Project report on Eye tracking interpretation system
UCI Study Finds Eliminating Left Turns Cuts Traffic Delay
1. UNIVERISTY OF CALIFORNIA,
IRVINE
Left-Turn Elimination to Improve Network Performance
THESIS
submitted in partial satisfaction of the requirement
for the degree of
MASTER OF SCIENCE
in Civil Engineering
by
Cunxiang Nicole Mi
Thesis Committee:
Professor Michael McNally, Chair
Professor Will Recker
Professor R.(Jay) Jayakrishnan
2013
3. ii
Table of Contents
List of Figures..................................................................................................................iv
List of Tables................................................................................................................... v
Acknowledgement...........................................................................................................vi
Abstract of the Thesis.....................................................................................................vii
Chapter 1 Literature Review.......................................................................................... 1
1.1 Access Management.............................................................................................. 1
1.2 Conventional Intersection Signal Operation.......................................................... 6
1.2.1 Signal Phasing................................................................................................. 6
1.2.2 Practice of Protected or Permissive Left-turn................................................... 7
1.2.3 Comparison of Signal Control and Innovative Design...................................... 9
1.3 Case of UPS Delivery Truck with No Left-turn Routing...................................... 10
1.4 Information Tech & Traffic................................................................................... 11
1.5 Motivation............................................................................................................ 12
Chapter 2 Assessment of Left-turn Delay.................................................................... 15
2.1 Intersection Delay Caused by Additional Left-turn Phasing ................................. 15
2.2 A Preliminary Study of Intersection Delay Related to Left-turn ........................... 16
Chapter 3. Methodology.............................................................................................. 19
3.1 Model Analysis Method........................................................................................ 19
3.2 Left-turn Elimination - Case of Right-In & Right Out............................................. 21
Chapter 4 Case Study of Grid Network ........................................................................ 24
4.1 Existing Network .................................................................................................. 24
4.2 Proposed Network .............................................................................................. 27
4.2.1 Proposed Network Before Application of Left-turn Elimination....................... 27
4.2.2 Proposed Application of Left-turn Elimination .............................................. 29
4.3 TransCAD and Synchro Models models ............................................................ 31
4.4 Preparation of OD-matrix .................................................................................... 32
4.5 Synchro Control Delay Evaluation ..................................................................... 35
4.5.1 Synchro User Guide Recommendation........................................................... 35
4. iii
4.5.2 Synchro User Guide Recommendation ....................................................... 36
4.6 Peak Hour Demand Models Before 1 and After 1 .............................................. 37
4.7 Peak Hour Demand Model After 2 With Improved Turn Penalty ........................ 43
4.8 Off Peak Demand Models with Final Turn Penalty (Light Traffic )..................... 46
4.9 Peak Hour Demand Models with No Turn Penalty............................................ 50
4.10 Model Network Performance Evaluation.......................................................... 54
4.10.1 Interpretation of Network Performance Parameters.................................... 56
4.10.2 Introduction of Network Performance in Measure of Fuel Consumption..... 58
4.11 Discussion of Model Assumption and Validation............................................... 61
4.12 Conclusion from Case Study............................................................................ 62
Chapter 5 . Future Study Recommendation ................................................................. 64
Biography........................................................................ Error! Bookmark not defined.
5. iv
List of Figures
Figure 1.1.2 Michigan Left (Barras, FHWA)...................................................................................................3
Figure 1.1.3 Partial DLT intersection. (FHWA) ..............................................................................................4
Figure 1.1.4 Paired Intersections (APPAT) ....................................................................................................5
Figure 1.1.5 Conflict movements diagram (FHWA) ......................................................................................6
Figure 3.1.1 Model Flow Chart...................................................................................................................21
Figure 3.2.1 Culver Plaza (Irvine, CA) Right-In & Right-Out Entry...............................................................22
Figure 4.1.1 Existing Network in Orange County, CA..................................................................................25
Figure 4.2.1.1 Existing Intersection Signal 8-phase Operation...................................................................27
Figure 4.2.1.2 Proposed 6 x 6 Node Network............................................................................................28
Figure 4.2.2.1 Proposed Application of Left-turn Elimination ...................................................................30
Figure 4.2.2.2 Intersection Signal 6-phase Operation after left-turn Elimination Applied.......................31
Figure 4.8.2 TransCAD Model After 3 Trip Assignment Result ..................................................................48
Figure 4.9.2 TransCAD Model After 4 Trip Assignment Result ..................................................................52
6. v
List of Tables
Table 1.2.1 8-Phase Operation.....................................................................................................................7
Table 1.2.2 6-Phase Operation.....................................................................................................................7
Table 1.2.3 4-Phase Operation .....................................................................................................................7
Table 2.2.1 Preliminary Study of Left-turn Delay......................................................................................17
Table 4.7.1 Synchro Model Before 1 & After 2 Cycle and Delay................................................................45
Table 3.6.3 Synchro Model Before 2 & After 3 Cycle and Delay...............................................................49
Table 4.6.3 Synchro Model Before 3 & After 4 Cycle and Delay...............................................................53
7. vi
Acknowledgement
I would like to express my deeply appreciation to my committee chair and graduate
advisor Professor Michael McNally for the opportunity and support of my research of
this project over the past year, with his knowledgeable advise, guidance, inspiration,
and encouragement. Without his persistent help, this thesis would have not be possible.
I also want thank other two professors on the committee, Professor Jay and Professor
Will Recker. I have learned from you in classes and you have always made yourself
available as a resource whenever I need your insight. Thank you very much for being of
such help and assistance to me. Thanks to the University of California Transportation
Center (UCTC) for sponsoring this study.
Lastly, I would like to thank two dear friends of mine, Yue Sun and Biling Liu, who have
help me in this study as well. Thank you very much for helping me on completing some
data process tasks. Your help is greatly appreciated.
