Guidelines on the Use of Auxiliary Through Lanes at Signalized Intersections (2011)

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Page 55 6. SAMPLE APPLICATION The purpose of this chapter is to demonstrate the application of the g uidelines through a practical example . Several principles discussed previously in these guidelines are critical for the practitioner to consider throug hout the ATL evaluation and design process: • T he ATL evaluation and design approach needs to account for the project’s contextual environment as well as for applicable local, state, and federal policies, standards, and guidelines. • An ATL has different operational characteristics from a CTL and should always be treated as a separate lane group, regardless of whether it is a shared lane or an exclusive lane. • T here is an iterative and dynamic relationship among geometric design choices, traffic operations performance, and the expected safety of an ATL. This application example guides practitioners through the steps involved with conducting an operational evaluation of the addition of an ATL on a signalized intersection approach. Volume - to - capaci ty ratios, average delays, levels of service, and 95 th percentile queue lengths under each alt ernative are computed on a lane - group basis according to standard Highway Capacity Manual 2010 ( 2 ) procedures for a signalized approach. This sample application makes use of the computational engine that is described in Appendix B. Exhibit 6-1 illustrates the evaluation process.

Sample ATL Functional Design Plan NOTES • • No additional data required beyond traditional intersection analysis Applicable to approaches with one or two continuous through lanes and an exclusive or shared right-turn lane Assess Multimodal Needs • Identify facility needs for pedestrians, bicyclists, and transit riders Evaluate Traffic Operations • • HCM analysis using statistical model to predict ATL Microsimulation Assess Safety Effects • • Qualitative evaluation Conflict prediction Calculate Design Elements • • • • Upstream Passive Taper Upstream ATL Length Downstream ATL Length Downstream Active Taper Lay Out Individual Segments • • • • Approaching ATL Approaching Signal Departing Intersection Merge at End of ATL Page 56 Exhibit 6-1 Evaluation Process

Page 57 ATL APPLICATION EXAMPLE DESCRIPTION The study intersection for this example is located at the crossroads of a principal arterial and a minor arterial in a suburban setting. Exhibit 6 - 2 identifies the key characteristics of the two roadways. The local highway agency has received a significant number of complaints from citizens about back - ups on the eastbound approach at the intersection and future volume forecasts show additional traffic growth on the approach will only continue to degrade its operational performance. Options for adjusting the signal timing are very limited due to the high volume of traffic on the principal arterial and the intersection ’s location on a coordinated arterial . The local highway agency is evaluating options for relieving the congestion on the eastbound approach. The minimum acceptable level of service for the design year is LOS E with a requirement that the volum e - to - capacity ratio for all lane groups be less than 1.0 for the peak - hour period . An arterial capacity analysis shows there is sufficient downstream capacity on the minor arterial to accommodate up to a 50 percent increase in through volume. However, bec ause of right - of - way and funding constraints, the local highway agency has ruled out certain capacity - enhancing solutions such as adding a second CTL, converting to an alternative intersection configuration, or constructing dual left - turn lanes on the appr oach. The volume - based data shown throughout this example represent the forecast demand on the eastbound approach , and not necessarily the volume measured through the intersection. This distinction is important as the final alternative selected should pro vide enough capacity to accommodate the full demand on the approach and not just the volume currently able to pass through the intersection. The term “volume - to - capacity ratio” will continue to be used, as it is the industry standard term for the performan ce measure of the volume or demand to the capacity of a movement, approach, or intersection. The local highway agency is evaluating several improvement alternatives for the minor arterial eastbound approach: • Alternative 0: Base case (do nothing) • Alternative 1: Add an exclusive right - turn lane • Alternative 2: Add a shared ATL • Alternative 3: Add an ATL and an exclusive right - turn lane The practitioner must be aware that these improvement alternatives are not the only alternatives that could be applie d to address the congestion issue; Characteristics Major-Street Roadway Minor-Street Roadway Classification Principal Arterial Minor Arterial Avg. Annual Daily Traffic 25,000 15,000 Posted Speed 45 mph 35 mph Lane Configuration 1 shared through/right lane 1 through-only lane 1 shared through/right lane 1 left-turn lane 1 left-turn lane Exhibit 6-2 Sample Application Roadway Characteristics

