Guidelines on the Use of Auxiliary Through Lanes at Signalized Intersections (2011) / Chapter Skim
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From page 54... ...
Page 55 6. SAMPLE APPLICATION The purpose of this chapter is to demonstrate the application of the g uidelines through a practical example .
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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
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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 .
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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
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From page 58... ...
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)
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From page 59... ...
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.
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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 .
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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.
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From page 62... ...
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.
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From page 63... ...
• 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.
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From page 64... ...
Page 65 Exhibit 6-8 Comparison of the Base Case to the Preferred Alternative Design
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