8. vii
Abstract of the Thesis
Left-Turn Elimination to Improve Network Performance
by
Cunxiang Nicole Mi
Master of Science in Civil Engineering
University of California, Irvine 2013
Professor Michael McNally, Chair
Left-turn movement volume takes small percentage of approach volume, however case
delay to the majority of traffic flow at an intersection. Left-turn movement has longest
averaged delay at an intersection itself. The idea of eliminating left-turn movement is to
force a small number of left-turn trips to re-routing, as a results, all other trips would
have less delay at an intersection. Elimination of some left-turn movements at a defined
network may help improving network performance in term of network travel time, thus, a
higher level of system optimization can be achieved. A network analysis is performed to
compare network performance before and after left-turn elimination applied. Traffic
assignment result is expected to be different because of trips re-routing after left-turn
elimination.. Intersection control delay is expected to decrease at the intersection that
where left-turn elimination is applied. As a result, network travel time is expected to drop
because of the saving at intersection control delay. Signalized intersection control delay
is a key of this study. Trips re-routing may happen because of prohibited left-turn
movement, as well as the difference in turn penalty per movement if it is assigned,
which would affect shortest path calculation. Trip re-routing may cause increase in total
turn movement volume in network.
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Chapter 1 Literature Review
1.1 Access Management
There are many ways of managing left -a turn movement at an intersection. Access
management is a way to resolve conflict movements with special design of intersection
in geometry. A short summary and review of few common intersection access
management designs is presented as follow.
Roundabout. It is a commonly used intersection design across the country. It is
efficient in moving low volume traffic which has low environmental impact that
vehicles do not have to stop at the intersection. It slows down movements from all
directions and allows vehicle to flow around the central median counter clock wide to
get through the intersection and reach drivers' destinations. Compare with STOP
sign intersection, the advantage is that traffic can continue moving without a
complete stop. Traffic in all directions can move the same time.. However, the
disadvantage of roundabout is that it can only manage a low volume of traffic flow.
When traffic is heavy, geometry design gets complicated and both safety and
efficiency may become issues. The other concern about roundabout is that it is
difficult to accommodate pedestrian crossing the same time.
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Figure 1.1.1 Round-about (FHWA)
Michigan Left-turn. As one of innovative strategies in moving left-turn, Michigan
Left-turn has been practiced for years in the state of Michigan. Michigan Left-turn
have no left turn movements at the center of the intersection. All left-turn
movements are "offset" in location before the intersection of two approaches.
Thus, left-turn can be achieved by a combination of movements in series of right-
turn, u-turn and then through. As a result, the conflict movements has eliminated
to through and right-turn only. By eliminating the number of conflict movements,
delay can be reduced significantly. Michigan left-turn can efficiently handle heavy
vehicle demand at an intersection with little delay. However, there are some
shortcomings of Michigan left-turn. First of all, it takes larger space than normal
intersections do. Michigan left-turn takes place along a corridor. Therefore, the
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whole arterial needs large space. For the reason, they have to be design since
transportation planning phase. It is difficult to re-design later on to change an
intersection into Michigan left-turn because of the right-of-way constraint.
Secondly, Michigan left-turn turns all left turn movements into right-turn, as a
result all four right-turn movement volume would substantially increase, which
conflict with pedestrian movements. High volume of pedestrian demand would
cause large delay for all right-turn movements.
Figure 1.1.2 Michigan Left (Barras, FHWA)
Displaced Left-turn. Similar to Michigan left-turn, Displaced left-turn (DLT)
eliminates the number of conflict movements right at the intersection. However,
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there's an essential difference of DLT comparing with Michigan left-turn., wihch is
that Michigan Left has physically eliminated all left-turn movements by turning it
into right-turn, while DLT only relocate left-turn by offsetting it to happen before
intersection. In the other words, DLT still has the conflict between left-turn and
through movements, but the conflicts happen before the intersection. Compare
with Michigan left-turn, DLT has the advantage that it does not create additional
turning movement volumes at an intersection.
Figure 1.1.3 Partial DLT intersection. (FHWA)
Paired Intersection. There are some other unconventional intersections, such
as Jug-handle, Superstreet, Split Intersection, Paired Intersections, and so on.
They all have similar limitations that cause the popularity in practice. However, it
worthies to mention Paired Intersections that because it has similarities with
proposed network in the case analysis in this study.
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Paired Intersection proposes to pair up two or more intersections so that
alternative movements can be eliminated in one or other intersection. As it is
shown in the Figure1.1.4 , for a major corridor, through movement from side
street can be eliminated at the first intersection, while its allowed in the next
intersection where major street left-turn movement is eliminated instead. In such
case, the first intersection is an incomplete Superstreet (without u-turn portion)
and it’s complemented with the second intersection.
Figure 1.1.4 Paired Intersections (APPAT)
The core of access management strategies is to displace and diverse left turns
movements off the main intersection, thus, reduce the number of conflict movements,
as a result to achieve improvements in safety and efficiency under the number of
conflict movement at an intersection can is presented in Figure 1.1.5. As shown in
picture, there are total 16 crossing conflicts in an conventional intersection not including
pedestrian crossing. Many access management strategies are trying to reduce these 16
conflicts into only 4, which are 4 through movements in each direction.
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Figure 1.1.5 Conflict movements diagram (FHWA)
1.2 Conventional Intersection Signal Operation
1.2.1 Signal Phasing
The most common practise in California is a conventional four legs signalized
intersection, with having left-turn movement protected or permissive. Depending on
how a left turn movement is managed, generally speaking a 4-legs signalized
intersection may have 8, 6 or 4 phases with NEMA 2 rings phasing design. A left-turn
could be protected and permissive (5 signal heads) at some intersections. With 8-
phase operation, all four approaches left-turns are protected; 6-phase operation only
have two left-turn movements are protected and the other two are permissive; and 4-
phase operation all left-turn movements are permissive only. See Table 1.2.1, 1.2.2
and 1.2.3 for corresponding phase sequence and barriers for the three cases.
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Table 1.2.1 8-Phase Operation
Φ 1, EB-Left Φ 2, WB-Thru Φ 3, SB-Left Φ 4, NB-Thru
Φ 5, WB-Left Φ 6, EB-Thru Φ 7, NB-Left Φ 8, SB-Thru
Table 1.2.2 6-Phase Operation
Φ 1, EB-Left Φ 2, WB-Thru Φ 4, NB-Thru
Φ 5, WB-Left Φ 6, EB-Thru Φ 8, SB-Thru
Table 1.2.3 4-Phase Operation
Φ 2, WB-Thru Φ 4, NB-Thru
Φ 6, EB-Thru Φ 8, SB-Thru
1.2.2 Practice of Protected or Permissive Left-turn
Agencies may practise slightly different from one to the other for the warrants
check of protected left turn. Some may prefer to have protected left turn for every
signalized intersection at the first place unless there's obvious evidence showing
that it is absolute unnecessary, others may practise the opposite until a certain
warrants are satisfied.
