Guidance for the Use of Dynamic Lane Merging Strategies

Transcription

1 Guidance for the Use of Dynamic Lane Merging Strategies November 2012 This material is based upon work supported by the Federal Highway Administration under Grant Agreement No. DTFH61-06-G American Traffic Safety Services Association

2 Guidance for the Use of Dynamic Lane Merging Strategies Objectives of this Document: Introduce innovative merging strategies and their benefits Discuss when and how to implement the merging strategies Present other key aspects to consider before and during implementation The Federal Highway Administration (FHWA) estimates that work zones accounted for approximately 10 percent of all the traffic delay in the United States and up to 24 percent of non-recurring delay (Federal Highway Administration, 2009). In addition to delays, work zones can contribute to an unsafe driving environment. Data from the Fatality Analysis Reporting System indicates that work zone-related crashes claimed 576 lives in 2010 with drivers and occupants accounting for approximately 85 percent of those fatalities. In addition, over 30,000 people are injured each year in work zone crashes (Federal Highway Administration, 2011). The nature of work zones and lane closures indicate that some delay is inevitable; however, certain lane closure and merging techniques are able to improve operations, reduce delay, and increase safety when used in appropriate situations. Merging techniques tend to generate speed differentials between the open lane and the closed lane (Chen, Qin, & Noyce, 2006) which can lead to aggressive driving maneuvers as vehicles use the nearly-empty closed lane to pass queued vehicles before darting back into the open lane just before the merge point. This type of driving increases the probability of work zone crashes (Hallmark, Mudgal, Stout, & Wang, 2011). Furthermore, some drivers take matters into their own hands by straddling the broken white markings to create rolling roadblocks (McCoy & Pesti, 2001). Although their intent is to deter aggressive drivers, it often aggravates drivers thus increasing road rage and aggressive driving (Hallmark et al., 2011). This document shows how two merging strategies, known as early merging and late merging, can be used either individually or cooperatively to reduce delay and increase safety at highway lane closures.

3 The early merging strategy (Figure 1) advises drivers to move out of the closed lane well before the forced merge point and before traffic breaks down. To avoid congestion, the early merging strategy works best when there is a low traffic volume on the road combined with high average speeds. Figure 1: Early Merging Concept Diagram The late merging strategy (Figure 2) works best when the road has a high traffic volume and low average speed due to congestion. Drivers are instructed to remain in their respective lanes until they reach the designated merge point. This ensures that both lanes are being used to their full capacity, and traffic flow is smooth by minimizing unnecessary lane changes. Once at the merge point, late merging encourages an alternating merging style, or zippered merging, in which cars take turns moving into the open lane. Figure 2: Late Merging Concept Diagram Early and late merging methods can be implemented in a static or dynamic manner. Static merging systems utilize static signs to instruct motorists on where to merge. A static early merge system is very similar to the traditional merging strategy where drivers are generally warned approximately one mile in advance. Static late merging, although uncommon, involves placing signs that instruct drivers to remain in their lanes until they reach the merge point. These static late merging signs may have flashing lights that are activated at certain times of the day based on historic traffic conditions data. Dynamic merging systems can alternate which merging technique it displays. It uses technology to monitor real-time traffic characteristics and convey instructions to motorists through changeable message signs or flashing indicators on static signs. Based on a site-specific algorithm, once a dynamic merging system detects a change in traffic conditions, it is able to switch from early to late merge or from late to early merge. While the term dynamic merging often refers to a system that uses the same changeable message signs to switch between early and late merging strategies, dynamic early or late merging may be used on its own. In the case of dynamic early merging, the merge point moves along with the end of the queue. In applications of only dynamic late merge, the changeable messages signs either display a late merge message or it defaults to a blank or cautionary message. The remainder of this document provides guidance on when and how to use dynamic lane merging during closure of one inside or outside lane on a two or three lane road. Based on the current body of research, other lane closure configurations, such as middle lane closures or closures on roads with more than three lanes, cannot be recommended. 1

