchange. One terminal is formed by the intersection of the on- and off-ramps with the crossroad on the left side of the interchange. The other terminal is formed by the ramps and crossroad on the right side of the interchange. The off-ramp right-turn movement may be included in the ramp terminal signalization. That is, it may be signal controlled. Or, it may yield-controlled and excluded from the signalization.

Calculation Framework
Ramp Terminals Calculation Framework Step Diamond Parclo, DDI SPUI Yield Determine O-D and movement demands √ Determine lane groups Determine adjusted saturation flow rates Determine eff. green adj. due to interchange operations Determine eff. green adj. due to closely-spaced intersection Determine performance of yield-controlled turns Determine v/c ratio and queue storage ratio Determine control delay and travel time by O-D Determine LOS The methodology for evaluating interchange ramp terminals consists of nine steps. These steps are listed in the first column of the table. [read] However, only some of these steps are used for any given ramp terminal configuration. For ramp terminals at diamond, parclo, or DDI interchanges, eight steps are used. These steps are indicated in the second column of the table. The evaluation of a SPUI requires the use of only six steps. These steps are indicated in the third column of the table. The evaluation of a yield-controlled terminal (as may be found at a DDI) requires the six steps identified in the last column of the table.

Ramp Terminals New Input Data
Extra travel distance relative to the center line Travel speed for extra travel distance Default value estimated from speed limit Extra travel distance for O4 to D3 The ramp terminals methodology has been revised to include prediction of the diverted-path delay that occurs for some interchange configurations. The procedure for predicting this delay requires the analyst to provide two new inputs. One input is the extra travel distance for each interchanging movement. The second input is the travel speed associated with the extra travel distance. The concept of extra travel distance is show in the bottom half of this slide. The movement of interest is the westbound left turn. The geometry of the interchange requires the left-turning vehicle to past the main lanes by an amount equal to the distance shown by the green dimension line. This represents “extra” travel distance relative to an at-grade intersection where the left-turn vehicle does not pass by the main street and has no extra travel distance. (At an at-grade intersection, the driver does not travel any extra distance because he or she makes a right-angle turn right at the intersection of the cross street and main street).

Alternative Intersections
New Intersection Configurations Displaced left turn (DLT) Left turns cross over in advance of intersection Restricted crossing U-Turn (RCUT) Minor-street left turn and through vehicles re-routed Now let’s switch over to Part C of Chapter 23 and discuss the alternative intersections methodology. This methodology is entirely new. At this time, it provides procedures for evaluating three alternative intersection configurations. Two of these configurations are shown in this slide. They include... [read].

New Intersection Configurations Median U-turn (MUT) Major- and minor-street lefts are re-routed via a U-turn lane in the major-street median The third configuration that the new methodology addresses is shown in this slide. It is the... [read].

Calculation Framework Ten steps Determine O-D and movement demands Determine lane groups Determine lane utilization Signal progression adjustments Additional control-based adjustments Estimate junction-specific perf. meas. Calculate extra distance travel time Applies to RCUT and MUT (not DLT) Estimate weaving delay Applies to RCUT (not MUT or DLT) Calculate experienced travel time Calculate LOS Perform for each individual junction The methodology for evaluating an alternative intersection consists of ten calculation steps. They are completed in sequence (from top to bottom) for a complete evaluation. They are... [read] Steps 2 to 6 are focused on the evaluation of each junction within the alternative intersection. A “junction” in this sense is any one point of intersection between two or more conflicting movements where at least one of the movements is stop, yield, or signal controlled. The signalized intersection methodology in Chapter 19 or the two-way stop-controlled intersection methodology in Chapter 20 is used to evaluate each junction, depending on the junction’s control mode. Once the delays are computed for each movement at each junction, Steps 7 through 9 are completed once for each movement entering the intersection. During these steps, the delays from Step 6 are combined with the extra distance travel time and weaving delay to determine the total experience travel time through the intersection. I should note that Steps 7 and 8 only apply to some configurations, as indicated in the subbullets. Perform for each individual link

New Input Data Same data as needed to evaluate each of the junctions that comprise the alternative int. Chapter 19, Signalized Intersections Chapter 20, Two-way Stop-Controlled Intersections Additional input data... Volume of U-turns on red at a signalized crossover Median width at a signalized U-turn crossover Distance from the U-turn crossover to main junction Free-flow speed along major street The alternative intersections methodology uses the signalized intersections methodology and the two-way stop-controlled intersection methodology. As a result, the evaluation of an alternative intersection requires the same input data as needed for the Chapter 19 and Chapter 20 methodologies. In addition to that data, the alternative intersections methodology needs data that describe the U-turn movements and the extra travel distance. Specifically, these data include...[read]

Questions on Methodology Basics?
Calculation framework New input data Questions on Methodology Basics? This concludes the session on Methodology Basics. Let’s take a few minutes to answer some questions.