There is a general guidance for warrants check to justify the need of protected
which provides a reference for agencies practices in California DOT provides
guidance for such warrants check in CA MUTCD 2012, Section 4D.19 Protected
16. 8
left turn phases should be considered where such alternatives couldn’t be
utilized, and one or more of the following conditions exist:
1. Collisions - Five or more left turn collisions for a particular left turn
movement during a recent 12-month period.
2. Delay - Left-turn delay of one or more vehicles, which were waiting at
the beginning of the green interval and are still remaining in the left turn
lane after at least 80% of the total number of cycles for one hour.
3. Volume - At new intersections where only estimated volumes are
available, the following criteria may be used. For pre-timed signal or a
background-cycle-controlled actuated signal, a left turn volume of more
than two vehicles per approach per cycle for a peak hour; or for a traffic-
actuated signal, 50 or more left turning vehicles per hour in one direction
with the product of the turning and conflicting through traffic during the
peak hour of 100,000 or more.
4. Miscellaneous. Other factors that might be considered include but are
not limited to: impaired sight distance due to horizontal or vertical
curvature, or where there are a large percentage of buses and trucks.
The warrants can be summarized into two main concerns: safety and efficiency.
Accident rate is one standard criteria that engineers base on to judge the need of
protected left turn. Such accidents mostly happen because of conflict movements
along with poor driver judgement calls, or limited sight distance. Other similar
17. 9
warrants, such as sight distance and conflict movements are all coming from the
safety concern, they can be absorb into this criteria. The judgement is that
protected left is necessary when it is no longer safe for driver to make left turn
movements or pedestrian crossing because of the conflicts.
The second major concern is delay. They can be expressed as conflict movement
counts or the measure of single vehicle delay. The judgement is that when there
is a long wait for a driver to find the time gap to make a left turn, and they are
good number of vehicles are queues for the same movement, it is necessary to
have a protected left turn signal head. In this case, both the safety and efficiency
are parallel, both improved by adapting a protected left turn phase when left turn
volume is adequately high. However, most of the time, safety and efficiency are
competing interests. When left turn volume is low, a protected left turn can be
very inefficient in moving both through movement and left turn itself. In
conclusion, the use of protected left turn could improve or harm efficiency
depending on the volume of left turn movements verse through movement
volumes.
1.2.3 Comparison of Signal Control and Innovative Design
In the state of California, existing infrastructure has most intersections signalized
control without special access management design. It is unrealistic to change
this infrastructure to cooperate the usage of many access management designs.
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First of all, it's costly to do road way improvement. Secondly, adequate space is
required for many design.
Compare with access management strategy, application of left-turn elimination
requires nearly no road way improvement done to the existing infrastructure. It
can be done through signal operation and a little bit of regulation, such as
signing. Study of the effectiveness of left-turn elimination is worthy because the
application is practical and less costly.
1.3 Case of UPS Delivery Truck with No Left-turn Routing
The case of UPS no left-turn routing has become a topic of discussion since its
implementation a few years ago. The public is surprised by how much UPS can save
with their deliveries by simply eliminating left-turns in routing by delivery trucks. In
2010, UPS had reduced 20.4 million miles off their routes, on top of that, 350,000
more packages were delivered. The environmental impact is that CO2 emissions
was diminished by 20,000 metric tons. Instead of making left-turn, right-turn
decrease safety hazards and delay, said Bob Stoffel, Senior VP of UPS. UPS has
proven that eliminating left turns in routing can result in reduction in both delay and
possibly vehicle-mile-travelled. This left-turn eliminating strategy is actually not new
to many experienced other taxi and truck drivers. Many expressed that they often try
to eliminate left-turn in their routing, especially at signalized intersections with heavy
traffic movements.
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1.4 Information Tech & Traffic
Information technology today provides drivers necessary information needed to
search for alternative route. GPS technology has been a tool for routing for drivers
for a long time. Now because of user population growth (especially with smart
phones), GPS also has become a technology that can provide information about
traffic conditions to drivers, such as traffic congestion, speed, etc. Smart phone is
the other recent technology that has changed many people life, especially their
driving behaviours with easily access information available in fingertips. When
driving to unknown area or with unknown traffic condition, many drivers would use
smart phone and GPS to get their route choices.
On the other end from the supply side, traffic signal controller technology has
evolved to response to demand changes. The new generation of controllers are
much more powerful with memory, functions, and other features, such as Intelligent
ACT, Econolite Cobalt, and so on. They are more standardized as a regular PC, or
even running a standard Android system. This means that a traffic controller
(intersection performance) information could be easier access by smart phone users
if agency allow sharing of controller information. Maybe in the near future, a smart
phone app can read controller information and transfer it to meaningful traffic
information for nearby smart phone users, or can even do routing base on all these
additional information. A smart phone app could read if a controller has protected
20. 12
left-turn phase in a certain approach (either permissive left-turn exist or no left-turn
allowed for approach).
Google driverless car has been a proven case of technology growth that has or will
have huge impact in traffic. It is not a dream any more. Driverless car can read and
responses to a traffic signal control, detect, response and adapt to driving
complicated driving environment. In such an environment, assumption can be made
that drivers would have known if a left-turn movement is prohibited through device
before route choice. Local drivers would adapt very quickly; and non-regional
drivers can always find out from their devices when they do route search.
1.5 Motivation
In the research proposed submitted to UCTC, the stated motivation for left-turn
elimination study as follow,
"Over the past century, the automobile has evolved to dominate transportation not only from a
behavioral perspective but from an infrastructure perspective. Thoroughfares that evolved over
millennia to serve many users were transformed in decades to the near exclusive use by motor
vehicles. The reasons for this evolution are well documented; alternatives to the behavioral
dominance, while numerous in terms of proposals and promise, are nevertheless constrained by
the infrastructural dominance. One option that has not been systematically studied but that has
the cost advantage of maintaining current infrastructure while addressing associated performance
impacts is a significant reduction in allowed arterial left turns. For current arterial infrastructure,
left turns consume a disproportion share of intersection capacity, pose the greatest restrictions on
non-automotive movements (pedestrians, in particular), and can cause excessive delay in low
volume operation. Driver behavior has already become habitual, with the ability to turn left
assumed at every intersection, thus limiting the potential to remove left turns, even at problematic
locations. The evolving technology that accommodates this proposal is the growing presence of
21. 13
GPS devices in automobiles and the growing familiarity of drivers with communication
technology. The question to be addressed is to assess the potential for performance
improvements, direct and indirect, from the systematic elimination of left turns."
