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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 6-3 Turning-Movement Volumes Exhibit 6-4 provides the specific input parameters to use in the operational evaluation of the approach alternatives. Exhibit 6-4 Input Parameter Description Input Parameters 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 Page 58
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The remaining sections describe the analysis and design steps 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 parameters remain consistent across all four treatment options. 1-CTL ONLY Exhibit 6-5 COMPUTATIONS OF ATL LENGTHS (UPSTREAM AND DOWNSTREAM)-- FOR VARIOUS LANE CHOICES Computational Engine Input Screen 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 III 1-CTL+SHARED ATL 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. Page 59
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Exhibit 6-6 95 th % Traffic Operations Analysis Delay Queue Results for the Eastbound Condition Lane VTH VRT VTot v/c (sec) LOS (ft) Approach Base Case CTL+RT 425 75 500 1.25 174 F 800 Add Right- RT 0 75 75 0.21 36 D 100 Turn (RT) CTL 425 0 425 Lane 1.04 97 F 500 Add Shared ATL 138 75 213 0.55 43 D 200 ATL CTL 287 0 287 0.70 49 D 300 Add ATL & RT 0 75 75 0.21 36 D 100 RT Lane 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 under 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, the CTL continues to operate at LOS F and slightly above capacity 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 through 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 95th 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. Given that the total through traffic in the CTL remains the same the performance of the CTL is identical to the shared ATL alternative. Evaluate Anticipated Safety Effects In addition to evaluating intersection operations, the potential safety impacts of installing 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 capacity 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 presents an assessment of safety considerations: · Access control. There are no driveways in this example. Page 60
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· Sight lines. The subject approach has adequate sight lines in this example. There are no visual obstructions within the sight lines. · Queuing downstream of 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 additional design needs for the ATL: · Pedestrians. Pedestrian volumes in this example 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 perception. 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 alternatives are deemed feasible from user consideration, safety, and cost perspectives, 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 approach 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 61