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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
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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