There are some other challenges of traffic engineering maybe resolved by the
alternative of eliminating some left-turn, such as corridor coordination problems.
One issue with coordination is the dilemma of if ped phase split have to cover ped-walk
time. Covering pedestrian crossing time may result in an inefficient large length, while
not covering would result in drop-off of coordination whenever pedestrian place a call.
For a large intersection (size large in geometry), such as many major intersections in
city of Irvine, with new pedestrian speed of 3.5 ft/s, a FDW time can easily go high up to
35 seconds, as a result:
7 s (Walk) + 35s (FDW) + 4 s (Yellow) + 1 s (AR) = 47 second (min split per thru phase)
This means that disregard of how light side street traffic may be (or both approach), a
thru phase has to have min. of 47 seconds splits, which would result in up to 150
second of cycle length for a fully 8 phase intersection. In order to coordinate signals,
every intersection has to have the same cycle length. 150 second cycle is inefficient in
general because with large cycle, every approach has to wait for long time for service
green time, especially with a very small side street intersection with the major corridor,
150 s cycle is very unnecessary. However, it has to be done if coordination is needed to
run for the corridor. Some people would suggest to run smaller cycle length while give
less than pedestrian crossing time for each pedestrian phase. The problem with that is,
with decent pedestrian demand, the coordination would be drop off and signal run free
every moment when a pedestrian place a call, as a result, coordination can rarely be
22. 14
performed. This may even benefit left-turn vehicles by forcing them to find alternative
route, rather than wait for a 150s cycle. Usually, left-turn has the longest delay because
of short split of cycle and service rate of once per 150 seconds. City of Irvine, CA has
implemented coordination plans on Culver and Jeffrey with high cycle length of 150
seconds.
Eliminating left-turn movements for or both street would change an 8-phase operation to
6 or 4 phases, as a result, a large cycle length can be reduced by the left-turn phase
eliminations.
The other concern/ issue with coordination is that, along a corridor, some intersections
have very high Volume/Capacity ratio and poor performance while others may have
lower V/C ratio. In the other words, the ICU (Intersection Capacity Utility?) are not
balanced along the corridor. By eliminating left-turn in some heavy traffic intersection
(high ICU) while allowing neighbouring intersection (low ICU) left-turn, would push the
left-turn demand toward the low ICU intersections, which have the capacity to take in
higher demand. Elimination of left-turn for selected intersections along a corridor would
improve the overall performance.
As mentioned in earlier, at an intersection that has heavy through traffic while left-turn
traffic is light, it does not make sense to let the whole intersection movements to
experience the delay just for that few left-turn movement, in the mean time while they
can find alternative routes easily instead of making the left-turn at the intersection. Thus,
left-turn elimination is reasonable solution for this situation.
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Chapter 2 Assessment of Left-turn Delay
2.1 Intersection Delay Caused by Additional Left-turn Phasing
It may be obvious to many people that left turn movement has the most delay itself,
and it causes delay to other movements. It takes up a big portion of intersection
control delays. However, for a signalized intersection, depending on the control type,
not all the left turn movements would cause the same level of delay to an
intersection. Left turns that are controlled by Protected, permissive or protected&
permissive, the three type of phases, have different impacts of delays on an
intersection. (Here, assume all the three types of phasing used are all feasible to
demands.) Normally, permissive left turn has the least delay impact to the
intersection while protected left turn causes the most delay. For protected left turn,
the minimum delay caused by the additional left-turn phasing can be estimated as
follow:
Delay per Phase = min. green (4 s) + detector extension (3 s) + Yellow (4 s) +All Red
(1 s) = 12 seconds
In the other word, if 1 phase less, cycle length can be reduced by at least of 12
seconds. Therefore, fully 8 phases operated signal has the most delay of all
(comparing with 6 phases and 4 phases). As a result, it's the case that has the most
potential of reducing the maximum delay by eliminating left turn movements.
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2.2 A Preliminary Study of Intersection Delay Related to Left-turn
A study of intersection delay caused by left turn movements is done for cases of 8
phases signal control intersections. Intersections are sampled to cover various
arterial and demand levels from real world. Synchro 8, HCM 2010 standard is used
for this study. Delays and LOS are evaluated for an intersection with the case of
with and without left turn.
For comparison purpose, when left turn movements are eliminated from an
intersection, the volume on left turns are added to corresponding through movement
volume. In other word, comparison of same intersection with "same" demands are
done for the cases of left turn allowed and left turn not allowed. Peak hour turn
movement data for intersections listed in table were from City of Irvine and SANBAG
projects order of turn movement counts.
Results from the study shows that by eliminating left turn movements, delays for
every other movements are reduced and the whole intersection LOS can be
improved greatly. There are many factors would affect the exact improvements. It is
not necessary a linear relationship with the number of left turn movement eliminated.
It also related to how the geometry of intersection, exact timing operation, relative
and absolute volumes of each movements of the intersection. However, the table
below summarizes the results of some category calculations, which can provides
some information of the impacts of left turn movements to intersection delays. See
Table 2.2.1 for summary of results.
25. 17
Table 2.2.1 Preliminary Study of Left-turn Delay
IntersectionwithVolume
at Time Period
Intersection
DelaywithLT
Intersection
Delaywithout
LT
Delay
Reduction
(%)
Delay
Reduction
(s/veh)
Cumulative
Delay
Reduction(veh-
Rate =DelayReduction(s)/
EliminatedMovement
Volume (veh/hr)
LOSwithLT LOSWithout LT
Culver & UniversityDr AM 35.8 24.4 31.8% 11.4 17.5 71.4 D C
Culver & UniversityDr OffPeak 18.4 9.3 49.5% 9.1 7.4 61.3 B A
Micheson & JeffreyRd PM 37 19.5 47.3% 17.5 23.6 187.4 D B
Campus & UniversityDr PM 44.4 14.8 66.7% 29.6 38.8 170.1 D B
Campus & UniversityDr OffPeak 20.3 8.8 56.7% 11.5 7.3 88.7 C A
Harvard & UniversityDr PM 24.7 13.2 46.6% 11.5 13.7 75.0 C B
Harvard & UniversityDr OffPeak 19.4 8.2 57.7% 11.2 4.4 98.9 B A
Euclid AM& Chino Ave AM 16 10.1 36.9% 5.9 2.9 68.9 B B
Madrugada Dr & Grand AM 69.4 6.7 90.3% 62.7 34.7 389.3 E A
Peyton Dr & Valle Vista Dr AM 20.1 5.5 72.6% 14.6 5.1 186.0 C A
Edison Ave & Fern Ave AM 16 14 12.5% 2.0 0.7 38.5 B B
Peyton Dr & Eucalyptus Ave AM 23.9 13.2 44.8% 10.7 5.3 77.0 C B
26. 18
As indicated in Table 2.2.1, for a congested large intersection, the delay reduction
per hour per intersection can be up to 38.8 vehicle-hour (or more). Thus, the
potential of saving by eliminating left-turn movements at a heavy traffic large
intersection is large.