4 When and How to Implement Each of the merging strategies can be successful when implemented under the right circumstances. Figure 3 should be used in combination with engineering judgment to aid in the decision making process for when to use static merging strategies (early or late) or a dynamic combination of early and late strategies. Figure 3: Decision diagram indicating guidelines for when to use various merging strategies for either two-to-one lane closures or three-to-two lane closures. Static techniques are most effective when there is a steady flow of traffic because they always display the same message throughout the duration of the lane closure. Under the right conditions, this less expensive method can produce similar results compared to the dynamic methods. Dynamic merging techniques are more useful in work zones that experience fluctuating traffic demands. Dynamic systems are programmed to change their message and merging strategy based on measurements of real-time traffic conditions such as speed, density, and occupancy. 2

5 Figure 4 demonstrates the successful implementation conditions of early and late merging associated with static and dynamic techniques. Figure 4: Conditions for effective application of merging strategies. Researchers and practitioners who have implemented dynamic lane merging systems utilized various methods to monitor real-time traffic conditions. The list below contains methods that have been used to test, evaluate, and implement dynamic lane merging strategies. Non-intrusive vehicle presence detectors o Microwave signals to identify traffic volume (Chen et al., 2006; Datta, Hartner, & Grillo, 2007; Datta, Schattler, Kar, & Guha, 2004; Grillo, Datta, & Hartner, 2008; Harb et al., 2009; Harb, Radwan, & Shaaban, 2010; Radwan, Harb, & Ramasamy, 2009) o Video analysis to track traffic volume, lane occupancy, and queue (Datta et al., 2007; Kang, Chang, & Paracha, 2006) o Doppler radar to determine average speed (Datta et al., 2007; Grillo et al., 2008) Pneumatic tubes to calculate traffic volume (Sperry, McDonald, Nambisan, & Pettit, 2009) Computer simulations to optimize dynamic merging algorithms (Beacher, Fontaine, & Garber, 2005; Harb, Radwan, & Dixit, 2010; Yulong & Leilei, 2007) 3

6 The cost of dynamic merging systems varies immensely based on the detection technology, number of changeable message signs, and product manufacturer. In general, a basic dynamic merging system could cost between $50,000-$60,000 to purchase and between $40,000-$45,000 to rent for one year (Datta et al., 2007; Radwan et al., 2009). One research study stated that the dynamic merging system increased the cost of a lane closure by $120/day when compared to standard lane closure methods (Beacher, Fontaine, & Garber, 2004); however, it has the potential to reduce delay and increase safety in the work zone. Recommendations for Implementation of Dynamic Merging Systems Two-to-One Lane Closures Most of the research on dynamic lane merging techniques has been completed on lane closures on roads which have two lanes of travel in one direction reduced down to one lane. This configuration is often referred to as a two-to-one lane closure. Dynamic late merging works best when traffic volumes are between 1200 and 1800 vehicles per hour with the most common implementation volume set at 1500 vehicles per hour (Datta et al., 2007; Grillo et al., 2008; Sperry et al., 2009; URS Corporation, 2004), although one study fell just outside that range by suggesting late merging strategies when volumes are anything greater than 1000 vehicles per hour (Radwan et al., 2009). Another study stated that results of the dynamic late merge strategy were only statistically significant when the percentage of heavy vehicles was greater than 20% due to the trucks slow acceleration rates (Beacher et al., 2004, 2005). Other researchers have cited low truck volumes to explain why their field results produced nonsignificant results (Harb, Radwan, & Dixit, 2010) or used higher truck volumes to design statistically significant simulation runs (Hallmark et al., 2011; Kang & Chang, 2011a, 2011b). Signing Early Merge Early merging systems consist of static or portable changeable message signs that typically say DO NOT PASS / WHEN FLASHING, as shown in Figure 5, to indicate that once a vehicle has merged into the open lane they should remain in that lane (Pesti et al., 2008; Tarko & Venugopal, 2001). Late Merge Late merging systems may consist of static or portable changeable message signs. One research group utilized a set of static signs prior to the changeable message signs at the beginning of the work zone warnings to inform trucks on which lanes to use (Datta et al., 2007; Grillo et al., 2008); however, the most widespread application uses only changeable message signs. The following messages used on changeable messages signs were found to be operationally successful, yet other messages may be used as long as they abide by MUTCD standards. STAY IN YOUR LANE / MERGE AHEAD (Radwan et al., 2009) USE BOTH LANES / TO MERGE POINT ; Last variable message sign always flashes TAKE YOUR TURN / MERGE HERE (International Road Dynamics, 2005; Kang et al., 2006) SLOW TRAFFIC AHEAD / USE BOTH LANES, STAY IN YOUR LANE / MERGE AHEAD XX MILES, TAKE YOUR TURN / MERGE HERE (Datta et al., 2007; Grillo et al., 2008) USE BOTH LANES, STOP TRAFFIC AHEAD, MERGE HERE, and TAKE TURNS (Chen et al., 2006) STAY IN YOUR LANE / MERGE AHEAD (Radwan et al., 2009) Figure 5: Dynamic early merge sign with flashing beacons (source: Minnesota Department of Transportation, 2008) Several researchers have also mentioned the importance of having a passive, or non-activated, mode for when the merge strategy is no longer necessary such as conditions with very little traffic flow. As an example, the passive mode could say LEFT (or RIGHT) LANE CLOSED / XX MILES AHEAD and LEFT (or RIGHT) LANE CLOSED (Datta et al., 2007; Grillo et al., 2008). 4