Session 1 – Overview and Background Session 2 – Content and Structure Session 3 – Methodology Basics Session 4 – New Capabilities OK, let’s continue with Session 4 - New Capabilities

Session 4 New Capabilities LOS Framework Ramp Terminals
Diverging diamond interchange (DDI) evaluation Alternative Intersections RCUT and MUT computational steps Displaced left turn (DLT) computational steps For this session, we will briefly look at the procedures that were added, or underwent major revision, for the HCM 6th Edition. Notably, there was a change to the measure used to determine level of service. The ramp terminal methodology was modified to include procedures for evaluating the DDI. Finally, the alternative intersections methodology was added. We will take a look at all of these changes in the next few slides.

LOS Framework New Capabilities LOS Framework
Service measure is extra travel time (ETT) ETT = control delay (d) + extra distance travel time (EDTT) LOS thresholds for signalized interchanges No change LOS thresholds for alternative intersections Same values as for signalized intersections A: ≤10 s/veh; B: >10-20; C: >20-35; D: >35-55; E: >55-80 LOS thresholds for interchanges with roundabouts For the HCM 6th Edition, the service measure has changed from control delay to extra travel time, or “ETT.” Extra travel time is the sum of control delay and extra distance travel time. This change is intended to better represent the service provided motorists traveling through an interchange or alternative intersection. There were no changes to the LOS threshold values for signalized interchanges, even though the service measure changed (from control delay to ETT). For example, for the HCM 2010, the threshold control delay for LOS A is 15 s/veh. For the HCM 6th Edition, the threshold ETT for LOS A remains at 15 s/veh. The take-away here is that the service measure changed but the numbers defining the LOS thresholds did not change. The LOS thresholds for the new alternative intersections methodology are the same as those provided in Chapter 19 for signalized intersections. But, again, the service measure is ETT (not control delay, as it is in Chapter 19). The LOS thresholds for interchanges with roundabouts did not change for the HCM 6th Edition. However, again, the service measure is now ETT (not control delay).

Diverging Diamond Interchange
Ramp Terminals Diverging Diamond Interchange Saturation flow adjustment factor fDDI = 0.913 Lane utilization Additional lost time Yield-controlled movements The ramp terminal methodology was modified to include procedures for evaluating the DDI. One of the modifications was to include a new saturation flow rate adjustment for DDI thru movements. This factor is less than 1.0 and indicates that the saturation flow rate through a DDI is lower than at other interchanges. The modified procedures for lane utilization, additional lost time, and yield-controlled movements are summarized in the next few slides.

Ramp Terminals Diverging Diamond Interchange Lane utilization Significant differences in lane volume Equation provided to predict lane volume Five lane configurations addressed Similar to other interchange types, DDIs exhibit differences in lane volume on the external crossroad approaches (indicated here using red boxes). Equations are provided for predicting the lane volume distribution. These equations address five typical DDI lane configurations. They are: 2-lane shared, 3-lane shared, 3-lane exclusive, 3-lane exclusive with middle lane shared, and 4-lane exclusive.

Ramp Terminals Diverging Diamond Interchange Additional lost time due to... Presence of a downstream internal queue Back of downstream queue does not start moving until several seconds after start of green If internal storage is filled and the upstream phase starts before back of downstream queue is moving, then additional lost time is incurred by upstream phase The HCM 6th Edition provides a procedure to quantify the lost time cause by the following conditions: Presence of a downstream queue on the segment between the two ramp terminals. Additional clearance time needed for the off-ramp movements. The lost time associated with the first condition is described in this slide. The lost time for the second condition is described in the next few slides. So, the additional lost time due to the presence of a downstream queue is a lost time incurred by an “external” movement, which is a movement entering the segment between the two ramp terminals. The procedure recognizes that the last queued vehicle does not start moving until several seconds after the start of green. If the internal segment is full of queue vehicles and the upstream signal phase starts before the back of a downstream queue is moving, then the upstream queue cannot move. Even though the signal is green, drivers must wait until the downstream queue starts moving before they can move. This wait is modeled as “additional lost time” in the ramp terminal methodology.

