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Dynamic Late Merge Control Concept for Work Zones on Rural Freeways

by

Patrick T. McCoy

Department of Civil Engineering

W348 Nebraska Hall

University of Nebraska-Lincoln

Lincoln, Nebraska 68588-0531

(402) 472-5019

FAX (402) 472-8934

email: pmccoy@unlinfo.unl.edu

Geza Pesti

Department of Civil Engineering

W348 Nebraska Hall

University of Nebraska-Lincoln

Lincoln, Nebraska 68588-0531

(402) 472-1992

FAX (402) 472-8934

email: gpesti@unl.edu

Introduction

The single lane closure is a common application of temporary traffic control on rural freeways. It is used where one lane is closed to traffic to provide a work space. It is also used in advance of median crossovers where one roadway is closed to traffic to provide a work space and two-lane, two-way traffic is maintained on the other roadway.  Typically, drivers are advised of the lane closure by advance lane closed signs placed on both sides of the roadway at 1 mile and ½ mile in advance of the taper. In addition, lane reduction symbol signs are placed on both sides of the roadway at 1,500 feet in advance of the taper and a flashing arrow panel is usually placed at the beginning of the taper.

This traffic control plan normally works well during most hours of the day when traffic demand is less than the capacity of the open lane.  However, when the demand exceeds the capacity, congestion develops and problems occur. The shock wave associated with the developing congestion increases the potential for rear-end accidents, especially when the congestion extends upstream beyond the advance lane closure signs.  When this happens, unfamiliar drivers may not be prepared to stop, because they have not passed by the advance warning signs. They also have not been informed about which lane is closed. Therefore, unfamiliar drivers in the closed lane are not  prepared to move to the open lane and they may be enraged when blocked by slower vehicles attempting to prevent them from merging into the open lane ahead. Also, drivers, who are in the open lane, may be upset when passed by drivers in the closed lane.

There are two basic approaches used to address these problems (footnote 1). One approach is the Early Merge, which is designed to encourage drivers to merge into the open lane sooner than they would with the conventional merge. The other approach is the Late Merge, which is designed to encourage drivers to remain in their lanes until they reach the merge point at the lane closure taper.

Early Merge

Several schemes have been used to encourage drivers to merge into the open lane farther in advance of the lane closure. These strategies are of two basic types, static and dynamic.

Static Early Merge Strategies

Static forms of the Early Mergeprovide advance notice at a fixed distance ahead of the lane closure. These strategies include the placement of additional advance lane closed signs at approximately 1-mile intervals for several miles in advance of the lane closure.  The additional signs reduce the chances of drivers encountering congestion without knowing which lane is closed. This knowledge enables them to merge into the open lane before arriving at the end of the queue and having to squeeze into the open lane between vehicles in the queue.  Also, the early advance lane closure notice may reduce rear-end accident potential by alerting drivers to the possibility of congestion farther in advance of the lane closure. Simulations conducted by Nemeth and Rouphail (footnote 1)found that early merge control strategies significantly reduced the frequency of forced merges, especially at higher traffic volumes. On the other hand, simulations by Mousa et al (2)found that early merge control strategies increased the travel times through the work zone, because vehicles are more likely to be delayed over greater distances by slower vehicles ahead of them in the open lane. This may in turn increase the likelihood of drivers in the open lane attempting to use the closed lane to pass slower vehicles, which would increase the potential of lane-change accidents.  Some states (footnote 1)use lane drop arrows, rumble strips, and/or no-passing zones for distances of up to 1 mile or more in advance of the lane closure to discourage drivers from using the closed lane to pass.

Dynamic Early Merge Strategies

Dynamic forms of the Early Mergeprovide advance notice over a variable distance ahead of the lane closure based on real-time measurements of traffic conditions. One example is the Indiana Lane Merge developed by the Indiana Department of Transportation. This system creates a dynamic no-passing zone to encourage drivers to merge into the open lane before reaching the end of a queue caused by congestion, and to prohibit them from using the closed lane to pass vehicles in the queue and merge into the open lane ahead of them. The system uses sonic detectors to determine the presence of a queue in the open lane.  The detectors are mounted on DO NOT PASS signs with two flashing strobes and WHEN FLASHING supplementary plates. The signs are installed adjacent to the closed lane at ¼- to ½-mile intervals for up to 2.5 miles or more in advance of the lane closure. When stopped vehicles are detected in the open lane next to a sign, a signal is transmitted to turn on the flashing strobes on the next sign upstream. When vehicles are moving again, the strobes are shut off. In this way, the length of the no-passing zone is tailored to the length of congestion present.