7 Figure 6: Example of static late merge signing (Hallmark et al., 2011) Figure 7: Example of four dynamic late merge messages using variable message signs (International Road Dynamics, 2005) Sample Layout Figure 8 presents a sample layout application of combination early and late merging for two-to-one lane closures, adapted from Typical Application 33 in the 2009 MUTCD. Additional signs and portable changeable message signs may be used, but this example intends to show a very basic layout. Further example layouts from published research studies are presented in the appendix. It should be noted that the layouts show the locations of traffic control features, but not detectors as detector use will vary from site to site. 5

8 Figure 8: Sample layout of a two-to-one dynamic merge design. Figure 9 depicts the use of static regulatory signs with flashing LED lights to restrict lane changes after an early merge point. The flashing lights essentially dictate when drivers should merge into the open lane. The signs can be spaced at large intervals to accommodate early merging, or they can be spaced closer together to assist with late merging. This example layout is intended to show the basic concept of using static regulatory signs in dynamic merging layouts; supplemental signs, such as MERGE HERE, may be added as desired or needed. 6

9 Figure 9: Example of a dynamic merge design utilizing regulatory signs with LED lights (Beacher et al., 2004). System Activation/Deactivation Thresholds When implementing a dynamic merging system, the most common traffic characteristics to monitor are speed and occupancy. Researchers used these characteristics as indicators for when to activate or switch between dynamic early and dynamic late merge. The University of Maryland found that changeable message signs used in late merging applications were most efficient when the sensors detected a vehicle at least 15% of the time; however, it should be deactivated when occupancy reduces to below 15%. Their research suggests that regardless of vehicle detection readings, the changeable message sign closest to the merge point should always display TAKE YOUR TURN / MERGE HERE (Kang et al., 2006). Researchers from the Korea Transportation Institute expanded upon the Maryland study by using simulation to establish a recommended activation threshold for early merging techniques. They found that early merging systems were optimized when the traffic occupancy was less than five percent (Kang & Chang, 2011a). This suggests that merging strategy algorithms can be programmed to implement a dynamic early merging system when occupancy is less than five percent, implement traditional merging when occupancy is greater than five but less than fifteen percent, and then utilize late merging when occupancy is greater than fifteen percent. Measuring the average speed of the traffic is another common method for determining when to activate dynamic merging strategies. No hard and fast rule exists as to how much the average speed should drop before activating late merging signs. In general, late merging strategies were activated (triggered) when the average speed dropped five to ten miles per hour below the posted speed limit for the work zone. The average speed was calculated in two to five minute intervals, and some required that the signs had minimum activation time of five minutes in order to avoid unnecessary switches between strategies (Datta et al., 2007; Grillo et al., 2008; Harb et al., 2009; Radwan et al., 2009). The late merging systems deactivated and returned to their default messages when the average speed rose above the trigger speeds. 7

