Use of Auxiliary Lanes at Isolated Freeway On-Ramp Junctions

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InSTITuTE oF TrAnSPorTATIon EngInEErS 1627 Eye Street, NW, Suite 600, Washington, DC 20006 USA P: 202.785.0060 F: 202.785.0609 www.ite.org VoLuME 5 | NuMBER I | JuLY 2013 Journal of TransporTaTion of the institute of Transportation Engineers use of auxiliary lanes at isolated Highway 1 on-ramp Junctions By Yubian Wang, E.I.T., Ruey Long Cheu, Ph.D., P.E., M.I.T.E., Yi Qi, Ph.D., and Xiaoming Chen, Ph.D. a methodology for Evaluating Centerline 17 markings in Temperate Climates By Wayne Sarasua, Ph.D., P.E., William J. Davis, Ph.D., P.E., Joseph Robertson, E.I.T., and Joshua Johnson, E.I.T. reducing Conflicts between pedestrians 31 and left-Turning Vehicles By Michael P. Pratt, P.E., Praprut Songchitruksa, Ph.D., P.E., and James A. Bonneson, Ph.D., P.E. CoNTENTS Journal of TransporTaTion 1 This paper uses the 2010 Highway Capacity Manual’s analysis procedure to investigate the operational benefits of having auxiliary lanes at isolated highway on-ramp junctions. The study scenarios cover only one-lane on-ramps but with different numbers of lanes on the highway, different highway and ramp volumes, and different lengths of auxiliary lanes. The first part of the study quantifies the reduction in density and improvement in the level of service in the merge influence area before and after the addition of auxiliary lanes of different lengths. The second part of the study uses level-of-service-improvement criteria to identify the design scenarios in which auxiliary lanes should be incorporated in on-ramp junctions. Tables of minimum lengths of auxiliary lanes provide guidance for design engineers. The outcomes of this research provide quantitative justifications for engineers to add auxiliary lanes at on-ramp junctions and suggest minimum lengths for these lanes. use of auxiliary lanes at isolated Highway on-ramp Junctions By Yubian Wang, E.I.T., Ruey Long Cheu, Ph.D., P.E., M.I.T.E., Yi Qi, Ph.D., and Xiaoming Chen, Ph.D. Introduction The american association of state Highway and Transportation officials’ (aasHTo’s) 2004 edition of A Policy on Geometric Design of Highways and Streets defines an auxiliary lane as “the portion of the roadway adjoining the traveled way for speed change, turning, turning storage, weaving, truck climbing, and other purposes supplementary to through-traffic movement”.1 according to this definition, an auxiliary lane at a highway on-ramp junction refers to the acceleration lane immediately downstream of the on-ramp that terminates after some distance. for the rest of this paper, this lane is referred to as an auxiliary lane, although it is also known as an acceleration lane. figure 1 shows the geometric design of an on-ramp junction with and without an auxiliary lane. The auxiliary lane gives a merging vehicle additional roadway to adjust its speed and seek a desirable gap 2 THE insTiTuTE of TransporTaTion EnGinEErs before merging into the highway. The auxiliary lane also provides an opportunity for an approaching vehicle in the right lane of the highway to adjust its speed to accommodate a merging vehicle. one of the benefits of including auxiliary lanes at highway on-ramps is to improve the traffic operations at the junctions. However, very few studies have quantified the operational benefits of auxiliary lanes, and there are very few national or state guidelines for the conditions under which auxiliary lanes should be implemented at highway on-ramp junctions. This paper has two objectives. The first is to compare the operational performance of isolated highway on-ramp junctions with and without an auxiliary lane. from this comparison, the benefits of installing auxiliary lanes at isolated highway on-ramp junctions can be quantified. The second objective is to evaluate the operational performance of on-ramp junctions with different geometry (for example, number of lanes, length of auxiliary lane) and traffic volumes (for example, highway and ramp volumes). on the basis of the operational performance in the design scenarios, the authors recommend the conditions under which auxiliary lanes should be included in the on-ramp junction design and the optimal length of these lanes to improve traffic flow and safety. The evaluation of operational performance is based on the wellestablished analysis procedure described in the 2010 edition of the Highway Capacity Manual (HCm2010).2 