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OPERATIONAL PRINCIPLES Results from extensive field observations and data analyses have demonstrated that ATL lane usage is governed by two primary factors: (1) the through movement's degree-of-saturation on the subject approach, and (2) the prevailing approach geometry. The supporting data and evidence is presented in the following section. Summary of Field Data A total of 22 ATL approaches across the United States were selected for field investigation and data collection based on the results of a web-based survey of transportation practitioners. All study approaches add ATLs to the right of the CTL and have a right-hand merge downstream. The sites have limited obstructions to ATL use (e.g., driveways, transit stops, work zones, etc.) and represent a range of geographic locations, geometric conditions, and levels of congestion. Exhibit 3-2 displays a diagram of a typical ATL. Exhibit 3-2 ATL Diagram Exhibit 3-3 and Exhibit 3-4 list the 14 one-CTL and 8 two-CTL approaches observed in this study. The two exhibits display location information, ATL length components, whether the ATL is exclusive or shared, and the level of ATL utilization by through traffic. ATL utilization in this context is defined as the percentage of approach through traffic, calculated by dividing ATL through volume by the total through volume on the approach. As shown in Exhibits 3-3 and 3-4, nearly all ATLs are underutilized compared to the lane utilization factors in the HCM 2010. According to the HCM 2010, the theoretical utilization should be 47.5 percent for an ATL with one CTL and 31.7 percent for an ATL with two CTLs (2). This information confirms the earlier statement regarding the need to consider the ATL as a separate lane group. Page 16
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Down Average Exhibit 3-3 Upstream stream- Min. ATL ATL Max . ATL Summary of Study Approach ATL ATL Utilization Utilization Utilization Characteristics for 1-CTL Sites Length Length % % % Approach Location Type (feet) (feet) Through Through Through EB Walker at Murray Beaverton, OR Shared 570 150 21 28 35 WB Walker at Murray Beaverton, OR Shared 220 350 23 29 32 EB NC 54 at Fayetteville Durham, NC Exclusive 1650 450 19 23 27 NB La Canada at Magee Tucson, AZ Shared 780 430 15 19 25 SB La Canada at Magee Tucson, AZ Shared 580 720 11 18 27 EB Magee at La Canada Tucson, AZ Shared 620 390 14 19 25 WB Magee at La Canada Tucson, AZ Shared 700 500 9 14 19 NB La Canada at Orange Tucson, AZ Shared 640 590 11 19 25 Grove SB La Canada at Orange Tucson, AZ Shared 730 560 18 19 24 Grove EB Walker at 185th Beaverton, OR Exclusive 410 220 34 40 44 WB Walker at 185th Beaverton, OR Shared 350 310 13 15 17 SB Sunset Lake at Holly Holly Springs, Shared 420 950 3 9 13 Springs NC NB Garrett at Old Chapel Hill Durham, NC Exclusive 320 300 13 19 24 SB Garrett at Old Chapel Hill Durham, NC Exclusive 330 380 15 23 27 EB = Eastbound, WB = Westbound, NB = Northbound, SB = Southbound Down - Exhibit 3-4 Upstream stream Min . ATL Average ATL Max . ATL Summary of Study Approach Right - Turn ATL Length Length Utilization Utilization Utilization Characteristics for 2-CTL Sites Approach Location Type (feet) (feet) % Through % Through % Through NB MD 2 at Arnold Annapolis, MD Exclusive 800 300 15 19 22 SB MD 2 at Arnold Annapolis, MD Exclusive 1670 1060 13 20 31 EB MD 214 at Kettering Bowie, MD Exclusive 830 510 2 5 8 NB IL 171 at IL 64 Melrose Park, IL Shared 890 1000 13 18 24 SB IL 171 at IL 64 Melrose Park, IL Shared 1150 830 14 18 23 NB IL 171 at Roosevelt Melrose Park, IL Shared 290 230 4 6 9 SB IL 171 at Roosevelt Melrose Park, IL Exclusive 450 360 21 26 30 SB US 1 at New Falls of Wake Forest, NC Exclusive 470 1040 11 13 15 Neuse EB = Eastbound, WB = Westbound, NB = Northbound, SB = Southbound Effect of Traffic Congestion on ATL Utilization The primary