An important note of this study relative to proposed research topic is that, this result
provides information about the magnitude of intersection delays that is caused by left
turn movements. The result answers the question that "Can elimination of left turn
movements improve an intersection performance?" The answer is that "almost
absolutely yes." It does not answer the question that, "can left turns movement be
eliminated, or should it be eliminated for the intersection". Furthermore, this study
only limited to an intersection level, not a network level. In a network setting, the
system performance could be worse, and no conclusion can be made at this point.
27. 19
Chapter 3. Methodology
3.1 Model Analysis Method
Fewer left-turn movements at a signalized intersection contribute to the delay of majority
of other vehicles. The benefit of little volume of left-turn does not seem justify the huge
cumulative delay from other movements because of it. Assumption can be made that a
better system optimization can be achieved if some Left-turn movements are eliminated
in selective locations. The eliminated Left-turn movements have to re-routed with a new
path that either with same travel distance or longer. Therefore, a total vehicle mile
traveled may increase slightly in term of network as a whole, however, vehicle hour
travel is expected to decrease because of expected reduction in delay at intersection for
the rest of movements.
In general, a vehicle trip travel cost can be expressed as a summation of vehicle-mile-
travel and vehicle-hour-travel,
Travel Cost = VMT + VHT
In order to take into account of the delay difference at an intersection, vehicle-hour-
travel can be calculated as total of link travel time and intersection control delay time,
Travel Time = Link Travel Time + Intersection Control Delay
In order to evaluate network performance before and after left-turn elimination applied,
traffic demand applied has to be the same. A general OD-Matrix is generated to the
level of peak hour demand for selected network.
28. 20
TransCAD is utilized in this study to perform traffic assignment to predict the resulting
traffic demand due to the re-routing after left-turn elimination applied. Because of left-
turn elimination, the corresponding control delay is expected to reduce. In order capture
the assignment difference due to the reduction in delay at certain location, turn penalty
is used in traffic assignment. For the intersections that have left-turn elimination, turn
penalty is reduced.
Synchro is used to analysis intersection control delay. It has been demonstrated in
literature review that that intersection signal control delay usually decrease big
percentage after left-turn elimination applied. However, traffic demand is different after
left-turn elimination applied because of re-routing. Therefore, the assumption of reduce
in delay due to left-turn elimination has to be examined with new traffic demand after
left-turn elimination applied. Using TransCAD model output of turn movement, Synchro
model can estimated intersection signal control delay, which the result should be
compared with TransCAD model turn penalty that was assumed at beginning. They
should be relatively close when the models converge.
The general procedure of this analysis can be summarized in the flow-chart presented
in Figure 3.2.1, which model inputs and outputs are shown:
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Figure 3.1.1 Model Flow Chart
3.2 Left-turn Elimination - Case of Right-In & Right Out
There may be many cases that left-turn elimination could benefit the network
performance and worth an analysis. It worth to mention that the case of right-in and
right-out is a success application of left-turn elimination in some locations. For
comparison purpose, pictures below for the two cases that where the traffic environment
is similar for an entry to a shopping plaza from a major arterial, however, have different
geometry design.
The first case is the entry to Culver Plaza from Culver Dr. This is an un-signalized T-
intersection that only Right-In & Right-Out is allowed. An exclusive North-bound left-turn
lane is designed for permissive left-turn. The left-turn movement out from driveway is
eliminated. However, this left-turn movement can be easily re-routed with other
alternatives routes available within the plaza. Only one left-turn movement coming out
from plaza is eliminated at this intersection, as a result, the major flow of traffic on
Culver are free from interruption. The rest of movements are free to move whenever it
30. 22
is safe. Because of the previous signal control intersection, there's time gap that both
the right-turn out from driveway and the northbound left-turn can happen without much
of waiting. See Figure 3.2.1 for the case geometry (picture from Google Maps).
Figure 3.2.1 Culver Plaza (Irvine, CA) Right-In & Right-Out Entry
The second case is the other T-intersection of plaza entry driveway and Jeffrey Dr.
Jeffrey Dr has about ten times of traffic volume than side street has during peak hour.
However, because that it is a signalized control intersection, traffic flow on Jeffrey Dr
31. 23
would get interrupted every signal cycle for any random call of traffic that going in or out
of the plaza at this intersection. Because the intersection is so close to the adjacent
signalized intersections, it is usually operated in full cycle in coordination to avoid
queue built up during peak hour. As the result, both left-turn movements in and out to
and from the plaza cause delay to the major traffic flow and have long delay
themselves. Figure 3.2.2 shows the geometry layout of this case (picture from Google
Maps).
Figure 3.2.2 Trabuco Grove Plaza (Irvine, CA) Entry -Signalized T-Intersection
32. 24
Chapter 4 Case Study of Grid Network
A key element of left-turn elimination is that alternative routes must be present that
driver can easily re-route. A grid network is selected as a case study for the reason that
drivers have many options to re-route. Once a left-turn movement is eliminated at an
intersection, assuming that GPS is available to all drivers, the best route can be found
within alternatives.
4.1 Existing Network
Picture 4.1.1 shows the layout of an existing grid network in Orange County, California.
The plan of this study is to fabricate such a network in models and carry out a network
performance study. Due to limitation of scope of study, only half-mile link is capture in
model network. There are many other minor street links that are not presented in
models.
