National Academies Press: OpenBook

Guidelines on the Use of Auxiliary Through Lanes at Signalized Intersections (2011)

Chapter: Chapter 3 - Operational Analysis

« Previous: Chapter 2 - ATL Characteristics
Page 14
Suggested Citation:"Chapter 3 - Operational Analysis." National Academies of Sciences, Engineering, and Medicine. 2011. Guidelines on the Use of Auxiliary Through Lanes at Signalized Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14617.
×
Page 14
Page 15
Suggested Citation:"Chapter 3 - Operational Analysis." National Academies of Sciences, Engineering, and Medicine. 2011. Guidelines on the Use of Auxiliary Through Lanes at Signalized Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14617.
×
Page 15
Page 16
Suggested Citation:"Chapter 3 - Operational Analysis." National Academies of Sciences, Engineering, and Medicine. 2011. Guidelines on the Use of Auxiliary Through Lanes at Signalized Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14617.
×
Page 16
Page 17
Suggested Citation:"Chapter 3 - Operational Analysis." National Academies of Sciences, Engineering, and Medicine. 2011. Guidelines on the Use of Auxiliary Through Lanes at Signalized Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14617.
×
Page 17
Page 18
Suggested Citation:"Chapter 3 - Operational Analysis." National Academies of Sciences, Engineering, and Medicine. 2011. Guidelines on the Use of Auxiliary Through Lanes at Signalized Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14617.
×
Page 18
Page 19
Suggested Citation:"Chapter 3 - Operational Analysis." National Academies of Sciences, Engineering, and Medicine. 2011. Guidelines on the Use of Auxiliary Through Lanes at Signalized Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14617.
×
Page 19
Page 20
Suggested Citation:"Chapter 3 - Operational Analysis." National Academies of Sciences, Engineering, and Medicine. 2011. Guidelines on the Use of Auxiliary Through Lanes at Signalized Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14617.
×
Page 20
Page 21
Suggested Citation:"Chapter 3 - Operational Analysis." National Academies of Sciences, Engineering, and Medicine. 2011. Guidelines on the Use of Auxiliary Through Lanes at Signalized Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14617.
×
Page 21
Page 22
Suggested Citation:"Chapter 3 - Operational Analysis." National Academies of Sciences, Engineering, and Medicine. 2011. Guidelines on the Use of Auxiliary Through Lanes at Signalized Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14617.
×
Page 22
Page 23
Suggested Citation:"Chapter 3 - Operational Analysis." National Academies of Sciences, Engineering, and Medicine. 2011. Guidelines on the Use of Auxiliary Through Lanes at Signalized Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14617.
×
Page 23
Page 24
Suggested Citation:"Chapter 3 - Operational Analysis." National Academies of Sciences, Engineering, and Medicine. 2011. Guidelines on the Use of Auxiliary Through Lanes at Signalized Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14617.
×
Page 24
Page 25
Suggested Citation:"Chapter 3 - Operational Analysis." National Academies of Sciences, Engineering, and Medicine. 2011. Guidelines on the Use of Auxiliary Through Lanes at Signalized Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14617.
×
Page 25
Page 26
Suggested Citation:"Chapter 3 - Operational Analysis." National Academies of Sciences, Engineering, and Medicine. 2011. Guidelines on the Use of Auxiliary Through Lanes at Signalized Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14617.
×
Page 26
Page 27
Suggested Citation:"Chapter 3 - Operational Analysis." National Academies of Sciences, Engineering, and Medicine. 2011. Guidelines on the Use of Auxiliary Through Lanes at Signalized Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14617.
×
Page 27

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Page 15 3. OPERATIONAL ANALYSIS This chapter presents the operational guidelines for evaluating ATLs at signalized intersections. It is founded on a method to predict the volume of traffic that will use an ATL. In addition to evaluating ATL and intersection performance, t he results from the operational analysis are needed to determine the recommended minimum design length s for the upstream and downstream ATL described in Chapter 5. This method supplements the Highway Capacity Manual 2010 (2) procedures. The HCM 2010 does not directly account for the effects of short lane additions (including ATLs) in its lane utilization factor f LU . Instead, it directs users to apply a simulation tool to predict the through - movement volume in the short lane. While some ATL characteristics can be modeled through micro - simulation, the models presented here ar e based pri marily on empirical observations taken at a number of existing signalized ATL approaches across the United States. A detailed discussion of how microsimulation may be used to evaluate ATL performance is provided in Appendix A. Appendix B describes the comp utational engine that incorporates the ATL lane use prediction model presented in this chapter. Given its unique operating characteristics and lower utilization, an ATL should always be treated as a separate lane group, regardless of whether the lane is sh ared with right turns or is exclusive. Thus, an approach that consists of a separate left - turn lane, one CTL, one ATL, and one exclusive right - turn lane would have four lane groups as shown in Exhibit 3 - 1 . While the HCM 2010 treats a shared through - right lane as a separate lane group (which is different from how the HCM 2000 lane groups were designated), it still combines all exclusive through lanes into a single lane group. This process is not appropriate in the presence of an ATL because the lane utilization of the ATL is much lower than that of a traditional CTL. This chapter presents the following information: • Principles of ATL operations • Data collection required for analysis • Analysi s techniques for ATL operations Exhibit 3-1 Exclusive ATL Lane Group Diagram