10 Three-to-Two Lane Closures Research on three-to-two lane closures was less abundant compared to the two-to-one closures; however, this section summarizes the available information. Signing Practitioners may use the same or similar sign messages in three-to-two lane closures as were presented in Figure 5{Formatting Citation}, Figure 6, and Figure 7 {Formatting Citation}in the previous section, Two-to-One Lane Closures; however, instead of instructing drivers to use both lanes the changeable message sign might say USE ALL LANES / TO MERGE POINT. As a part of the Midwest Smart Work Zone Deployment Initiative, the Kansas Department of Transportation approved the development of a system that utilized both early and late merging strategies for a three-to-two lane closure (Meyer, 2004). While the lane closure was in place, standard work zone traffic control was supplemented by changeable message signs that either displayed a merging message pertaining to early or late merging; there was no passive mode in this application. For this design, sensors were placed downstream from each sign. The signs were capable of accommodating early merge messages and two types of late merge. One late merge strategy was used when the average speed of the vehicles was less than the requirements for early merging but when there is no queue. The second late merge strategy was used when a queue existed up to the speed sensors. The decision to display early or late merge messages depended on the speed of the vehicles at the downstream detectors. When the average speed of the vehicles was high, indicating no queue at the sensor, the associated changeable message sign displayed a message that stated CURRENT AVERAGE SPEED / XX MPH 1 MILE AHEAD. If traffic backed up past the speed sensors (i.e., low average speeds), the corresponding changeable message signs instructed drivers to USE ALL LANES TO MERGE / DO NOT PASS. Typical Layout Figure 8 can be modified to fit a three-to-two lane closure. Switching Thresholds In three-to-two lane closure configurations, average speed of the vehicles was the most common metric used to determine when to switch between merging strategies. Some studies employed a trigger speed which would switch merging strategies once the average speed dropped below the threshold for more than two to five minutes. In these types of strategies, there is usually a minimum activation time period of around five minutes (Radwan et al., 2009). One system with an estimated free flow speed of around 75 mph utilized a five mile per hour overlap between switching from late merge to early merge. If the system was operating as a late merge and the average speeds increased to above 51 mph, the message would switch from late merge to early merge. It would then switch back to late merge when the average speed decreased below 46 mph (Meyer, 2004). This ensured that the system was not constantly switching between early and late merging when the average speed was near the threshold. Another methodology for switching between merging strategies is to use sensors to detect a backup downstream (Datta et al., 2004). If there is a backup, then the merging strategy would switch from early or traditional merge to late merge. Benefits of Dynamic Merging Strategies Table 1 presents the recorded benefits of early and late merging strategies based on a conglomeration of current research. Cells with a green background indicate that there was a positive change from traditional merging strategies to either dynamic early merging or dynamic late merging and cells with a red background indicate a negative change. Blank cells simply indicate that the information was not a part of the project s analysis. As evident in the table, most of the current body of literature focuses on dynamic late merge applications compared to traditional merging resulting in more empty cells in the early merge section 8

11 than in the late merge section. It is also important to note that this table combines all data regardless of lane configuration or other work zone characteristics. Tests for statistical significance were uncommon; however, if tested, the results either were statistically significant or they were deemed to be of practical significance. Before implementing the strategies outlined in Table 1, strict care should be taken to ensure that work zone circumstances fall within the recommended guidelines presented in the subsequent sections{formatting Citation}. Table 1: Benefits of early and late merging strategies as presented in the literature. The numbers in the first row correspond to the number reference list in the appendix. Only studies that reported benefits are listed in this table. Special Considerations There are several extra aspects to consider prior to implementing a dynamic merging system such as where to place each sign, whether or not to involve police enforcement, and how to inform the driving public. Sign spacing, such as placement recommended in Figure 8, should be modified according to the geometry of the road, the expected queue length, and the average expected speed of the vehicles. Additionally, temporary traffic control plans should take into account how familiar the drivers are with the roadway and whether entrance or exit ramps are in close proximity to the merging area. Two studies suggested changeable message signs on both sides of the road at the taper to ensure that both lanes understand that the lanes are merging together (Datta et al., 2007; Grillo et al., 2008; Kang et al., 2006). Researchers have devised several strategies to improve public compliance with these new merging techniques. Public information campaigns can be used to alert the public about the lane closures and educate them on how the new merging systems work. Truck drivers should be advised to drive in the open lane in order to reduce the prevalence of rolling road blocks 9