Page 58 however, for the purposes of this example, these are the agency’s desired alternatives. OPERATIONAL EVALUATION Input Data Exhibit 6 - 3 illustrates the design - year turning - movement volumes at the example intersection. Exhibit 6 - 4 provides the specific input parameters to use in the operational evaluation of the approach alternatives. Input Parameter Description VTH = 425 vph Total approach through demand STH = 1,800 vph Through-movement saturation flow rate VRT = 75 vph Total approach right-turn volume SRT = 1,550 vph Right-turn movement saturation flow rate VA = 35 mph Approach speed IW = 110 ft Intersection width, from stop bar to far curb GE = 25 sec Approach effective green time C = 110 sec Intersection cycle length LVEH = 20 ft Average length between vehicles under stop condition AVEH = 10 ft/sec2 Average vehicle acceleration rate from stop condition TC = 6 sec Critical gap in neighboring CTL traffic lane RT = 1 sec Driver reaction time Exhibit 6-3 Turning-Movement Volumes Exhibit 6-4 Input Parameters

Page 59 The remaining sections describe the analysis and design s t e p s for the sample application. Inser t Input Data into the Computational Engine Exhibit 6 - 5 illustrates the input data for this example as entered in the one- CTL computational engine. It is important to understand when using the computational engine that input parameters should remain consistent through the evaluation of all alternatives unless site -specific data are collected to demonstrate otherwise. In this example, all of the input pa rameters remain consistent across all four treatment options. Evaluate Alternative ATL Configurations This step involves toggling the input values in Line 2 and Line 3 to match the desired ATL configuration. To analyze an ATL with an exclusive right - turn lane, the input parameter in Line 2 is set to “Y”, otherwise it is set to “N”. If the ATL is exclusive and a right - turn lane is present, Line 3 is set to “Y”, otherwise it is set to “N”. The base case condition of a single CTL with shared right - turn movements is analyzed automatically. Exhibit 6 - 6 displays the results from the evaluation of the four alternatives for the eastbound approach as reported from the computational engine. Exhibit 6-5 Computational Engine Input Screen 1-CTL ONLY COMPUTATIONS OF ATL LENGTHS (UPSTREAM AND DOWNSTREAM)-- FOR VARIOUS LANE CHOICES INPUT DATA HERE - CASE I IS THE BASELINE (SINGLE SHARED THRU+RIGHT LANE) ALL CELLS EXCEPT INPUT CELLS ARE PROTECTED 1 ENTER THE CASE STUDY ID OR TITLE IN YELLOW BOX HYPOTHETICAL CASE STUDY 2 ENHANCEMENT: EXCLUSIVE RIGHT TURN LANE (Y/N)? N Please enter data in CAPS for first two entries 3 ENHANCEMENT: ADDITIONAL EXCLUSIVE ATL (Y/N) ? N This entry cannot be "Y" if previous enry is "N" 4 TOTAL APPROACH THROUGH VOLUME= 425 VPH 5 RIGHT TURN VOLUME= 75 VPH 6 APPROACH SPEED (MPH)= 35 MPH 7 THRU SATFLOW PER LANE= 1800 VPH 8 RIGHT SATFLOW PER LANE= 1550 VPH 9 APPROACH EFFECTIVE GREEN= 25 SEC 10 INTERSECTION CYCLE LENGTH= 110 SEC 11 APPROACH EFFECTIVE GREEN WITH ATL / OTHER ADDS= 25 SEC DEFAULT 12 AVERAGE VEHICLE SPACING AT STOP= 20 FT 20 13 AVERAGE ACCELERATION RATE FROM STOP = 10 FT/SEC/SEC 10 14 INTERSECTION WIDTH (STOPLINE TO FAR CURB)= 110 FT 40 15 CRITICAL GAP IN NEIGHBORING CTL TRAFFIC LANE= 6 SEC 6 16 DRIVER REACTION TIME= 1 SEC 1 BASED ON THE INPUTS- THIS IS ANALYSIS CASE I I I 1-CTL+SHARED ATL