Page 16 OPERATIONAL PRINCIPLES Results from extensive field o bservation s 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 approach es 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 - 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 divid ing 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. Exhibit 3-2 ATL Diagram

Page 17 Approach Location Type ATL Length (feet) - Down stream ATL Length Min . ATL Utilization % Through Average ATL Utilization % Through Max . ATL Utilization % 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 Grove Tucson, AZ Shared 640 590 11 19 25 SB La Canada at Orange Grove Tucson, AZ Shared 730 560 18 19 24 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 Springs Holly Springs, NC Shared 420 950 3 9 13 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 Approach Location Right - Turn Type Upstream ATL Length (feet) Down - stream Length (feet) Min . ATL Utilization % Through Average ATL Utilization % Through Max . ATL Utilization % 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 Neuse Wake Forest, NC Exclusive 470 1040 11 13 15 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 avoid ing waiting at the light for mo re than one signal cycle (cycle failure). Thus, t he 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 s everal general studies of short - lane use ( 7 , 8 , and 9 ). • Signal timing . The lower the ratio of effective green for the approach to in tersection cycle length, the lower is the capacity of the approach . C onsequently, drivers are motivated to switch to the ATL to avoid a cycle Exhibit 3-3 Summary of Study Approach Characteristics for 1-CTL Sites Exhibit 3-4 Summary of Study Approach Characteristics for 2-CTL Sites Upstream (feet)

Page 18 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 u se than one with a random arrival pattern. • Right - turning vehicles and d riveways . 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, th ey must first be divided by the peak - hour factor (PHF) to yield the corresponding 15 - minute pe ak 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. Th is 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 predicte d from percentage of utilization.

Page 19 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. Exhibit 3-5 ATL Through-Movement Flow vs. Total Through-Movement Flow 0 500 1000 1500 2000 2500 3000 Total Through-Movement Flow (vph) Total Through-Movement Flow (vph) 0 100 200 300 400 500 600 700 800 900 1000 0 50 100 150 200 250 300 350 NB Garrett - E1 SB Garrett - E1 NB La Canada at Magee - S1 SB La Canada at Magee - S1 EB Magee at La Canada - S1 WB Magee at La Canada - S1 NB La Canada at Orange Grove - S1 SB La Canada at Orange Grove - S1 NC 54 - E1 Sunset Lake - S1 EB Walker at 185 - E1 WB Walker at 185 - S1 EB Walker at Murray - S1 WB Walker at Murray - S1 2 CTLs 0 100 200 300 400 500 600 NB IL 171 at IL 64 - S2 SB IL 171 at IL 64 - S2 NB IL 171 at Roosevelt - S2 SB IL 171 at Roosevelt - E2 NB MD 2 - E2 SB MD 2 - E2 MD 214 - E2 US 1 - E2 1 CTL A TL T hr ou gh -M ov em en t F lo w (v ph ) A TL T hr ou gh -M ov em en t F lo w (v ph )

Page 20 Exhibit 3 - 6 displays a plot of ATL through - movement flow against X T , the level of through - movement congestion, with X T defined as the app roach t h rough volume - to - capacity ( v /c) ratio: w here: V T = Through - movement demand flow rate on the ATL approach , N = Number of CTLs on the approach , S T = Through - movement adjusted saturation flow rate per lane , g = Effective green time for the approach through movement, and C = I ntersection cycle length. E quation 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 Exhibit s 3 - 5 and 3 - 6 imply that separate models are necessary to predict the ATL flow for one - CTL and two - CTL approache s . 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 ) , ha d excellent signal progression , which inhibited the use of the ATL, even though through traffic flows were quite high. T he remaining models should be considered valid primarily under random arrival conditions and should be used with caution under other cir cumstances . 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 inhibitin g 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. Equation 3-1