12 (Datta et al., 2007). Police enforcement has proven to be very helpful when implementing new techniques, but they are especially helpful with early merging applications (Datta et al., 2007). Police should focus on reducing the number of aggressive drivers who are disobeying the signs, creating rolling road blocks, or driving on the shoulders. Throughout the deployment, it is pertinent to ensure that the changeable message signs are displaying reasonable messages otherwise the driving public will not believe in the reliability or reasonability of the system. This is where real-time sign feedback is beneficial. Lastly, many researchers have found it important to keep static merging signs separate from dynamic merging (Datta et al., 2007; Grillo et al., 2008; Kang et al., 2006). Not doing so could confuse the drivers and create a dangerous situation for the workers in the work zone and the other vehicles around them. 10

13 APPENDIX A References: 1. Beacher, A., Fontaine, M., & Garber, N. (2004). Evaluation of the Late Merge Work Zone Traffic Control Strategy. Charlottesville. 2. Beacher A, Fontaine M, Garber N. Field Evaluation of Late Merge Traffic Control in Work Zones. Transportation Research Record, 1911, Beacher, A., Fontaine, M., & Garber, N. (2005). Guidelines for Using Late Merge Traffic Control in Work Zones. Transportation Research Record, 1911, Chen, Y., Qin, X., & Noyce, D. A. (2006). Evaluation of Speed Management Strategies in Highway Work Zones. Proceedings of the 2006 Mid- Continent Transportation Research Symposium. 5. Datta, T., Hartner, C., & Grillo, L. (2007). Evaluation of the Dynamic Late Lane Merge System at Freeway Construction Work Zones. Detroit. 6. Datta, T., Schattler, K., Kar, P., & Guha, A. (2004). Development and Evaluation of an Advanced Dynamic Lane Merge Traffic Control System for 3 to 2 Lane Transition Areas in Work Zones. Detroit: Wayne State University, Transportation Research Group, Dept. of Civil & Environmental Engineering. 7. Federal Highway Administration. (2009). 10th Annual National Work Zone Awareness Week. Work Zone Safety and Mobility Fact Sheet. Retrieved June 14, 2011, from 8. Federal Highway Administration. (2011). Work Zone Safety Fact Sheet for Motorists. National Work Zone Awareness Week. Retrieved October 4, 2011, from 9. Grillo, L., Datta, T., & Hartner, C. (2008). Dynamic Late Lane Merge System at Freeway Construction Work Zones. Transportation Research Record, 2055, Hallmark, S., Mudgal, A., Stout, T., & Wang, B. (2011). Behavior Study of Merge Practices for Drivers at Work Zone Closures, Harb, R., Radwan, E., & Dixit, V. (2010). Comparing Three Lane Merging Schemes for Short Term Work Zones - A Simulation Study. Transportation Research Board 89th Annual Meeting, Harb, R., Radwan, E., & Shaaban, K. (2010). Two Simplified Dynamic Lane Merging Systems (SDLMS) for Three-to-Two Short Term Work Zone Lane Closure Configuration. Transportation Research Board 89th Annual Meeting, Harb, R., Radwan, E., Ramasamy, S., Abdel-aty, M., Pande, A., Shaaban, K., & Putcha, S. (2009). Two Simplified Dynamic Lane Merging System (SDLMS) for Short Term Work Zones. Transportation Research Board 88th Annual Meeting, Hicks, T., & Paracha, J. (2006). Guidelines for Late Lane Merge Concept, International Road Dynamics. (2005). Dynamic Lane Merge Systems. MdSHA. 16. Kang, K., & Chang, G. (2011a). An Integrated Control Algorithm of the Advanced Merge and Speed Limit Strategies at Highway Work Zones. Transportation Research Board 90th Annual Meeting, Kang, K., & Chang, G. (2011b). Evaluation of the Work Zone Merge Control Strategies Using the Extensive Simulation Experiments. Transportation Research Board 90th Annual Meeting, Kang, K., Chang, G., & Paracha, J. (2006). Dynamic Late Merge Control at Highway Work Zones: Evaluations, Observations, and Suggestions. Transportation Research Record, 1948, McCoy, P., & Pesti, G. (2001). Dynamic Late Merge-Control Concept for Work Zones on Rural Interstate Highways. Transportation Research Record, 1745, Meyer, E. (2004). Construction Area Late Merge (CALM) System. 21. Minnesota Department of Transportation. (2008). IWZ Toolbox: Guideline for Intelligent Work Zone System Selection (pp. 1-25). Retrieved from 11

