**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 Merge* provide 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 Merge* provide 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 Merge* and 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 Merge* than they did with the *conventional merge*. Also,
the merging operations with the *Indiana Lane Merge* occurred 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
Merge* study site; whereas forced merges were observed at the rate of 20
or more per hour under comparable levels of traffic volume at the
*conventional merge* study site.

**LATE MERGE**

The *Late Merge* is opposite of the *Early Merge* in 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 Merge* is
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 Merge* developed 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 Merge* is 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 Merge* seems to address many of the problems experienced
with the *conventional merge* during congestion, the UNL also conducted
field studies *(footnote 1)* to compare the safety and operational
effects of the PennDOT *Late Merge* and 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 Merge* to be nearly 20 percent higher than that of
the *conventional merge*.

Conceptually the *Late Merge* addresses 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 Merge* during 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 Merge* during 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 Merge* and *Late Merge* provide 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 Merge* was found
to have a higher capacity than the *conventional merge* and 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 Merge* reduces congestion delay; whereas, the *Early
Merge* has 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 Merge* may 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 Merge* during periods of
congested flow. In other words, a *Dynamic Late Merge* would be needed.

**DYNAMIC LATE MERGE CONCEPT**

The concept of the *Dynamic Late Merge* is 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 merge* and the *Late Merge*,
based on real-time measurements of traffic conditions. It would operate
as the *conventional merge* during periods of uncongested flow, and as
the *Late Merge* during congested flow conditions.

It is envisioned that the *Dynamic Late Merge* would 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 Merge* addresses 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 Merge*
improves 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 Merge* concept 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 Merge* concept. Also, protocols for the
transition between the *conventional merge* and *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 Merge* concept.

**ACKNOWLEDGMENT**

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 Merge* and *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 Record* 869. 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 Record* 1254. 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.