Page 60 Condition Lane V TH V RT V Tot v/c Delay (sec) LOS 95 th % Queue (ft) Base Case CTL+RT 425 75 500 1.25 174 F 800 Add Right- Turn (RT) Lane RT 0 75 75 0.21 36 D 100 CTL 425 0 425 1.04 97 F 500 Add Shared ATL ATL 138 75 213 0.55 43 D 200 CTL 287 0 287 0.70 49 D 300 Add ATL & RT Lane RT 0 75 75 0.21 36 D 100 ATL 138 0 138 0.34 38 D 100 CTL 287 0 287 0.70 49 D 300 Results from the analysis indicate the following: • Under the base - case alternative , the eastbound approach has a volume - to - capacity ratio of 1.25 and operates at LOS F . These indicators show that the approach has more demand than the existing lane configuration and signal timing scheme can discharge through the intersection u nder forecast traffic conditions and that vehicles will experience high delays . This finding reinforces the need for a capacity improvement. • With the addition of an exclusive right - turn lane , t he CTL continues to operate at LOS F and slightly above capacit y at a volume - to - capacity ratio of 1.04. Removing the right - turn demand from the CTL is not a sufficient improvement for this approach as the through demand is too high to be served by a single CTL. • With the addition of the shared ATL, 138 out of 425 throu gh vehicles (32 percent) are forecast to use the ATL. The result is that both the ATL and CTL operate below capacity and at LOS D. The 95 th percentile back of queue is estimated to be 300 feet for the CTL. • With the addition of an exclusive right - turn lane and an ATL , all of the right - turn volume shifts to the exclusive right - turn lane resulting in further improved operation of the ATL over the previous alternative. G iven that the total through traffic in the CTL remains the same the performance of the CTL is identical to the s hared ATL alternative. Evaluate Anticipated Safety Effects In addition to evaluating intersection operations, the potential safety impacts of in stalling an ATL will be investigated. As stated in Chapter 4, ATLs add lane - changing activity and this may lead to an increase of sideswipe crashes, especially in the downstream merge area. On the other hand, the forecast increase in through - movement capac ity demonstrated in the above operational analysis may help prevent some rear - end and other congestion - related crashes on the ATL approach, especially since the approach is forecast to operate in a congested condition without the ATL . The following list pr esents an assessment of safety considerations: • Access control . There are no driveways in this example. Exhibit 6-6 Traffic Operations Analysis Results for the Eastbound Approach

Page 61 • Sight lines . The subject approach has adequate sight lines in this example. There are no visual obstructions within the sight lines. • Queuing downstream o f the ATL merge . There are no downstream bottlenecks causing queue spillback to the ATL. Evaluate Multimodal Effects This step evaluates effects on non - auto users to identify addition al design needs for the ATL: • Pedestrians . Pedestrian volumes in this exam ple are low and the additional crossing time required to accommodate the ATL can be accommodated within existing signal timing. • Bicyclists . Bicycle lanes are not being accommodated in this example. • Transit Vehicles . No bus stops are included in the ATL in this example. Select a Preferred Alternative The selection of the alternative should consider multiple factors, including user considerations, operational performance, safety performance, cost, environmental impacts, time to implement, and public percept ion. There are multiple ways to evaluate alternatives including but not limited to criterion rating, benefit - cost analyses, and best - value that meets the desired operational standard. For the purposes of this example application, the best - value alternative will be implemented, meaning the alternative with the least negative impact that satisfies operational performance standards will be selected as the preferred alternative, provided that it does not significantly compromise safety or other modes of travel. In this example, it is assumed that all of the alternat ives are deemed feasible from user consideration, safety, and cost perspective s , and that the local highway agency is therefore focused on identifying the alternative with the lowest negative impact that meets its operational standard. The consideration of alternatives was conducted as follows: • Alternative 0 . Does not meet the local highway agency operational standard; therefore, this alternative is eliminated from further consideration. • Alternative 1 . Improves the operational performance of the eastbound approach, but still falls short of meeting the local highway agency operational performance standard; therefore, this alternative is also eliminated from further consideration. • Alternative 2 . Improves the operational performance of the eastbound approach to a satisfactory condition. • Alternative 3 . Improves the operational performance of the eastbound approa ch to a satisfactory condition, but requires additional lane widening to accommodate an exclusive right - turn lane in comparison to Alternative 2. Based on this evaluation, and the earlier statement that each alternative is adequate from the user needs, safety, and cost perspective, Alternative 2

Page 62 provides the best value as it requires the least amount of widening, while still meeting the local highway agency’s operational standards. Alternative 2, the addition of a shared through plus right - turn ATL, will be carried forward into the preliminary horizontal geometric design process. PRELIMINARY HORIZONTAL GEOMETRIC DESIGN The following section describes a step - by - step process to develop a preliminary horizontal design for the preferred alternative, the addition of a shared ATL. Refer to the exhibit at the end of this chapter for a detailed illustration of the preliminary horizontal design of the shared ATL in comparison to the existing eastbound approach configuration. The theory behind the design process is described in detail in Chapter 5. Design Input Data • Lane width: W = 11 feet; • Approach design speed: S = 35 mph; • Intersection width: I W = 110 feet. Step 1: Calculate the Length of the Design Elements T o gain an idea of the overall picture of the design needs for the eastbound approach and potential property, slope, drainage, and infrastructure impacts, t he first step is to calculate the length of each of the four sections of the ATL . Steps 1a - 1d identify the procedures for calculating the length of each of the ATL design elements : • Passive taper • Upstream ATL length • Downstream ATL length • Active taper Chapter 5 indicates that the minimum passive taper rate should be 10:1. Using an 11 - foot lane width and applying the passive taper rate of 10:1, the minimum length for the passive taper is 11x10 = 110 feet. The upstream ATL length should be sufficient to accommodate the maximum 95 th percentile maximum vehicle queue on the approach along with any additional distance that is desired for deceleration. As shown in Exhibit 6 - 6 , the maximum 95 th percentile back of queue is 300 feet in the C TL. Given the 95 th percentile back of queue in the ATL is expected to be 200 feet and prevailing speeds on the approach , a dista nce of 300 feet is deemed sufficient to accommodate safe deceleration (which is assumed to begin in the taper) for a vehicle that departs the CTL and reaches the back of queue in the ATL. Step 1a: Determine the Length of the Passive Taper Step 1b: Determine the Upstream ATL Length