The system was field tested during the 1997 construction season by the Indiana Department of Transportation. It was found to smooth the merging operations in advance of the lane closure. Drivers merged when they were supposed to merge and flow in the open lane was uniform with very few rear-end accidents. However, the system did not increase throughput, and the results of a simulation study by Purdue University (footnote 4)indicated that travel times through the work zone are longer.  The field tests also determined that the spacing of the signs should be logarithmic instead of uniform in order to account for the reduction in speed as traffic approaches the lane closure. Preliminary benefit-cost estimates by the Indiana Department of Transportation indicate that implementation of the system is justified at lane closures where the capacity of the single lane will be exceeded at least 15 to 20 times per week.

The University of Nebraska-Lincoln (UNL) conducted field studies (footnote 1)to compare the Indiana Lane Mergeand the conventional merge. The study sites were right lane closures. The lane distributions within 3000 feet of the lane closures observed in the field studies indicated that vehicles moved into the open lane sooner with the Indiana Lane Mergethan they did with the conventional merge. Also, the merging operations with the Indiana Lane Mergeoccurred more uniformly over a much longer distance than they did with the conventional merge, which were concentrated over a 500-foot section approximately 1200 feet in advance of the lane closure. Spreading the merging over a longer distance produced smoother merging operations. Only seven traffic conflicts were observed in 16 hours of data collection during moderate to high traffic volumes at the Indiana Lane Mergestudy site; whereas forced merges were observed at the rate of 20 or more per hour under comparable levels of traffic volume at the conventional mergestudy site.

Late Merge

The Late Mergeis opposite of the Early Mergein that it is designed to encourage drivers to use either the open or closed lane until they reach the merge point at the lane closure taper rather than merging as soon as possible into the open lane. One example of the Late Mergeis the system developed by the Pennsylvania Department of Transportation (PennDOT). This system was implemented as a means to reduce the road rage engendered between drivers who merge into the open lane early and those who remain in the closed lane and merge into the open lane near the front of the queue at the last minute. Approximately 1.5 miles in advance of the lane closure, USE BOTH LANES TO MERGE POINT signs are placed on both sides of the roadway. These signs are followed by conventional ROAD WORK AHEAD and advance lane closed signs. Finally, MERGE HERE TAKE YOUR TURN signs are placed on both sides of the roadway near the beginning of the taper.

The primary intent of the Late Mergedeveloped by the PennDOT is to reduce the road rage between early and late mergers by informing drivers that it is permissible for traffic to travel in both lanes to the merge point. Although it is not standard practice, the Late Mergeis used regularly at work zones on freeways by one PennDOT district office. No problems have been reported with its use, and it seems to be well received by drivers. A study of its operational effects found that it increased the capacity of the merging operations by as much as 15 percent (footnote 5).

Since the Late Mergeseems to address many of the problems experienced with the conventional mergeduring congestion, the UNL also conducted field studies (footnote 1)to compare the safety and operational effects of the PennDOTLate Mergeand the conventional merge. The results of these studies revealed that the conflict rates are substantially lower with the Late Merge. At higher densities, about 75 percent fewer forced merges and 30 percent fewer lane straddles were observed for the Late Merge; and, at densities below 25 vpm, no conflicts were observed for the Late Merge, whereas conflicts were observed for the conventional merge.  The studies also found the capacity of the Late Mergeto be nearly 20 percent higher than that of the conventional merge.