motivation for a driver to use an ATL is to save travel time by either avoiding long queues by moving around slower vehicles or avoiding waiting at the light for more than one signal cycle (cycle failure). Thus, the level of ATL use is controlled by operational elements of the intersection, including: · Approach through-movement flow. Higher through flow rates on the approach encourage more vehicles to move to the ATL. This result has been confirmed in several general studies of short-lane use (7, 8, and 9). · Signal timing. The lower the ratio of effective green for the approach to intersection cycle length, the lower is the capacity of the approach. Consequently, drivers are motivated to switch to the ATL to avoid a cycle Page 17
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failure. Additionally, a longer cycle length creates longer queues in the CTL(s), which also encourages drivers to switch to the ATL. · Arrival type. Most drivers choose to use the ATL while traffic is still queued on the approach. Therefore, an ATL approach with a high number of arrivals on green--usually achieved by good signal progression--would experience lower ATL use than one with a random arrival pattern. · Right-turning vehicles and driveways. Driveways along either the upstream or downstream length of the ATL create the potential for right- turning vehicles to block the passage of ATL drivers, which discourages ATL use. A heavy flow of right turns from a shared ATL has the same effect, as shown in previous research (7, 8). Additional details on the effects of right turns are provided in the ATL volume-estimation section later in this chapter. Exhibit 3-5 displays a plot of ATL through-movement flow rate against total approach through-movement flow rate for all sites (each with a different marker), broken down by the number of CTLs. All the indicated flow rates are based on 15-minute counts expanded to an hourly rate. In the event that only hourly volumes are available, they must first be divided by the peak-hour factor (PHF) to yield the corresponding 15-minute peak flow rate. The relationship shown in the exhibit between congestion and ATL use is relatively strong, and more evident than a relationship between through traffic and the percentage of ATL utilization, expressed as a fraction of all through traffic. This latter relationship is weaker because ATL flow rate increases as the overall through flow rate increases, making the ratio of the two more or less a constant. For this reason, ATL flow in vehicles per hour (vph) was modeled directly rather than predicted from percentage of utilization. Page 18
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Exhibit 3-5 1 CTL ATL Through-Movement Flow vs. NB Garrett - E1 350 Total Through-Movement Flow SB Garrett - E1 300 NB La Canada at Magee - S1 ATL Through-Movement Flow (vph) SB La Canada at Magee - S1 250 EB Magee at La Canada - S1 200 WB Magee at La Canada - S1 NB La Canada at Orange Grove - S1 150 SB La Canada at Orange Grove - S1 NC 54 - E1 100 Sunset Lake - S1 50 EB Walker at 185 - E1 WB Walker at 185 - S1 0 0 100 200 300 400 500 600 700 800 900 1000 EB Walker at Murray - S1 Total Through-Movement Flow (vph) WB Walker at Murray - S1 2 CTLs 600 NB IL 171 at IL 64 - S2 SB IL 171 at IL 64 - S2 500 ATL Through-Movement Flow (vph) NB IL 171 at Roosevelt - S2 400 SB IL 171 at Roosevelt - E2 300 NB MD 2 - E2 SB MD 2 - E2 200 MD 214 - E2 100 US 1 - E2 0 0 500 1000 1500 2000 2500 3000 Total Through-Movement Flow (vph) EB = Eastbound, WB = Westbound, NB = Northbound, SB = Southbound E1 = Exclusive, one CTL; S1 = Shared, one CTL; E2 = Exclusive, two CTLs; S2 = Shared, two CTLs Many of the figures and tables in this report have been converted from color to grayscale for printing. The electronic version of the report (posted on the web at www.trb.org) retains the color version. Page 19