Literature Review The few published reports on the design of auxiliary lanes at on-ramp junctions focus on the length of the auxiliary lane, which is defined slightly differently in different reports. an auxiliary lane should have sufficient length to enable vehicles to accelerate comfortably to the speed at which they Figure 1. Highway on-ramp junctions with and without auxiliary lane. (a) with auxiliary lane (AL) (b) without auxiliary lane Journal of TransporTaTion 3 merge into the highway. The american association of state Highway and Transportation officials (aasHTo) set the minimum length for an auxiliary lane on the basis of the following inputs:1 ■■ The speed at which drivers enter the upstream end of the auxiliary lane; ■■ The acceleration behavior of vehicles at the on-ramp; and ■■ The speed at which drivers merge with highway through traffic. The length of the auxiliary lane, denoted by L, is measured from the point at which the left edge of the ramp joins the right lane on the highway to the beginning of the downstream taper, as shown in figure 2. Figure 2. Length of auxiliary lane (adopted from AASHTo1). according to the abovementioned design inputs, the minimum L values are shown in Table 1. The first two columns list the design speed of the highway (V) and the speed reached by vehicles on the ramp after acceleration (Va). Va is always less than V because vehicles always enter the ramp at much lower speeds (denoted by initial speed V'a) than the ramp design speed. Depending on the combination of (V or Va) and (V'a or entrance curve design speed), the minimum L can be read from the table. (note that aasHTo uses the term “entrance curve” instead of “on-ramp.” Thus, the term “entrance curve design speed” in Tables 1 and 2 is equivalent to “on-ramp design speed.”) Table 1. Minimum length of auxiliary lane as recommended by AASHTo.1 Minimum L (ft.) Highway Entrance curve design speed (mph) 0 15 20 25 30 35 40 45 50 Design speed V (mph) Speed reached Va (mph) Initial speed V'a (mph) 0 14 18 22 26 30 36 40 44 30 23 180 140 – – – – – – – 35 27 280 220 160 – – – – – – 40 31 360 300 270 210 120 – – – – 45 35 560 490 440 380 280 160 – – – 50 39 720 660 610 550 450 350 130 – – 55 43 960 900 810 780 670 550 320 150 – 60 47 1200 1140 1100 1020 910 800 550 420 180 65 50 1410 1350 1310 1220 1120 1000 770 600 370 70 53 1620 1560 1520 1420 1350 1230 1000 820 580 75 55 1790 1730 1630 1580 1510 1420 1160 1040 780 4 THE insTiTuTE of TransporTaTion EnGinEErs fitzpatrick and Zimmerman developed the minimum L using the same concept but with updated speed and acceleration data.2 Their minimum L values are listed in Table 2, without the Va and V'a values. a comparison of Tables 1 and 2 shows that fitzpatrick and Zimmerman recommended a longer minimum L than aasHTo. Table 2. Minimum length of auxiliary lane recommended by Fitzpatrick and Zimmerman.2 Minimum L (ft.) Highway design speed (mph) Entrance curve design speed (mph) 0 15 20 25 30 35 40 45 50 30 389 292 216 – – – – – – 35 529 432 357 259 – – – – – 40 691 594 519 421 303 – – – – 45 875 778 702 605 486 346 – – – 50 1080 983 908 810 691 551 389 – – 55 1301 1210 1134 1037 918 778 616 432 – 60 1556 1459 1383 1286 1167 1026 864 681 475 65 1826 1729 1653 1556 1437 1297 1134 951 746 70 2118 2020 1945 1848 1729 1588 1426 1243 1037 75 2431 2334 2258 2162 2042 1902 1740 1556 1351 in report 505, the national Cooperative Highway research project (nCHrp) incorporated large trucks into the development of a similar table for minimum L.3 The nCHrp project team used the truck speed profile model to derive the minimum L criteria. The team concluded that although the minimum L recommended by aasHTo did not fully accommodate loaded large trucks, there was no indication of any safety issue. Therefore, the team recommended no change in the aasHTo design guidelines for minimum L that would apply specifically to trucks. although this study has identified potential operational issues of a loaded large truck merging into the highway at the minimum L recommended by aasHTo, no research has been conducted to date. Gattis, Bryant, and Duncan collected field data and performed an analysis of the acceleration of large trucks at five on-ramps downstream of weighing stations on interstate highways in arkansas and missouri.4 They used the field data to develop minimum L values for large trucks; these values are longer than the values specified by fitzpatrick and Zimmerman and by nCHrp project 15-21. recently, liu