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APPENDIX B COMPUTATIONAL ENGINE The computational engine described in this appendix implements the procedures described in Chapter 3 in a Microsoft® Excel spreadsheet environment for both one-CTL and two-CTL approaches. The base geometric configuration for one-CTL approaches is a shared CTL and an exclusive left-turn lane, as illustrated in the top portion of Exhibit B-1. Three possible design scenarios can be evaluated in the computational engine as indicated in the exhibit: · Add an exclusive right-turn pocket (no ATL), · Add an ATL with shared right turns, or · Add an exclusive ATL and an exclusive right-turn pocket. Exhibit B-1 Design Options for One-CTL Case A similar set of design options is provided in the same spreadsheet for a two- CTL configuration. The Input Dialog Box The input dialog box for a one-CTL case is depicted in Exhibit B-2. Users enter information for the geometric configuration and signal timing in Lines 15 consisting of the number of CTLs (1 or 2), whether an ATL is present (Yes or No), if an exclusive right-turn lane is present (Yes or No), the effective green time for Page B-1
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the through movement, and the cycle length for the intersection. Users can input information for up to two scenarios to perform a comparative analysis. Exhibit B-2 I. GEOMETRIC CONFIGURATION AND SIGNAL TIMING Input Dialog Box Scenario 1 Scenario 2 1 Number of CTLs (1 OR 2) 1 1 2 ATL (Y/N)? N Y 3 Exclusive right-turn lane (Y/N)? N N 4 Effective green time for through movement(s) (sec) = 25 25 5 Cycle length (sec) = 110 110 II. APPROACH CHARACTERISTICS User Input (Applies to Both Scenarios) Comment 6 Total approach through volume (vph) = 425 7 Total saturation flow rate for CTL(s) (vph) = 1800 Add sat flow across both lanes for 2 CTL approaches 8 Right-turn volume (vph) = 75 9 Right-turn lane saturation flow rate (vph) = 1550 10 Prevailing approach speed (mph) = 35 Speed at which vehicles approach intersection during green phase 11 Average vehicle spacing at stop bar (ft) = 25 Default value = 25 ft 12 Average acceleration rate from stop bar (ft/sec/sec) = 10 Default value = 10.0 ft/sec/sec 13 Intersection width measured from stop bar to far curb (ft) = 110 14 Critical gap in adjacent CTL (sec) = 6 Default value = 6.0 seconds 15 Driver reaction time (sec) = 1 Default value = 1.0 seconds 16 Confidence level for calculation of downstream length = 0.85 Express as decimal between .85 and .95 Approach characteristics are entered in Lines 616. These values apply to both scenarios 1 and 2. The input requirements include volumes and adjusted saturation flow rates for the through and right-turn movements (note that all procedures assume that through movements are free of any impedance caused by left-turn movements if left-turn movements are present). Additional input information consists of prevailing approach speed, average vehicle spacing at stop bar, acceleration rate from stop bar, intersection width, critical gap for merging from ATL into CTL, driver reaction time, and a confidence level for calculating the downstream ATL length. Note that items 1216 are primarily related to determining the downstream ATL length and can be defaulted to the values shown on the right-hand column. Summary Results Tabulation Results from the computational engine are summarized at the lane-by-lane and approach levels, as shown in Exhibit B-3. III. LANE-BY-LANE RESULTS Exhibit B-3 Summary of Results Layout TH Vol RT Vol TH + RT Vol XALL Avg. Delay LOS 95th % Queue Lane Configuration (vph) (vph) (vph) (sec/veh) (ft) Scenario 1 1 SHARED CTL 425 75 500 1.25 174.2 F 1000 2 3 4 TOTAL 425 75 500 Scenario 2 1 CTL 287 0 287 0.70 48.7 D 400 2 SHARED ATL 138 75 213 0.55 43.1 D 300 3 4 TOTAL 425 75 500 IV. APPROACH RESULTS Estimated Min. ATL Length Avg. Delay LOS ATL Utilization Upstream ATL Downstream ATL (sec/veh) (ATL TH/Total TH) (ft) (ft) Scenario 1 174.22 F N/A NA NA Scenario 2 46.33 D 32% 400 230 Page B-2
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The computational engine provides lane-by-lane analysis for two user- defined geometric scenarios. Measures include the lane volume allocation, the degree of saturation (XALL), average control delay, LOS, and 95th percentile queue by lane (except for the two exclusive CTLs, which are treated as a single lane group as per the HCM 2010). The computational engine also summarizes average control delay, LOS, ATL utilization, and the estimated minimum length for the upstream and downstream ATL at the approach level based on the procedures described in the guidelines. In the case shown in Exhibit B-3, a one-CTL approach was analyzed without an ATL (Scenario 1) and with an ATL (Scenario 2). In this example, a total of 138 vehicles per hour out of 425 vehicles per hour for the through movement are expected to use the ATL, which results in an ATL utilization of 32 percent. This reduces the approach control delay for the through and right-turning movements from 174 seconds per vehicle (LOS F) to 46 seconds per vehicle (LOS D). The right-hand side of the approach results table shows the estimated design length for the upstream and downstream ATL using the procedures described in Chapter 5. The upstream length needed to provide queue storage for unimpeded access to the ATL is estimated at 400 feet, based on the ATL 95th percentile queue. The minimum downstream length for this example is estimated to be 230 feet. These values do not include taper lengths. Page B-3