Conceptually the Late Mergeaddresses many of the problems that are associated with traffic operations in advance of lane closures at work zones on rural freeways, especially during periods of congestion.  In particular, the lengths of the queues that form as a result of congestion are reduced by about 50 percent, because the queued vehicles are stored in two lanes instead of only one. The shorter queue lengths reduce the likelihood of them extending back beyond the work zone's advance warning signs and surprising approaching drivers, which in turn reduces the potential of rear-end accidents. In addition, driver experience less anxiety about knowing which lane is closed, because either lane can be used to reach the merge point. The availability of both lanes also reduces the frustration levels of drivers. Drivers in the open lane are less likely to be irritated by others passing by them in the closed lane, because this maneuver is permissible with the Late Merge.  Drivers are able to select the lane with the shortest queue and not be concerned about others blocking their path to the merge point.

However, despite the obvious advantages of the Late Mergeduring peak traffic flow conditions, there is a concern about the safety of its operation during off-peak periods when traffic demand is below the capacity of the open lane and traffic speed is high. Under these conditions, it may be more difficult for drivers to decide who has the right-of-way.  This indecision could increase the potential for collisions at the merge point. This was not found to be a problem at the Late Merge study site in the UNL study (footnote 1).  At this location, drivers merged according to the Early Merge concept during off-peak periods, because the USE BOTH LANES TO MERGE POINT signs were followed by a series of advance lane closed signs which enabled them to merge into the open lane before reaching the merge point.  Although these signs may have improved the safety of the merging operation during the off-peak periods, they also seemed to reduce the effectiveness of the Late Mergeduring peak periods, because some drivers did not stay in the closed lane until the merge point. Instead, they slowed to merge into the open before the merge point. This behavior often resulted in some forced merges, which reduced both the safety and capacity of the merging operation.

Dilemma

The results of the UNL study (footnote 1) indicate that both the Early Mergeand Late Mergeprovide safer merging operations than the conventional merge. Both systems were observed to have lower merging conflict rates than the conventional merge. But, there is a concern about the potential for driver confusion at the merge point of the Late Merge, especially under high-speed, low-volume conditions, which could adversely affect safety. On the other hand, the Late Mergewas found to have a higher capacity than the conventional mergeand the Early Merge(footnote 1). The Late Merge's higher capacity and larger queue storage area reduce the probability of congestion extending back beyond the advance warning signs; thus, reducing the potential of rear-end collisions on the approach to the work zone. The higher capacity also reduces the duration of congestion, which in turn reduces the exposure to rear-end collisions. In addition, because of its higher capacity, the Late Mergereduces congestion delay; whereas, the Early Mergehas been found to increase travel times, especially under high traffic volumes (footnotes 3,4).

Based on these findings, the best system of merge control during peak periods is the Late Merge. However, because of the safety concerns regarding its operation under high-speed, low-volume conditions, the Late Mergemay not be the best system during off-peak periods.  Therefore, in order to maintain optimum merging operations at all times, it would be necessary to convert from the conventional merge during periods of uncongested flow to the Late Mergeduring periods of congested flow. In other words, a Dynamic Late Mergewould be needed.

Dynamic Late Merge Concept

The concept of the Dynamic Late Mergeis intended to resolve the aforementioned dilemma. Its goal is to provide the safest and most efficient merging operations at all times in advance of the lane closure by switching between the conventional mergeand the Late Merge, based on real-time measurements of traffic conditions. It would operate as the conventional mergeduring periods of uncongested flow, and as the Late Mergeduring congested flow conditions.

It is envisioned that the Dynamic Late Mergewould consist of a series of advance signs that would be activated to advise drivers to "use both lanes to the merge point" when congestion is detected in the open lane adjacent to the signs in a manner similar to the Indiana Lane Merge. A sign would also be placed at the merge point advising drivers to "merge and take their turn doing so."  When the congestion clears, the signs would be deactivated, or changed, to advise drivers of the lane closure and effect the conventional merge. The signs could be variable message signs equipped with traffic. Or, perhaps, the signs could be static signs equipped with traffic detectors and flashing strobes like the sign used in the Indiana Lane Merge.  Research is needed to determine the most effective sign message, type, and spacing. The length of signing in advance of the lane closure should be longer than the longest backup expected for the design flow rate and capacity of the work zone.