and Wang provided an analysis framework to calculate the minimum L (they called it “length of the on-ramp weaving/merging section”), incorporating human factors, vehicle performance characteristics, and road surface conditions.5 for an on-ramp vehicle to merge ahead of or behind a vehicle in the highway’s main lane, they derived the minimum on-ramp weaving/ merging section length as the function of the speed differential between the on-ramp vehicle and the highway vehicle. The length of the on-ramp weaving/merging section recommended by liu and Wang is shorter than the minimum L recommended by fitzpatrick and Zimmerman but longer than that recommended by aasHTo. Journal of TransporTaTion 5 These studies focused on the microscopic interactions between the merging vehicle (from the on-ramp) and the highway vehicle (in the rightmost main lane) with the objective of deriving the minimum L for a successful merge. none of the studies investigated how the absence of an auxiliary lane affects traffic operations in the vicinity of an on-ramp junction. This research examines how the presence and length of an auxiliary lane influence macroscopic traffic performance, such as density and level of service (los), at on-ramp junctions. Analysis Methodology a recent survey of design engineers in Texas and in other state departments of transportation found that the software tools most commonly used for analyzing operational impacts of auxiliary lanes are Highway Capacity Software (HCs), Vissim, and Corsim.6 HCs is the most popular tool; therefore, this study used the 2010 version to assess the operational performance of on-ramp junctions.7 HCs2010 follows the los analysis procedure described in Chapter 13 of HCm2010.8 This procedure applies to isolated on-ramp junctions with one-lane ramps on the right-hand side of the highway (hereafter simply on-ramp junctions). The analyzed influence area covers a distance of 1,500 ft. downstream from the merge point in the two right-hand lanes of the highway plus the auxiliary lane, as indicated in figure 3. according to HCm2010, this is the area where most lane-changing events occur, including vehicles in the right-hand highway lane moving to the left to avoid merging vehicles. Figure 3. Merge influence area (from HCM20102). The HCm2010 analysis procedure for on-ramp junctions essentially consists of the following steps: 1. Estimate the demand flow rate at the upsteam end of the on-ramp junction in the two right-hand lanes on the highway; 2. Estimate the capacity of the merge area (in the two right-hand lanes on the highway immediately downstream of the merge point) and compare that capacity with the demand flow rates (including the demand flow rate from the on-ramp); and 3. if the total demand flow rate is greater than the capacity, assign los f; otherwise, estimate the density within the merge influence area and convert it into los. Table 3 lists HCm2010 los criteria for on-ramp junctions. 12 THE insTiTuTE of TransporTaTion EnGinEErs figure 5 shows the scenarios in which auxiliary lanes of 500 ft. and 1,000 ft. should be included as part of the on-ramp junctions. The regions of NF, vR, and vF values are developed, and can be interpreted similarly to those in figure 4. in both figures, with a fixed NF, the shaded regions become larger as LA increases from 500 ft. to 1,000 ft. and then to 1,500 ft. The increase is mainly due to the lowering of the solid lines. This reflects the fact that with a higher value for vR or vF, or both, only longer auxiliary lanes are recommended. Minimum Length of an Auxiliary Lane figures 4 and 5 illustrate only the design scenarios in which an auxiliary lane of 500 ft., 1,000 ft., or 1,500 ft. should be considered as part of an on-ramp junction. Consider the design scenario of NF = 3 lanes, vR = 800 pc/h/ln, and vF =1,500 pc/h/ln. from figure 5, it is clear that LA = 500 ft. will not meet the los improvement criteria but LA = 1,000 ft. will. Thus, the minimum LA that will meet the los improvement criteria is somewhere between 500 ft. and 1,000 ft. in this research, the design Figure 5. Design scenarios in which auxiliary lanes of 500 ft. and 1,000 ft. should be added. LA = 500 ft. LA = 1,000 ft. NF=2 lanes vF (pc/h/ln) vR (pc/h/ln) vF (pc/h/ln) vR (pc/h/ln) 100 200 300 400 500 600 700 800 900 1000 100 200 300 400 500 600 700 800 900 1000 500 500 750 750 1000 1000 1250 1250 1500 1500 1750 1750 2000 2000 NF=3 lanes vF (pc/h/ln) vR (pc/h/ln) vF (pc/h/ln) vR (pc/h/ln) 100 200 300 400 500 600 700 800 900 1000 100 200 300 400 500 600 700 800 900 1000 500 500 750 750 1000 1000 1250 1250 1500 1500 1750 1750 2000 2000 NF=4 lanes vF (pc/h/ln) vR (pc/h/ln) vF (pc/h/ln) vR (pc/h/ln) 100 200 300 400 500 600 700 800 900 1000 100 200 300 400 500 600 700 800 900 1000 500 500 750 750 1000 1000 1250 1250 1500 1500 1750 1750 2000 2000 Journal of TransporTaTion 13 scenarios actually covered LA from 100 ft. to 1,500 ft. in increments of 100 ft. Thus, it is possible to construct figures similar to figures 4 and 5 for all the LA values studied and to deduce the minimum LA values for any given combination of NF, vR, and vF. Table 6 shows minimum LA values at the resolution of 100 ft. as mentioned before, LA < 500 ft. may not be practical. Therefore, if an auxiliary lane has been determined to be necessary, the design engineer may want to set the minimum LA to at least 500 ft. in Table 6, the cells without value near the top left-hand corner indicate the design scenarios in which no auxiliary lane is necessary (from an operational point of view), while the cells without value near the bottom right corner indicate the design scenarios in which other geometric improvements should be considered. it is difficult, if not impossible, to compare the minimum LA values in Table 6 with the minimum L values in Tables 1 and 2. first, the definitions of length of auxiliary lane are different in these tables. Figure 5. Design scenarios in which auxiliary lanes of 500 ft. and 1,000 ft. should be added. LA = 500 ft. LA = 1,000 ft. NF=2 lanes vF (pc/h/ln) vR (pc/h/ln) vF (pc/h/ln) vR (pc/h/ln) 100 200 300 400 500 600 700 800 900 1000 100 200 300 400 500 600 700 800 900 1000 500 500 750 750 1000 1000 1250 1250 1500 1500 1750 1750 2000 2000 NF=3 lanes vF (pc/h/ln) vR (pc/h/ln) vF (pc/h/ln) vR (pc/h/ln) 100 200 300 400 500 600 700 800 900 1000 100 200 300 400 500 600 700 800 900 1000 500 500 750 750 1000 1000 1250 1250 1500 1500 1750 1750 2000 2000 NF=4 lanes vF (pc/h/ln) vR (pc/h/ln) vF (pc/h/ln) vR (pc/h/ln) 100 200 300 400 500 600 700 800 900 1000 100 200 300 400 500 600 700 800 900 1000 500 500 750 750 1000 1000 1250 1250 1500 1500 1750 1750 2000 2000 14 THE insTiTuTE of TransporTaTion EnGinEErs second, the tables use different input variables. Tables 1 and 2 use design speeds as inputs, while Table 6 uses NF and design volumes. This is not surprising, as Tables 1 and 2 were developed from the aasHTo design concept (using individual vehicle dynamics), while Table 6 was developed from the HCm2010 analysis procedure (using macroscopic analysis of traffic flow). The authors recommend that during the on-ramp junction design process, both L and LA are checked against their respective design criteria. Table 6. Minimum length of auxiliary lane. Minimum LA (ft.) for NF = 2 lanes vF (pc/h/ln) vR (pc/h/ln) 100 200 300 400 500 600 700 800 900 1000 500 –––––––––– 750 –––––––––– 1000 –––––––––100 1250 – – – – 200 300 400 500 600 700 1500 300 400 500 700 800 900 1000 1100 1200 1400 1750 900 1000 1200 1300 1400 1500 – – – – 2000 –––––––––– (a) NF = 2 lanes Minimum LA (ft.) for NF = 3 lanes vF (pc/h/ln) vR (pc/h/ln) 100 200 300 400 500 600 700 800 900 1000 500 –––––––––– 750 –––––––––– 1000 –––––––––– 1250 – – – – – – – 100 200 400 1500 – – – 200 300 500 600 700 900 1000 1750 400 600 700 800 1000 1100 1300 1400 – – 2000 1100 1300 1400 1500 – – – – – – (b) NF = 3 lanes Minimum LA (ft.) for NF = 4 lanes vF (pc/h/ln) vR (pc/h/ln) 100 200 300 400 500 600 700 800 900 1000 500 –––––––––– 750 –––––––––– 1000 –––––––––– 1250 –––––––––100 1500 – – – – – 100 300 400 500 600 1750 100 200 300 400 500 600 800 900 1000 1100 2000 600 700 800 900 1000 1100 1300 1400 1500 – (c) NF = 4 lanes Journal of TransporTaTion 15 Conclusions and Recommendations in this study, the operational performance of on-ramp junctions with and without an auxiliary lane under different design scenarios has been evaluated by means of HCs2010, which implements the los analysis procedure in Chapter 13 of HCm2010. on the basis of an analysis of traffic density and los in the merge influence area before and after the inclusion of an auxiliary lane, the operational benefits of adding auxiliary lanes are visualized and quantified (see Tables 3 and 4). The results showed that after the addition of an auxiliary lane, the density in the merge influence area declined by 3.0 pc/mi/ln for LA = 500 ft. and by up to 9.4 pc/mi/ ln for LA = 1,500 ft. This research