Page 21 EB = Eastbound, WB = Westbound, NB = Northbound, SB = Southbound E1 = Exclusive, one CTL; S1 = Shared, one CTL; E2 = Exclusive, two CTLs; S2 = Shared, two CTLs Exhibit 3-6 ATL Flow vs. Level of Through- Movement Congestion (XT) 0.0 0.0 0.2 0.2 0.4 0.4 0.6 0.6 0.8 0.8 1.0 1.0 1.2 1.2 1.4 1.4 XT XT 0 50 100 150 200 250 300 350 NB Garrett - E1 SB Garrett - E1 NB La Canada at Magee - S1 SB La Canada at Magee - S1 EB Magee at La Canada - S1 WB Magee at La Canada - S1 NB La Canada at Orange Grove - S1 SB La Canada at Orange Grove - S1 NC 54 - E1 Sunset Lake - S1 EB Walker at 185 - E1 WB Walker at 185 - S1 EB Walker at Murray - S1 WB Walker at Murray - S1 2 CTLs 0 100 200 300 400 500 600 NB IL 171 at IL 64 - S2 SB IL 171 at IL 64 - S2 NB IL 171 at Roosevelt - S2 SB IL 171 at Roosevelt - E2 NB MD 2 - E2 SB MD 2 - E2 MD 214 - E2 US 1 - E2 1 CTL A TL T hr ou gh -M ov em en t F lo w (v ph ) A TL T hr ou gh -M ov em en t F lo w (v ph )

Page 22 Effect of ATL Geometry T he 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 . Th e 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 Maximum ATL Utilization vs. Downstream Length Downstream Length (ft) Downstream Length (ft) 2 CTLs 1 CTL Avg ATL % Avg ATL % A TL U til iz at io n (% ) A TL U til iz at io n (% ) 0 5 10 15 20 25 30 35 0 200 400 600 800 1000 1200 0 5 10 15 20 25 30 35 40 45 50 0 100 200 300 400 500 600 700 800 900 1000

Page 23 The exhibit shows the minimum, average , and maximum ATL utilization per site, which are also distinguished by the number of CTLs. It is clear from the data that downstream length plays little , if any , role in enticing drivers to use the ATL . From a design perspective, the downstream ATL length should be long enough to enable drivers starting from a stopped queue in the ATL to accelerate to a safe merging speed. It should also allow drivers traveling through the intersection during the green phase to find a suitable gap for merging into the adjacent CTL traffic stream. Still, as ATLs are inherently an interim capacity improvement at an intersection, the ultimate length may be limited based on available ri ght - of - way, environmental constraints, and construction costs. ATL design elements are discussed further in Chapter 5. Other Factors Affecting ATL Use The following operational and design characteristics should also be considered during ATL design, alth ough their effects were not fully quantified in the statistical lane - use models because of the limited number of observations : • Downstream congestion . A bottleneck downstream of the ATL merge area due to the presence of a signalized intersection, a lane dr op , or heavy driveway traffic onto the roadway may cause queued traffic to spill back onto the ATL and affect its operations. • Posted speed . The higher the posted speed limit, the greater is the speed differential between queued vehicles in the CTL that begin to accelerate when the signal turns green and vehicles arriving on green that may pass more easily in the ATL. This situation may encourage greater ATL use, but likely require s a longer downstream length for safe merging. • Sight distance at the intersection approach . Drivers feel more comfortable using an ATL when they can see there are no obstructions in the merge area. DATA COLLECTION REQUIREMENTS Conducting a traffic operations analysi s for an ATL requires the same input data as needed for a signalized intersection analysis performed using the HCM 2010 method. These data include 15 - minute peak - period flow rates and heavy vehicle percentages, geometric data, and signal timing data. If d riveways are present in the ATL , the driveway volume should be estimated and added to the right - turn movements at the intersection. L eft turns are assumed to operate from one or more exclusive turn lanes and t o not influence the operation of the adjacent C TL or ATL. Such was the case at each site visited for this research. The following bullet items summarize the data that must be measured in the field or estimated in order to predict the through - movement approach volume that will use the ATL: • Through - move ment demand flow rate on the approach, in vehicles per hour

