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Page B-1 APPENDIX B COMPUTATIONAL ENGINE The computational engine described in this appendix implement s 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 exc lusive right - turn pocket (no ATL), ⢠Add an ATL with shared right turns, or ⢠Add an exclusive ATL and an exclusive right - turn pocket. 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 1 â 5 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 ( Y es or N o) , the effective green time for Exhibit B-1 Design Options for One-CTL Case
Page B-2 the through movement, and the cycle length for the intersection . Users can input information for up to two scenarios to perform a comparative analysis. Approach characteristics are entered in Lines 6 â 16. The se 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 moveme nts 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 reacti on time, and a confidence level for calculating the downstream ATL length. Note that items 12 â 16 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 computationa l engine are summarized at the l ane - by - lane and a pproach levels, as shown in Exhibit B-3. I. GEOMETRIC CONFIGURATION AND SIGNAL TIMING Scenario 1 Scenario 2 1 1 1 2 N Y 3 N N 4 25 25 5 110 110 II. APPROACH CHARACTERISTICS User Input (Applies to Both Scenarios) 6 425 7 1800 8 75 9 1550 10 35 11 25 12 10 13 110 14 6 15 1 16 Confidence level for calculation of downstream length = 0.85 Default value = 1.0 seconds Express as decimal between .85 and .95 Default value = 25 ft Default value = 10.0 ft/sec/sec Default value = 6.0 seconds Number of CTLs (1 OR 2) ATL (Y/N)? Exclusive right-turn lane (Y/N)? Total approach through volume (vph) = Comment Total saturation flow rate for CTL(s) (vph) = Right-turn volume (vph) = Right-turn lane saturation flow rate (vph) = Prevailing approach speed (mph) = Effective green time for through movement(s) (sec) = Driver reaction time (sec) = Add sat flow across both lanes for 2 CTL approaches Speed at which vehicles approach intersection during green phase Cycle length (sec) = Average vehicle spacing at stop bar (ft) = Average acceleration rate from stop bar (ft/sec/sec) = Intersection width measured from stop bar to far curb (ft) = Critical gap in adjacent CTL (sec) = TH Vol RT Vol TH + RT Vol XALL Avg. Delay LOS 95th % Queue (ft) Lane Configuration (vph) (vph) (vph) (sec/veh) 1 SHARED CTL 425 75 500 1.25 174.2 F 1000 2 3 4 TOTAL 425 75 500 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 Avg. Delay LOS ATL Utilization (ATL TH/Total TH) Upstream ATL Downstream ATL (sec/veh) (ft) (ft) 174.22 F N/A NA NA 46.33 D 32% 400 230 III. LANE-BY-LANE RESULTS Scenario 2 Scenario 1 Scenario 2 Estimated Min. ATL Length Scenario 1 Exhibit B-2 Input Dialog Box Exhibit B-3 Summary of Results Layout
Page B-3 The computational engine provides lane - by - lane analysis for two user - defined geometric scenarios . Measures include the lane volume allocation, the degree of saturation (X ALL ) , average control delay, LOS, and 95th p ercentile 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 g uidelines. 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 sec onds per veh icle (LOS F) to 46 sec onds per veh icle (LOS D). The right - hand side of the a pproach r esults table shows the estimated design length for the upstream and downstream ATL using the procedure s described in Chapter 5 . The upstream length needed to prov ide queue storage for unimpeded access to the ATL is estimated at 4 00 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.