has also developed recommendations for the minimum LA under different combinations of NF, vF, and vR design scenarios. The minimum LA is the length at which, after the addition of an auxiliary lane, the los in the merge influence area will improve from D or worse to C or better. a look-up table (Table 6) for the minimum LA was developed for design engineers to use as a quick reference. This research is believed to be one of the first to quantify the operational improvements in terms of density and los in the merge influence area before and after the addition of auxiliary lanes at on-ramp junctions and to develop recommendations for the minimum LA based on the HCm2010 analysis procedure combined with los improvement criteria. The research considered only isolated highway on-ramp junctions with one-lane ramps at zero percent grade. other geometric conditions (such as two-lane on-ramps, although they are rare) were not considered and could be a direction for future research. Crash analysis at on-ramp junctions with and without an auxiliary lane and with different LA values should also be performed to establish the minimum LA values from a safety perspective. Acknowledgment and Disclaimer This research is supported in part and based on an earlier work conducted by the authors under Texas Department of Transportation Contract 0-6706. The contents of the paper and the views expressed are the sole responsibility of the authors and do not necessarily reflect the policy of the Texas Department of Transportation. References 1. american association of state Highway and Transportation officials (aasHTo). A Policy on Geometric Design of Highways and Streets, 5th edition. Washington, DC: aasHTo, 2004. 2. fitzpatrick, K. and K. Zimmerman. “potential updates to 2004 Green Book’s acceleration lengths for Entrance Terminals.” Transportation Research Record, Journal of the Transportation Research Board, no. 2023 (2007): 130–139. 3. Harwood, D. W., D. J. Torbic, K. r. richard, W. D. Glauz, and l. Elefteriadou. nCHrp report 505: Review of Truck Characteristics as Factors in Roadway Design. Washington, DC: Transportation research Board, 2003. 4. Gattis, J., m. Bryant, and l. K. Duncan. Acceleration Lane Design for Higher Truck Volumes. report no. mBTC fr 2094/3003. mack-Blackwell Transportation Center, university of arkansas, 2008. 16 THE insTiTuTE of TransporTaTion EnGinEErs 5. liu, C. and Z. Wang. “Determination of onramp Weaving length for resolving merging Dilemma.” International Journal of Transportation Science & Technology, Vol. 1, no. 1 (2012): 49–60. 6. Texas Department of Transportation. Technical Memorandum for Task 2—Survey of Transportation Engineers. Design and Scope of Impact of Auxiliary Lanes. project 0-6707. Texas Department of Transportation, 2011. 7. Center for microcomputers in Transportation (mcTrans). Highway Capacity Software 2010, Weaving Version 6.1. mcTrans, university of florida, 2010. 8. Transportation research Board of the national academies (TrB). Highway Capacity Manual. Washington, DC: TrB, 2010. 9. aasHTo. A Policy on Geometric Design of Highways and Streets, 6th edition. Washington, DC: aasHTo, 2011. Yubian Wang, E.I.T., is a Ph.D. research associate in the Department of Civil Engineering at the university of Texas at El Paso. Her research interests include traffic operation and safety. She has been involved in several research projects that investigated the operations and safety of different roadway and signal designs. Ruey Long Cheu, Ph.D., P.E., M.I.T.E., is an associate professor with the Department of Civil Engineering at the university of Texas at El Paso. He has served as the faculty advisor of the school’s student Institute of Transportation Engineers chapter. His research interests include intelligent transportation systems, traffic operations, and public transportation. He is a member of ITE. Yi Qi, Ph.D., is an associate professor and the chair of the Transportation Studies Department at Texas Southern university. She has been principal investigator for many research projects related to transportation safety analysis, roadway geometric designs, and traffic signal operations. Xiaoming Chen, Ph.D. is a research assistant professor at Texas Southern university, Houston, where he has served as a co-principal investigator for multiple research projects sponsored by the Texas Department of Transportation. His research interests include highway capacity, access management, and geometric design.