Ramp Terminals Diverging Diamond Interchange Additional lost time due to... Clearance lost time for each ramp phase Long clear time needed after through phase before ramp can discharge Modeled as lost time since overlap cannot be modeled using Chapter 19 method Phase 6: WB thru green The lost time associated with the additional clearance time needed for the off-ramp movements is described in the next few slides. The lost time is caused by the long clearance time needed for the off ramp movement after the through phase ends. It is described in the next few slides for the off-ramp right-turn, but, the concept also applies to the off-ramp left-turn. In practice, overlap phasing is used to separate the off-ramp and through movements, and also limit the added lost time to the ramp movements. However, the signalized intersections methodology in Chapter 19 does not specifically describe how this type of overlap should be evaluated. So, when the Chapter 19 method is used, the overlap is ignored and an additional lost time is used to replicate the overlap’s effect on operation. This slide illustrates one point in the signal cycle, where the westbound through movement is receiving green and the other two movements are shown a red indication. [click to next slide]

Ramp Terminals Diverging Diamond Interchange Additional lost time due to... Clearance lost time for each ramp phase Phase 7: WB thru clears ramp, EB thru green Long clear time needed after through phase before ramp can discharge safely Modeled in method as lost time The graphic here shows the signal indications during a later point in time. If fact, this slide shows the signal indication just after westbound through movement first receives a red indication. The eastbound through green starts at this moment, but the southbound ramp movement continues to face a red indication because the westbound traffic (shown by dashed green line) has not passed by the ramp entry point. The indications shown in this graphic stay in this state for several seconds, until the westbound vehicles have all passed the ramp entry point. The duration of this state equals the additional lost time for the southbound ramp movement.

Ramp Terminals Diverging Diamond Interchange Additional lost time due to... Clearance lost time for each ramp phase Long clear time needed after through phase before ramp can discharge safely Modeled in method as lost time Phase 8: EB thru & ramp green After the westbound through vehicles clear the ramp entry point, the southbound ramp movement receives a green indication. The additional lost time ends at this point in time.

Ramp Terminals Diverging Diamond Interchange Yield-controlled movements Ramp left-turn or right-turn can be yield Capacity dictated by three flow regimes If the off-ramp movements operate with yield control (instead of signal control), then the eastbound and westbound movements are evaluated as a two-phase signal. The off-ramp movements are evaluated as a yield-controlled junction. However, the conflicting traffic flows are not random. In fact, they proceed past the ramp entry lanes in pulses of flow that are created by the signal at the upstream ramp terminal. The HCM 6th Edition breaks the flow pulses down in to three flow regimes. These flow regimes dictate the capacity of the off-ramp movements.

Ramp Terminals Diverging Diamond Interchange Yield-controlled movements Capacity dictated by three flow regimes Regime 1 – turn blocked by major-street platoon Regime 2 – turn into gaps after platoon passes by Regime 3 – turn freely because upstream signal is restricting all conflicting flows Equations provided for estimating capacity of each regime Overall turn capacity is computed as probability-weighted average of three regime capacities The ramp terminal methodology provides a procedure for estimating the capacity of yield controlled movements. This procedure identifies three flow regimes.. [read]. Equations are provided for estimating the capacity of each regime. An additional equation is provided for combining the three capacity components into one representative capacity for the analysis period.

Diverging diamond interchange (DDI) evaluation Alternative Intersections RCUT and MUT computational steps Restricted crossing U-turn (RCUT) evaluation Median U-turn (MUT) evaluation Displaced left turn (DLT) computational steps OK, we are still working our way through Session 4. We have completed the discussion about LOS framework and the ramp terminal methodology. We are now going to look more closely at the new alternative intersections methodology. This methodology is actually presented in two sections in Part C of the HCM 6th Edition. The first section in Part C describes the computational steps for the RCUT and MUT. The second section describes the computational steps for the DTL. We will take a look at these changes in the next few slides.