14 22. Pesti, G., Wiles, P., Cheu, R. L. (Kelvin), Songchitruksa, P., Shelton, J., & Cooner, S. (2008). Traffic control strategies for congested freeways and work zones (Vol. 7). Texas Transportation Institute, Texas A & M University System. Retrieved from pdf 23. Radwan, E., Harb, R., & Ramasamy, S. (2009). Evaluation of Safety and Operational Effectiveness of Dynamic Lane Merge System in Florida. 24. Sperry, R., McDonald, T., Nambisan, S., & Pettit, R. (2009). Effectiveness of Dynamic Messaging on Driver Behavior for Late Merge Lane Road Closures. 25. Tarko, A. P., & Venugopal, S. (2001). Indiana Lane Merge System - Warrants for Use (pp ). West Lafayette. 26. URS Corporation. (2004). Evaluation of 2004 Dynamic Late Merge System for the Minnesota Department of Transportation, Wunderlich, K., & Hardesty, D. (2002). A Snapshot of Peak Summer Work Zone Activity Reported on State Road Closure and Construction Websites. Washington, D.C. Retrieved from Yulong, P., & Leilei, D. (2007). Study on Intelligent Lane Merge Control System for Freeway Work Zones IEEE Intelligent Transportation Systems Conference, IEEE. doi: /itsc

15 APPENDIX B Example Layout: Static Merge The 2009 MUTCD provides this example layout for static merge signing (TA-33). As discussed in this document, additional signs, such as DO NOT PASS may be applied to this setup. Figure 10: Example of a static merging layout without supplemental signs. 13

16 Example Layout: Static Late Merge The final report on the Evaluation of the Late Merge Work Zone Traffic Control Strategy contained an example of a basic late merging control layout utilizing mostly static signs (Beacher et al., 2004). Figure 11: Example of a late merging setup. 14

17 Example Layout: Dynamic Early Merge Wayne State University presented this layout of signing for dynamic early merge strategies in the Evaluation of the Dynamic Late Lane Merge System at Freeway Construction Work Zones (Datta et al., 2007). Figure 12: Dynamic early merge layout with flashing indicators on static signs. 15

18 Example Layout: Dynamic Late Merge International Road Dynamics presented this layout of a Maryland-based late merging system in a presentation on Dynamic Lane Merging Systems (International Road Dynamics, 2005; Kang et al., 2006). Other combination early-late merging systems may utilize a layout similar to this where the messages on the signs may be changed depending on whether the traffic requires early or late merging messages. Figure 13: Example of changeable message signs displaying the late merge messages. Figure 14: Typical layout of a late merging system 16

19 Example Layout: Dynamic Combination of Early and Late Merge Example of a combination early-late merging systems (Datta et al., 2007). Each cutout will be shown in detail on pages Figure 15a Figure 15c Figure 15b Figure 15d Figure 15: Example layout of a dynamic merging system that can switch between early and late merge messages. 17

20 Figure 15a: Dynamic merge system section one of four. Figure 15b: Dynamic merge system section two of four. Figure 15a: Dynamic merge system section one of four. 18

21 Figure 15c: Dynamic merge system section three of four. Figure 15b: Dynamic merge system section two of four. 19

22 Figure 15d: Dynamic merge system section four of four. Figure 15c: Dynamic merge system section three of four. 20

23 Example Layouts for Special Situations Signing Near Highway Ramps Wayne State University s final report gives sample layouts that consider adaptations for highway entrance and exit ramps (Datta et al., 2004). Figure Figure 15d: 15d: Dynamic merge system section four four of four. of four. Figure 16: Dynamic early merge signing at an on and off ramp. Figure Figure 15d: 15d: Dynamic merge system section four four of four. of four. Figure 16: Dynamic early merge signing at an on and off ramp. 21

24 Example Layouts for Special Situations Signing Near Highway Ramps Wayne State University s final report gives sample layouts that consider adaptations for highway entrance and exit ramps (Datta et al., 2004). Figure 16: Dynamic early merge signing at an on and off ramp

25 NOTES

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28 Developed by: The American Traffic Safety Services Association 15 Riverside Parkway Suite 100 Fredericksburg, VA Any opinions, findings, and conclusions or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the Federal Highway Administration.