From observation of such network, arterials or links are defined into 3 levels according
to geometry, capacity, traffic demand and others. The characters of each level is
summarized as following:
1st level: 3- through lanes in each direction, speed limit 45 mph, raised median
in the middle mostly no street parking (connected to freeway On/Off ramps)
2nd level: 2 - through lanes in each directions, speed limit 40 mph, raised
median in the middle (connected to freeway ON/OFF ramps)
3rd level: 2 - through lanes in each directions, speed limit 40 mph, no raised
median, street parking. (does not have freeway access)
34. 26
They are in order of: 1st - 3rd - 2ed - 3rd - 1st - 3rd - 2ed - 3rd ... in horizontal or
vertical. There is freeway access every 1 mile. 3rd level street does not have freeway
access, thus, has less demand than 2ed level, even tho they share the about the same
number of lanes and speed limits
Special note about Beach Blvd. Beach Blvd is used to be a highway which has 4 lanes
in each direction. The same level of arterial with 4 lanes each direction cannot be found
repeating in horizontal or vertical. Beach Blvd is special and it has higher demand than
normal 1st level defined above.
Information of real world turn movement counts or link flow data is available from OCTA
synchronization project order of count data. Magnolia (2nd level) peak hour count:
range 700-1100 VPH each direction. Beach Blvd (higher than 1st level) peak hour
count: range 1800-2400 VPH each direction.
Once a network is defined for analysis, an OD demand needs to be estimated or
assumed. According to the real world count data, the target final O-D demand for each
level arterial is decided to be as follow:
1. 1st level (3 lanes) demand: a bit less than Beach Blvd (4 lanes) demand, at
about 1600-2000 vph (link volume)
2.2ed level demand: same as Magnolia demand or very slight higher at 800-1200
vph (link volume)
3. 3rd level demand: a bit less than Magnolia demand at about 500-800 vph (link
volume)
35. 27
In the existing network, every half-mile link intersection is a signalized intersection with
8 - phase operation, which means that all movements are allowed at any intersection
and all left-turn movements are protected. See figure below for an example of
intersection signal control phasing:
Figure 4.2.1.1 Existing Intersection Signal 8-phase Operation
4.2 Proposed Network
4.2.1 Proposed Network Before Application of Left-turn Elimination
Network size is chosen to be 6 x 6 nodes. Each link length is pre-defined as 0.5 mile.
There are two centroids for the network. See figure 4.2.1.2 for a brief representation
of the network layout.
37. 29
4.2.2 Proposed Application of Left-turn Elimination
In the existing network, every half-mile link intersection is a signalized intersection
with 8 - phase operation, which means that every left-turn movement is protected.
A left-turn elimination is proposed to every intersection in this grid network system,
however, only left-turn movements on one of the two arterials - either Northbound
and Southbound left-turns eliminated or Eastbound and Westbound ones. There are
no permissive left-turn allowed, left-turn movements are simply prohibited on one of
the two intersected arterials. As a result, every intersection only has 6-phase
operations with the rest of two left-turn movements still protected. With such an
alternative direction of left-turn elimination, traffic is still free to circulate easily within
the half-mile block. Hint that alternative routes are available for the eliminated left-
turn movements.
See Picture 4.2.2.1 below for brief representation of proposed left-turn elimination
layout. Only left-turn movements marked are allowed for the intersection. For
analysis propose, the outside circle intersections still remain 8-phase operations, no
left-turn elimination applied, acting as a buffer of analysis zone.
39. 31
With left-turn elimination applied, a 6-phase intersection signal phasing diagram can be
presented in picture below: (the case of EBL and WBL are eliminated)
Figure 4.2.2.2 Intersection Signal 6-phase Operation after left-turn Elimination Applied
4.3 TransCAD and Synchro Models
TransCAD and Synchro Models are built with network and lane geometry as described
above, including signal control setting in Synchro models. A “Before” models is the
existing network without left-turn elimination applied. An “After” model is the model that
with left-turn elimination applied to alternative direction as described in proposed
network. For the left-turn eliminated intersection, Synchro model has only 6-phase
operation, and TransCAD model has turn penalty of turn movement prohibited applied.
40. 32
For the simplicity, U-turn movement is prohibited at any intersection to all models in this
study.
The goal of this study is to evaluate and compare the “Before” and “After” network
performance in term of network cumulative vehicle-mile-travel, vehicle-hour-travel and
intersection signal control delay.
4.4 Preparation of OD-matrix
The emphasis of this study is on the Assignment step of transportation planning that
can capture the route choice behaviour once a left-turn movement is prohibited in a
location. Trip generation, distribution and mode choice are not the focus of this study.
Considering the difficulties and accuracy of taking a small network from a countywide
network, the feasible solution is to get a OD - matrix of demand is to assume a
reasonable O and D volume for each external nodes and internal cancroids, and then
adjust the gravity factors of distribution model until an O-D matric is obtain that
reasonable hourly link volume resulted for each level of arterial from Assignment.
O-D between adjacent external nodes are zero out since they are essentially “U-turn”
volume that they do get into or pass through the network. The final assignment result
shows an hourly link volume that it is close to the peak hour volume with real world turn
count volume of the same level arterial. The final OD-matrix is used to both “Before” and
“After” network TransCAD assignments. The resulting turn movements from each
assignment are input turn movement volumes to the corresponding Synchro models.
41. 33
Table 4.4.1 is the finalized OD-Matrix (hourly) that shows the OD paired volume
between all external nodes and the two internal centroids. (See node location labelled in
trip assignment result pictures).
43. 35
4.5 Synchro Control Delay Evaluation
4.5.1 Synchro User Guide Recommendation
TransCAD model assume a set of turn penalty values in assignment process. To further
evaluate the assumed turn penalty, intersection signal delay is evaluated through
Synchro model using TransCAD resulting turn movements as inputs. In Synchro guide,
Synchro delay is recommended over HCM Signal report delay when evaluating
actuated signal parameters, optimizing offsets, detail modelling of coordination and
actuated signals are needed.
" Synchro's core delay calculation is called the Percentile Delay Method. The percentile delay
calculation looks at five levels of traffic arrivals so that actuated signal can be evaluated under
varying traffic loads. This allows the percentile delay method to capture and rationally model the
non-linear behavior of actuated signals.
The percentile Delay calculations in Synchro are also interval based. Vehicle arrivals from
adjacent intersections are evaluated in intervals to determine the influence of coordination.
The calculations for The percentile Delay Method can be quite complex, multiple intervals to be
evaluated with detailed information about arrival patterns from adjacent signals. The HCM Delay
equation (Webster's Formulation), can be calculated by hand. " (Synchro Studio 8 User Guide,
p262-263).