Restricted Crossing U-Turn Four-legged RCUT with signals Up to this point, we have shown schematics of the RCUT geometry. This slide provides a little more detail of the RCUT geometry. The major road lies is horizontal in the slide, such that major-road traffic travels in an east-west direction. Cross street traffic travel in a north-south direction. All cross street traffic must turn right at the intersection, as indicated by the black turn arrow at the bottom center of the slide. If the cross street drivers desire to make a left-turn or go through the intersection, they must make a U-turn at the median opening. The through driver then makes a right-turn from the major street to complete the “through” maneuver. This particular RCUT has signal control at four locations. These locations are indicated by a red circle. The two signals in the middle of the slide operate independently of one another.

Restricted Crossing U-Turn Four-legged RCUT w/merges and diverges This slide shows an unsignalized RCUT. The left-turns from the major street is yield-controlled. The U-turns on the major street are provided speed-change lanes from which they can merge and diverge from the major-street vehicles.

Median U-Turn Four-legged MUT with signals Can also have stop signs The MUT geometry is shown in this slide. This MUT is shown to have three signalized locations. However, a MUT can also be controlled using stop signs. Unlike the RCUT, the MUT will allow the cross street through movement to proceed straight through the intersection. However, the cross street left-turn movement includes a U-turn maneuver, which is the same at both the MUT and RCUT.

Restricted Crossing U-Turn, Median U-Turn Weaving delay Delay during merge can be reasonably estimated using Freeway Weaving methodology U-turn saturation flow rate adjustment factors Narrow median (< 35’): 0.80 Typical median (35-80’): 0.85 Wide median (>80’): 0.95 U-turn gap acceptance parameters Critical headway: 4.4 s Follow-up headway: 2.6 s As I mentioned previously, the alternative intersections methodology uses the methodology in the signals chapter and the methodology in the two-way stop-controlled chapter to evaluate the junctions that comprise an alternative intersection. We will not be discussing these two chapters in this presentation. We are focusing on the highlights of some of the key elements of the alternative intersections methodology that go beyond the signals and two-way stop methodologies. One element is the use of the Freeway Weaving methodology in Chapter 14. This methodology is used to estimate the additional weaving delay. The subject of this analysis is the median U-turn movement that merges with the major-street through traffic stream. If the U-turn is controlled by a signal, then there is an adjustment factor provided for estimating the saturation flow rate of the U-turn movement. The adjustment factor values are shown here for three different median width ranges. If the U-turn is controlled by a stop sign, then its capacity is dictated by the availability of acceptable gaps in the conflicting traffic stream. The methodology advises using the capacity prediction models in the Two-Way Stop-Controlled Intersections chapter. However, the critical headway and follow-up headway parameters for U-turns at RCUTs and MUTs are notably smaller than those provided in the Two-Way Stop-Controlled Intersections chapter.

Restricted Crossing U-Turn, Median U-Turn Extra distance travel time (EDTT) with Dt = distance from main junction to U-turn crossover Df = distance from U-turn crossover to main junction FFS = major-street free-flow speed a = delay associated with decel. or accel. in a turn that is completed via a merge maneuver a equals 0.0 for turn controlled by stop sign or signal As noted in a previous slide, the service measure for alternative intersections is experienced travel time, which is the sum of the control delay and extra distance travel time (EDTT). The EDTT is computed for the U-turn movements at the RCUT and MUT. The equation for computing this travel time is shown in this slide. The EDTT is a function of the distance traveled to and from the U-turn location, as well as the free-flow speed and acceleration delay.

Diverging diamond interchange (DDI) evaluation Alternative Intersections RCUT and MUT computational steps Restricted crossing U-turn (RCUT) evaluation Median U-turn (MUT) evaluation Displaced left turn (DLT) computational steps OK, we are still working our way through Session 4. We have one last topic to discuss. It is the computational steps for the DTL. Let’s get started...

Displaced Left Turn Configurations Partial DLT Full DLT Full DTL Partial DTL There are two general configurations of the displaced left-turn intersection. They are the partial DTL and the full DTL. A schematic for the full DTL is shown on the left side of this slide. It includes displaced left-turns for each of the four intersection approaches. The partial DTL is shown on the right side of the slide. The displaced left-turn are provided only for the northbound and southbound approaches.