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Exhibit 3-6 displays a plot of ATL through-movement flow against XT, the level of through-movement congestion, with XT defined as the approach through volume-to-capacity (v/c) ratio: Equation 3-1 where: VT = Through-movement demand flow rate on the ATL approach, N = Number of CTLs on the approach, ST = Through-movement adjusted saturation flow rate per lane, g = Effective green time for the approach through movement, and C = Intersection cycle length. Equation 3-1 computes the v/c ratio for the CTL s only because that factor was found to be the primary motivator for using the ATL. Also, all calculations should be based on 15-minute flow rates, including the average green and cycle time, in the event that the traffic signal is actuated. The relationships displayed in Exhibits 3-5 and 3-6 imply that separate models are necessary to predict the ATL flow for one-CTL and two-CTL approaches. A two-CTL site was omitted in the model development phase because it exhibited different characteristics than the other sites. That site, MD 214 (the data for which are circled at the bottom of Exhibit 3-5), had excellent signal progression, which inhibited the use of the ATL, even though through traffic flows were quite high. The remaining models should be considered valid primarily under random arrival conditions and should be used with caution under other circumstances. In summary, ATLs are more likely to be used by drivers on intersection approaches that operate near their through-movement capacity without an ATL in place. Lane utilization is also affected by arrival type, with improved progression inhibiting the use of the ATL due to limited queuing and delay on the approach, and by the volume of right-turning movements at the intersection or adjacent driveways. Page 20
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Exhibit 3-6 1 CTL ATL Flow vs. Level of Through- NB Garrett - E1 Movement Congestion (XT) 350 SB Garrett - E1 NB La Canada at Magee - S1 300 SB La Canada at Magee - S1 ATL Through-Movement Flow (vph) 250 EB Magee at La Canada - S1 WB Magee at La Canada - S1 200 NB La Canada at Orange Grove - S1 150 SB La Canada at Orange Grove - S1 NC 54 - E1 100 Sunset Lake - S1 EB Walker at 185 - E1 50 WB Walker at 185 - S1 0 EB Walker at Murray - S1 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 WB Walker at Murray - S1 XT 2 CTLs 600 NB IL 171 at IL 64 - S2 SB IL 171 at IL 64 - S2 500 ATL Through-Movement Flow (vph) NB IL 171 at Roosevelt - S2 400 SB IL 171 at Roosevelt - E2 300 NB MD 2 - E2 200 SB MD 2 - E2 MD 214 - E2 100 US 1 - E2 0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 XT EB = Eastbound, WB = Westbound, NB = Northbound, SB = Southbound E1 = Exclusive, one CTL; S1 = Shared, one CTL; E2 = Exclusive, two CTLs; S2 = Shared, two CTLs Page 21
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Effect of ATL Geometry The upstream storage length of the ATL should be long enough to adequately contain the maximum expected (i.e., 95th percentile) queue in the ATL. Ideally, it should also be longer than the maximum expected queue in the adjacent CTL to ensure that drivers have access to the ATL. The results of this research indicate that the downstream length of an ATL has virtually no impact on the level of ATL use, in spite of several earlier studies that hypothesized that a longer downstream length would encourage ATL use (7, 8, 9, 10, 11). Exhibit 3-7 displays a plot of the observed ATL utilization for each study approach against the corresponding downstream length. Exhibit 3-7 Minimum, Average, and 50 1 CTL Maximum ATL Utilization vs. Downstream Length 45 40 ATL Utilization (%) 35 30 25 Avg ATL % 20 15 10 5 0 0 100 200 300 400 500 600 700 800 900 1000 Downstream Length (ft) 2 CTLs 35 30 25 ATL Utilization (%) 20 15 Avg ATL % 10 5 0 0 200 400 600 800 1000 1200 Downstream Length (ft) Page 22