Page 24 • Right - turn flow rate on the approach (only for proposed shared through/right ATLs), i n vehicles per hour; also right - turn flow rates into downstream driveways if those are available and deemed to be significant • Effective green time for the approach , in seconds during the peak 15 - minute period • Intersection cycle length, in seconds during the peak 15 - minute period • Adjusted saturation flow rate for through and right - turn movements on the approach (us ing HCM 2010 methods), in vehicles per hour. ATL VOLUME ESTIMATION This section describes a step - by - step analytica l method to predict the through - movement volume that will use an ATL. The method is based principally on the demand - to - capacity relationship of an intersection approach without an ATL, and estimates the expected volume in the ATL based on various parameters. The base estimation method is founded on models built from fie ld data collected on ATL use. Separate prediction models were developed for one - CTL and two - CTL approaches. The field model estimates are constrained by upper bound estimates from the HCM 2010 model, which specify the maximum through volume to be expected in any exclusive or shared lane, where through traffic has a choice of lanes. The upper bound estimate also gu arantees consideration of right - turn traffic effects, even for those cases where field observations did not show an impact due to low right - turni ng movements. While the method predicts ATL volume, the utilization percent age can be calculated from the results if desired. Approaches with One CTL A key parameter for the analysis of an ATL facility with one existing CTL is X T , the ratio of through - move ment demand to capacity, also listed earlier in Equation 3 - 1: w here: V T = 15 - min ute through - movement demand flow rate on the approach, expressed in vehicles per hour; S T = A djusted through saturation flow rate per lane on the approach, in vehicles per hour; g = E ffective green time for the approach, in seconds; and C = I ntersection cycle length, in seconds. In Equation 3 - 2, V T is the total through demand flow rate, whereas S T is the per - lane saturation flow rate of the CTL. Once X T is computed, the through - movement flow rate in the ATL can be predicted using Equation 3 - 3 (R 2 = 0.781): Equation 3-2

Page 25 where: V ATL = T he predicted through - movement flow rate in the ATL (in vehicles per hour), and all other variables are as previously defined. The remaining flow rate in the continuous lane, V CTL , is obtained by subtracting V ATL from VT. This method can be used to estimate ATL use on approaches with one CTL in situations when the ATL will be an exclusive lane and when it will be a shared lane with right turns. Equation 3 - 3 does not contain a right - turn volume variable , because the measured right - turn volumes in the field were not high enough to impact the ATL through volume for observed shared ATL sites . This does not mean that right - turn effects will be ignored. In fact, those will be accounted for in the estimation of an upper - bound flow rate using HCM 2010 methods . AT L Approaches with Two CTLs For an approach with two CTLs and a proposed shared ATL, an additional parameter X R (the right - turn volume - to - capacity ratio) must be estimated as given in Equation 3 - 4 (R 2 = 0.768): w here: V R = R ight - turn flow rate for the proposed shared ATL (this term could include right - turn traffic entering the downstream driveways if that flow rate is available and deemed to be significant); and S R = A djusted right - turn saturation flow rate in the proposed shared ATL (usually defaults to 0.85 S T ) . In the case of an exclusive ATL, X R is set to zero in Equation 3 - 5. The through - movement flow rate (in vehicles per hour) in the ATL can then be predicted using Equation 3 - 5: Similar to the one - CTL case, V T represents the total approach through volume. Upon computing V ATL , the remaining volume in both continuous lanes (V CTL ) is again obtained by subtracting V ATL from V T . This research did not show Equation 3-3 Equation 3-4 Equation 3-5

Page 26 evidence of uneven lane utilization across the two continuous lanes, and V CTL is therefore assumed to be divided equally across the two CTLs. Upper-Bound Values for ATL Use Regardless of the predicted ATL flow rate derived f rom Equa tions 3 - 3 or 3 - 5, when given a choice, drivers will generally seek the lane that will minimize their own queue position service time. This upper bound on the typical through flow rate for an ATL is best represented by the equal v/s (volume -to-adjusted saturation flow rate) app roach adopted in the HCM 2010 ( 2 ). It essentially states that through traffic on an approach will divide itself across several eligible lanes in a manner that equalizes all lane v/s ratios serving the through traffic. Therefor e, if an exclusive ATL is contemplated, the upper bound for the ATL through flow rate for the single CTL case can be estimated using Equation 3 - 6: V ATL,MAX In the case of two CTLs, the upper bound is computed using Equation 3-7: V ATL,MAX = w here: V ATL,MAX = U pper bound for ATL through flow rate, in vehicles per hour; and f LU = HCM 2010 lane utilization factor (see HCM 2010; Exhibit 18 - 30 for default values). In the case of a shared through - right ATL, the lane utilization factor is not applicable because lane choice is governed by the possible impedance caused by right turns in the shared lane. Instead, the upper bound for ATL flow rate is estimated on the basis of the equal v/s concept, using Equation 3 -8: w here: N = N umber of CTLs and shared ATLs on the proposed approach , V R = T he right - turn flow rate from the shared ATL (in vehicles per hour including possibly right turn s onto downstream driveways), and Equation 3-6 Equation 3-7 Equation 3-8