Partial Displaced Left Turn Three signalized intersections Analyzed using.. Urban Street Segments method, and Signalized Intersections method Supplemental Intersection 1 Supplemental Intersection 2 Partial DLTs have one main and two supplemental signalized intersections. The alternative intersections methodology indicates that the intersections are to be evaluated using the signalized intersections methodology in Chapter 19. It also indicates the use of the urban street segments methodology in Chapter 19. Main Intersection

Partial Displaced Left Turn Guidance Signals are timed properly such that displaced left-turns have zero delay when crossing cross street Saturation flow rate adjustments Left-turns at supplemental intersections Recommend using right-turn saturation flow rate adjustment factor when computing left-turn sat. flow rate Offset adjustment Guidance provided to compute signal offset needed to allow displaced lefts to proceed across the cross street without stopping This slide summarizes some of the key elements of the computational steps for the partial DTL. For example, the guidance provided for the DTL evaluation indicates that the displaced left-turns can be considered to have zero delay at the point there they turn left across the cross street. This point is identified using the red oval in the schematic. At the two supplemental intersections, the left-turn radius is typically fairly sharp. As a result, the methodology indicates that the analyst should use the right-turn saturation flow rate adjustment factor for the left-turn movements. This approach is taken because the right-turn factor is appropriate for turns along a sharp radius. There is also guidance provided for computing the signal offset for the partial DTL. The offset produced by this guidance will allow the displaced left-turn to turn across the cross street without stopping. (This point ties back to the first bullet).

Full Displaced Left Turn Five signalized intersections Analysis approach Analyze as two DTLs DTL on N-S street DTL on E-W street Aggregate the results This slide summarizes the guidance provided for evaluating the full DTL, which has five signalized intersections. One main intersection and four supplemental intersections. Specifically, the guidance in the alternative intersections methodology is to evaluate each street as having a partial DTL. Then, the results from both streets would be aggregated to obtain results describing the overall operation of the DTL.

Full Displaced Left Turn Recall For partial DTL, displaced lefts not analyzed where they crossed the cross street Pseudo right-turn movement technique Displaced lefts analyzed as right-turns at main int. Technique used to ensure that left-turn volume departing main intersection arrives at downstream supplemental intersection Ensures flow balance at each end of link Delays to pseudo right-turns are not included in the approach or intersection delay There is one nuance to the evaluation of a full DTL. Recall that, for the partial DTL, the displaced left-turns are not analyzed at the point where they cross the cross street. It is assumed that the signal offset will be such that the displaced left-turns will experience zero delay at this point. However, unlike our evaluation of the partial DTL, we do need to include the left-turn volumes at the main intersection so that the volume on the cross street is correct. If the displaced left-turn volumes are ignored, then the volume arriving at the downstream supplemental intersection will be incorrect. The guidance recommends including the displaced left-turn volume as a “pseudo” right-turn movement at the main intersection. This approach will ensure a balance of flow between the main intersection and the downstream supplemental intersection. The next slide will illustrate this concept of “pseudo” right turns. Before we go to this slide, I need to note that the delay to the pseudo right-turns should be excluded from the results because it is not actually incurred by the displaced left-turn vehicles.

Full Displaced Left Turn Pseudo right-turn movements Example: NB lefts converted to SB rights Main intersection movements as they exist Main intersection movements as evaluated This slide illustrates the guidance regarding the use of pseudo right-turns. The graphic on the left side of this slide shows a plan view of the main intersection with a displaced left-turn movement. The graphic on the right side shows the main intersection as it is actually evaluated using the signalized intersections methodology. Note that the displaced left-turn movement is modeled at the intersection as a right-turn movement from an exclusive lane. Displaced left turns Left turns modelled as pseudo right turns

Software Availability
HCS7 – Streets (McTrans) Combines... Signalized intersections Urban street segments Urban street facilities Ramp terminals and alternative intersections Automatic... Supported users can download upgrade There are several software developers that have (or will) implement the new methodology in their software products. One of them, McTrans, is developing a software tool called “Streets”. It will ...[read]

Questions on New Capabilities?
LOS Framework Ramp Terminals Diverging diamond interchange (DDI) evaluation Alternative Intersections RCUT and MUT computational steps Displaced left turn (DLT) computational steps Questions on New Capabilities? This concludes the session on New Capabilities. Let’s take a few minutes to answer some questions.

Forthcoming Briefings
Closure Forthcoming Briefings Just a reminder of the remaining briefings. There are ___ left. The topics are: [read]

HCM 6th Edition: Ramp Terminals and Alternative Intersections

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HCM 6th Edition: Ramp Terminals and Alternative Intersections

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