Page 63 The minimum length for the section departing the intersection is computed using the computational engine and following the procedure described in Chapter 5. Exhibit 6-7 summarizes the geometric parameters for this design. The downstream ATL length calculations based on the two methodologies show the following results: DSL 1 = 220 feet DSL 2 = 250 feet The greater of the two distances should be used to determine the minimum downstream ATL length, which for this example is 250 feet. Given the posted speed (35 mph), lack of driveways, clear sight lines, and available right- of -way , the downstream ATL length of 275 feet is selected for this design. Condition Upstream ATL Length Based on Storage Q-Length (ft) Downstream ATL Length (ft) Downstream ATL Length Based on CTL-Gap Acceptance Distance (ft) Add Shared ATL 300 220 250 The minimum length for the active taper is calculated in a manner consistent with MUTCD ( 3 ) recommendations. In this example the width of the ATL is 11 feet and the speed of the minor arterial is 35 mph . T he downstream merge section length is calculated as: Step 2: Assess the Viability of Installing the ATL Once the design elements are defined, existing slopes, drainage areas, rights- of -way, utilities, and other infrastructure should be evaluated out to a width of approximately 15 to 25 feet from the existing curb or edge of pavement line to gain an idea of the effects that will occur due to the addition of the ATL. In this example, the cross section assumed for the ATL addition is an 11-foot wide ATL, with a 6-inch vertical curb, a 6-foot attached sidewalk, and a 4:1 cut/fill slope to tie into existing ground. In this example, there are no major conflicts within the cross-sectional area. Step 3: Design the Upstream Full-Width Lane Segment of the ATL This step involves identifying the signing and pavement markings for the upstream segment given the length of the passive taper and upstream ATL length described in Step 1. The following treatments are recommended: Place a 10 -foot skip stripe with 30-foot breaks along the entire length of the upstream full-width lane. Exhibit 6-7 Summary of Geometric Parameters • Step 1c: Determine the Downstream ATL Length Step 1d: Determine the Length of the Active Taper

Page 64 • At the upstream start of the full lane width of the ATL, place a lane configuration sign (side - mounted MUTCD R3 - 8 series sign) and add lane- use pavement markings to both the ATL and CTL lanes at this same point to provide lane - use confirmation for drivers. • Place overhead lane configuration signs for both the ATL and CTL on the signal mast arm for the approach to provide additional confirmation of lane use for drivers. Step 4: Design the Downstream Full-Width Lane Segment of the ATL Similar to Step 3, this step identifies the signing and pavement markings for the downstream ATL segment: • Place a 10 - foot skip stripe with 30 - foot breaks along the entire length of the downstream ful l - width lane. • Provide a side - mounted “Right Lane Ends” W9 - 1 sign approximately 50 feet minimum from the crosswalk (extension of opposite stop bar). • Place the “Lane Ends” W4 - 2 sign at a point halfway between the extension of the opposite stop bar and the e nd of the full - width lane segment, approximately 1 5 0 feet from the extension of the opposite stop bar. Note: Ensure a minimum distance of 100 feet between the W4-2 sign and the upstream “Right Lane Ends” W9 - 1 sign. Step 5: Design the Tie-ins at the Intersection Design the curb tie - ins on both the upstream and downstream sides of the interse ction to connect the ATL to the principal arterial cross street, as described in the following bullets: • Determine the appropriate design vehicle for the intersecti on based on local standards. In this example, the design vehicle is a WB - 50. • Place an appropriate radius for the curb return to connect the ATL to the cross street exit leg based on the WB - 50 envelope. In this example, a 50- foot curb return radius is used. • Place an appropriate radius for the curb return to connect the principal arterial approach to the ATL based on the WB - 50 envelope. In this example, a 50 - foot curb return radius is used. SUMMARY Exhibit 6 - 8 illustrates the result of the design process for the sample application of the one - CTL approach with a shared ATL. Remaining design steps for an ATL are similar to that of any full (continuous) - lane widening design process . They include developing plans for the vertical profile , drainage, and utility relocations (where necessary) and preparing final design plans, specifications, and a cost estimate for the ATL improvement.

Page 65 Exhibit 6-8 Comparison of the Base Case to the Preferred Alternative Design