Page 27 S R = T he right - turn saturation flow rate in vehicles per hour . NUMERICAL ILLUSTRATION OF ATL VOLUME PREDICTION A two - CTL approach carries a through traffic flow rate of 1 , 000 vehicles per hour along with 191 right turns per hour in an exclusive right - turn pocket during the peak 15 - minute period. The approach is signal controlled and receives an effective green time of 30 seconds in a 120 - second cycle. The traffic engineer is contemplating converting the short right - turn pocket into a shared ATL. The engineer is also interested in testing the effect of adding an exclusive through ATL on the approach. The ATL use and feasibility will be estimated for both scenarios. Shared ATL Scenario Using Equation 3 - 4 and assuming that the adjusted S T = 1,800 vph per lane and S R = 1 , 800 x 0.85 = 1,530 vph per lane, then The predicted ATL through fl ow rate from Equation 3 - 5 is: The relevant upper - bound value for through traffic in the shared ATL is computed from Equation 3 - 8, with N = 3 In this scenario, the vo lume prediction from Equation 3 - 5 is lower than the volume using the equal v/s criterion. The shared ATL is predicted to attract 157 existing through - movement vehicles per hour, or about 16 percent of the total through - mov ement f low. The lateral through - lane volume distribution will be 157 + 191 = 348 vph in the shared ATL, and (1,000 - 157)/2 = 422 through vehicles per hour in each of the two exclusive CTLs.

Page 28 Exclusive ATL Scenario In this case, Equation 3 - 4 is used with X R set to 0, yielding V ATL = 202 vph, which is higher than the shared lane case, as expected. The maximum value of through traffic that equalizes the queue service time is estimated fr om Equation 3 - 7 using a default HCM 2010 value of lane utilization f LU = 0 .908 ( 2 , Exhibit 18 - 30 ). Substituting into Equation 3 - 7 gives V ATL, MAX = 265 vph. The estimated exclusive ATL flow rate is the lower of the two estimates at 202 vph, and the per - lane through f low rate in the CTLs is (1,000 – 202)/2 = 399 vph . It is clear from the analysis that either the shared or exclusive ATL scenario will relieve the approach congestion considerably. By comparing the before - and - after v/c ratios, one can see that while right turns may experience a higher v/c ratio and higher delays in t he shared lane scenario, through traffic will benefit considerably from the ATL addition. A summary of the computed v/c ratios in the three scenarios is depicted in Exhibit 3 - 8 . Further estimates of delays, LOS , and queue lengths associated with ATL installations are provided with the computational engine described in Appendix B. A first - cut analysis of the example results would indicate that converting an exclusive ri ght - turn pocket to a shared ATL appears to be a sufficient treatment in the short term, assuming current flows and signal plans do not change appreciably , and that the ATL’s length is dimensioned properly . Movement Baseline v/c v/c After Conversion to Shared ATL v/c After Adding Exclusive ATL* Through Traffic 1.167 0.984 (in CTL) 0.50 in ATL 0.93 in CTLs Right-Turn Traffic 0.50 0.850 (traffic in shared ATL) 0.50 * Exclusive ATL scenario assumes maintaining the exclusive right turn lane Exhibit 3-8 Movement and Lane v/c Ratios Before and After ATL Conversion or Addition

Next: Chapter 4 - Safety »
Guidelines on the Use of Auxiliary Through Lanes at Signalized Intersections Get This Book
×
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF

TRB’s National Cooperative Highway Research Program (NCHRP) Report 707: Guidelines on the Use of Auxiliary Through Lanes at Signalized Intersections provides guidelines to help in the justification, design, and analysis of auxiliary through lanes (ATLs) at signalized intersections.

ATLs are lanes for through movements that begin upstream of a signalized intersection and end downstream of the intersection. ATLs are potentially a moderate-cost approach to increase intersection and overall corridor capacity.

A report that describes the research related to the development of NCHRP Report 707 has been released as NCHRP Web-Only Document 178: Assessment of Auxiliary Through Lanes at Signalized Intersections.

A spreadsheet-based computational engine is also available online.

Spreadsheet Disclaimer - This software is offered as is, without warranty or promise of support of any kind either expressed or implied. Under no circumstance will the National Academy of Sciences or the Transportation Research Board (collectively “TRB’) be liable for any loss or damage caused by the installation or operation of this product. TRB makes no representation or warranty of any kind, expressed or implied, in fact or in law, including without limitation, the warranty of merchantability or the warranty of fitness for a particular purpose, and shall not in any case be liable for any consequential or special damages.

  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  6. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  7. ×

    View our suggested citation for this chapter.

    « Back Next »
  8. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!