One important operations issue is the lane distribution between the open and closed lane prior to the switch from the conventional mode of merge control to the Late Merge. Under the conventional mode of merge control, drivers are encouraged to merge into the open lane.  Therefore, when the traffic volume approaches the capacity of the conventional mode of merge control, the speed of traffic in the open lane may be much lower than the speed of traffic remaining in the closed lane.  Consequently, when the system switches to the Late Merge, the accident potential may be high if drivers in the slower open lane attempt to merge into the higher speed closed lane before flow conditions in the two lanes are similar. Speed control and/or messages to advise drivers to stay in their lanes during the transition may be necessary to minimize this accident potential. Future research would determine the need for such measures.

Conclusion

Conventional traffic control plans for lane closures of rural freeways normally work well as long as congestion does not develop. But, when the traffic demand exceeds the capacity of the work zone, queues may extend back beyond the advance warning signs, often surprising approaching traffic and increasing the accident potential. Also, smooth and orderly merging operations may be lost when some drivers remain in the closed lane attempting to squeeze into the open lane at the head of queue while other drivers try to prevent drivers in the closed lane from passing them by straddling the centerline or traveling slowly in tandem with another vehicle in the closed lane. These erratic maneuvers tend to reduce the capacity of the merging operation and increase the accident potential and road rage among drivers.

The Late Mergeaddresses many of these problems, which occur during periods of congestion with the conventional mode of merge control.  Previous research (footnotes 1,5)has found that the Late Mergeimproves the safety and efficiency of merging operations, especially during congested flow conditions. However, when there is no congestion and speeds are high, potential confusion among drivers at the merge point becomes a concern. Therefore, the Dynamic Late Mergeconcept is proposed in the interest of providing the safest and most efficient merging operations at all times.

Research is needed to determine the driver information system necessary to effect the Dynamic Late Mergeconcept. Also, protocols for the transition between the conventional mergeand Late Merge need to be developed. These protocols would specify the traffic flow thresholds for transferring from one form of control to the other as well as the sequencing of the sign messages to effect the desired driver responses. Once a system prototype has been developed and found to perform successfully in operational tests, the roadway and traffic conditions necessary to justify the implementation of the concept should be identified.  It is the authors' opinion that this research is warranted by the improvements in the safety and efficiency of traffic operations in advance of lane closures on rural freeways that could potentially be realized by the deployment of the Dynamic Late Mergeconcept.

Acknowledgement

This paper was based on the results of Project No. SPR-PL-1(35) P513, Alternative Driver Information to Alleviate Work Zone-Related Delay, which was performed for the NDOR by the University of Nebraska-Lincoln.  The project monitors were Dan Waddle, Signing and Marking Engineer, and Dalyce Ronnau, Assistant Maintenance Engineer, NDOR. Dan Shamo, ITS Engineer, Indiana Department of Transportation, and Victor P. DeFazio, Assistant District Traffic Engineer, Engineering District 11-0, PennDOT, assisted the research team with the local arrangements for the conduct of the field studies of the Indiana Lane Mergeand Late Merge, respectively.

Disclaimer

The contents of this paper reflect the viewers of the authors who are solely responsible for the facts and accuracy of the data presented herein. The contents do not necessarily reflect the official views or policies of the NDOR or the PennDOT. The paper does not constitute a standard, specification, or regulation.

References

1. McCoy, P.T., G. Pesti, and P.S. Byrd. Alternative Information to Alleviate Work Zone-Related Delays: Final Report. NDOR Research Project No. SPR-PL-1(35)P513. Nebraska Department of Roads, Lincoln, Nebraska, February 1999.
2. Z.A. Nemeth and N.M. Rouphail. "Lane Closures at Freeway Work Zones: Simulation Study." Transportation Research Record869. 1982.
3. R.M. Mousa, N.M. Rouphail and F. Azadivar. "Integrating Microscopic Simulation and Optimization: Application to Freeway Work Zone Traffic Control." Transportation Research Record1254. 1990.
4. A. Tarko, S. Kanipakapatman, and J. Wasson. Modeling and Optimization of the Indiana Lane Merge Control System on Approaches to Freeway Work Zones. Final Report, FHWA/IN/JTRP-97/12. Purdue University, West Lafayette, Indiana. 1998.
5. Freeway Construction Work Zone Safety Enhancement Report. Orth-Rodgers & Associates, Inc, 230 South Broad Street, Philadelphia, Pennsylvania, January 1995.