Absolute values of a signal control delay is essentials in this study because it is used to
calculated network cumulative signal control delay as part of travel cost (travel time).
Since this study involves with signal timing parameters changes (from 8-phase to 6-
phase operation), Synchro signal control delay sounds a better fit for this purpose as
Synchro guide recommended.
44. 36
4.5.2 Synchro User Guide Recommendation
A further look of comparison of these two delay calculations is worthy when data is
available. Thanks to Advantec consultant engineers and project owning agency,
SANDBAG, of tiers 3 & 4 project, an access of some delay data is available.
A comparison of travel time delay, synchro control delay and HCM 2010 delay for same
intersections are present in table blow. Since travel time of a corridor was only carried
out through intersections, only through movement delay can be evaluated. Thus, all
delay values calculated using Synchro Control delay method and HCM 2010 are for
through movements only for the selected intersections.
A corridor travel time study is carried out through GPS data collection during peak
period. Drivers are driving along the corridor to collect trip data. Travel Time Delay (TT
Delay) at each intersection is calculated from GPS trip data. In this study, 10 round trips
were collected and the TT Delay at each intersection (per direction) is the averaged
over the 10 trips. For some intersections, because of special signal timing setting,
HCM2010 delay was not able to be reported. See Table below for delay value resulting
from the two methods comparing with real world travel time delay value. Over total 26 of
comparison, 17 of Signal Control Delay has closer values to TT Delay, which says that
73 % of Synchro control delay estimation is more accurate than HCM 2010 delay
estimation. See Table 4.5.2.1 for details of comparison.
This is just a random corridor travel time delay data that the meaning of this result may
not be big enough to draw a conclusion. A bigger scope of statistics can be done for
such a comparison in future. In this study, such a quick comparison just provides a
45. 37
further confirmation of what delay estimation method may be used in delay analysis in
Synchro model.
Table 4.5.2.1 Comparison of Delay Estimators of HCM 2010 Delay and Synchro Control Delay
4.6 Peak Hour Demand Models Before 1 and After 1
Peak hour traffic demand has been determined in section 4.2 for this network analysis.
This OD-matrix is used to do network assignments to all peak hour models. A set of turn
Corridor: Arrow Hwy
Jurisdiction:
Limit:
AM Peak After Study : 9/25/2013 10 Round Trips 10 Round Trips
Node Control Dealy TT Delay HCM 2010 Node Control Dealy TT Delay HCM 2010
to Vineyard Ave 53.7 29.7 42.6 to Hellman Ave 6.5 3.9 8.5
to Hermosa Ave 2.6 20.8 0.3 to Hermosa Ave 3.2 4.8 0.4
to Center Ave 4.7 4.0 15.2 to Center Ave 5.9 0.1 20.7
to Haven Ave 28 22.8 20.7 to Haven Ave 24.9 17.5 22
to Red Oak St 1.7 17.2 12.4 to Red Oak St 8.3 7.3 8.5
to White Oak St 1.6 23.8 0.1 to White Oak St 6.5 2.3 0.7
to Rochester Ave 4 34.7 2
Node Control Dealy TT Delay HCM 2010 Node Control Dealy TT Delay HCM 2010
to Rochester Ave 13.1 49.5 11.5 to White Oak St 7.5 11.9 1
to White Oak St 2.5 11.7 15.6 to Red Oak St 4 18.2 0.4
to Red Oak St 5.8 12.7 0.2 to Haven Ave 37.9 33.5 48
to Haven Ave 41.3 32.7 39.6 to Center Ave 5.1 3.7 0.5
to Center Ave 2.6 10.7 2 to Hermosa Ave 6.4 2.9 20.8
to Hermosa Ave 6.8 20.7 7.9
to Hellman Ave 5.6 21.7 7.4
to Vineyard Ave 33.5 34.2 38.2
Eastbound Trips
Westbound Trips
Eastbound Trips
Westbound Trips
Rancho Cucamonga
From Grove Ave to Etiwanda Ave
PM Peak After Study : 9/25/2013
46. 38
penalty values for left-turn, through and right-turn movements has to be determined in
order proceed the traffic assignment steps.
The first set of turn penalty values is an estimation based on delay information from
preliminary delay study section. For the existing network, Before model, with 8-phase
operation, Left-turn, through and Right-turn movements average delay in general is
estimated to be 45, 30 and 30 seconds, which are the global turn penalty values
assigned in TransCAD Before model. As shown in preliminary delay study section,
movement delays are expected to be reduced once some left-turn movements (and
phases) are eliminated. The estimated delay per movement after reduction are
assumed to be 35, 20 and 20 seconds for left-turn, through and right-turn movements.
The reduced delays are used as turn penalty for intersections that have left-turn
elimination applied (internal nodes 4 x 4) in After model. Nodes on the border of the
network still have the same turn penalty assigned as in Before model since no left-turn
elimination applied.
See Picture 4.6.1 for Before 1 network model trip assignment result; see Picture 4.6.2
for the After 1 network model trip assignment result with the first set of assumed turn
penalty applied
Note that, centroid connectors have zero turn penalty from and to network links and flow
capacities are set to relatively large for all models.
48. 40
Figure 4.6.2 TransCAD Model After 1 Trip Assignment Result
As assignment results observed, one main comment can be made is that traffic on
outside links and nodes seem to be attracted to use inside links more in After model
than in Before. The explanation can be of the reduced turn penalty applied to inside
nodes. With 10 seconds of reduction in travel time per intersection, some trips may
have changed the shortest path favour toward using internal links and nodes. Question
could be asked is that, is the assumption of 10 seconds reduction in turn penalty per
movement after left-turn elimination applied valid or not?
49. 41
Examination of such assumption of reduction in turn penalty can be done through
Synchro model output of control delay. Turn movements were output from assignments
of the two models, which provide volumes input to the corresponding Synchro models.
Synchro signal control delay of the internal 16 intersections (which has left-turn
elimination applied in After model) are compared in Table 4.6.1 for Before and After
models.
Table 4.6.1 Synchro Model Before 1 & After 1 Cycle and Delay
Model
Node ID
Nature
Cycle (s)
Control Delay
(s/veh)
Nature
Cycle (s)
Control Delay
(s/veh)
Delay
Difference (s)
14 100 23.2 85 27.6 4.4
35 100 28 85 28.2 0.2
36 100 28.6 85 29.5 0.9
47 100 25.4 85 22.7 -2.7
49 100 25.5 85 21.5 -4
51 100 26.7 85 29.6 2.9
54 100 26.9 85 28.1 1.2
72 100 25.9 85 26.9 1
74 100 29.3 85 28.4 -0.9
75 100 27.3 85 24.5 -2.8
77 100 26.8 85 27.9 1.1
222 100 25.7 85 21.7 -4
501 100 31 95 29.1 -1.9
1000 100 23.8 85 27.5 3.7
1022 100 20.5 85 23.7 3.2
6000 100 24.6 85 29.8 5.2
Average - 26.2 - 26.7 0.5
After 1Before 1
50. 42
In order to achieve an objective comparison, actuated uncoordinated signal control is
applied to all intersection signal timing setting. Coordination may have human factor
involved that a different level of coordination may affect the delay results. Thus, cycle
length is optimized with input turn movement volumes (resulting from the two TransCAD
models assignment result) and nature cycle is used. Cycle length is optimized per turn
volume, phasing, lane geometry and other standard timing setting.
Comparing nature cycle, eliminating 2 left-turn phases per intersection would achieve a
reduction in nature cycle length from 100 to 85. In general, a smaller nature cycle length
means it takes less time to serve and clear traffic in all direction. Since green time
serving rate is faster for all movement (from once per 100 s to once per 85 second),
average delay per intersection should be reduced. However, resulting delay values in
table does not consist with such expectation. The average delay per intersection remain
about the same for both models.
Synchro cycle length optimization is questioned here that how optimized does the
optimization do? The increase turn movement volumes in After1 model compare with
Before 1 may be one cause of delay remaining the same (as shown in TransCAD
assignment results above that internal links volume/capacity is higher in After and in
Before).
As the Synchro model estimation of intersection delay remaining the same Before and
After, assumption of 10 seconds reduction in turn penalty in After model to intersection
with left-turn elimination is not converged.
51. 43
4.7 Peak Hour Demand Model After 2 With Improved Turn Penalty
A new set of reduced turn penalty is assumed and applied to the After TransCAD model
assignment. Instead of 10 seconds reduction per movement for nodes with left-turn
elimination, left-turn movement turn penalty remains the same value of 45 seconds, and
though and right-turn movements have penalty reduction of only 5 seconds , which is 25
seconds. Global turn penalty is still remaining the same of 45, 30 and 30 for left-turn,
through and right-turn movements, which is applied to nodes without left-turn elimination
(border nodes).
See Picture 4.7.1 below for TransCAD assignment result, called this model After 2. After
2 model assignment result shows that less traffic is attracted to internal network with
less assumed reduction in turn penalty compare with After 1 result.
52. 44
Figure 4.7.1 TransCAD Model After 2 Trip Assignment Result
Now check convergence of Synchro delay and turn penalty applied. See Table 4.7.1
below for comparison of intersection signal control delay of Before1 and After 2 models
for inside intersections. With assumption of same penalty for left-turn movement and 5
seconds reduction for through and right-turn, the overall average intersection delay
would be slightly less than 5 seconds reduction per intersection.
53. 45
Synchro delay estimation shows that there is 5.2 seconds of reduction in intersection
control delay. This is very close to the assumed penalty value, can be concluded that
models converged, and turn penalty/control delay would decrease 5 seconds per veh
per intersection with 2 left-turn phases (movements) eliminated.
Table 4.7.1 Synchro Model Before 1 & After 2 Cycle and Delay
Model
Node ID
Nature
Cycle (s)
Control Delay
(s/veh)
Nature
Cycle (s)
Control Delay
(s/veh)
Delay
Difference (s)
14 100 23.2 85 19.1 -4.1
35 100 28 85 25.9 -2.1
36 100 28.6 85 23.3 -5.3
47 100 25.4 85 18.5 -6.9
49 100 25.5 85 19.9 -5.6
51 100 26.7 85 21.9 -4.8
54 100 26.9 85 18.4 -8.5
72 100 25.9 85 17.7 -8.2
74 100 29.3 85 22.5 -6.8
75 100 27.3 85 22.3 -5
77 100 26.8 85 24.4 -2.4
222 100 25.7 85 17.1 -8.6
501 100 31 85 25 -6
1000 100 23.8 85 23.1 -0.7
1022 100 20.5 85 17.5 -3
6000 100 24.6 85 18.9 -5.7
Average - 26.20 - 21.0 -5.2
Before 1 After 2
54. 46
4.8 Off Peak Demand Models with Final Turn Penalty (Light Traffic )
Taking 60% of trip number in the OD-Matrix used above may not necessary
representing an off-peak traffic demand. However, it can represent a light traffic demand
relatively close to mid-day traffic in such a network. To evaluate how effective is left-turn
elimination work in light traffic condition, assignments of such 60% of OD-Matrix traffic
demand are done to Before and After models. the turn penalty in After 2 is also
converged in such a light traffic model, it is applied to this After model with 60 % of peak
demand, called After 2- 60%. Global turn penalty remains the same.
See Figure 4.8.1 and Figure 4.8.2 for Before and After network assignment results for
light traffic demand.
56. 48
Figure 4.8.2 TransCAD Model After 3 Trip Assignment Result
Synchro delay estimation values are present in table below. The average intersection
delay may not be converged with turn penalty applied. Per scope of this study, models
are not re-ran with new penalty till convergence.
58. 50
4.9 Peak Hour Demand Models with No Turn Penalty
For peak demand models, the converged reduction in turn penalty is only 5
seconds/movement. TransCAD does capture such difference in calculation of shortest
paths, therefore affect the assignment results. However, question raised is that would
drivers in real life would react to such a little change in signal control delay difference,
would this difference be even observed y human or GPS tools?
GPS definitely would re-route drivers once some left-turn movements are prohibited in
certain locations. GPS route choice would also detect and reflect the traffic congestion
when it happens to avoid excessive delays. However, GPS traffic delay data has delay
itself and cannot reach an accuracy of 5 seconds.
Drivers may re-route to avoid a certain location or turn movements if they already have
pre-knowledge of excessive delay experience. However, a driver normally would not
sense a 5 seconds difference in signal control delay either.
As a result, modeling without assigning any turn penalty to the network may be carried
out and evaluated.
The same peak hour OD-matrix is assigned to Before and after networks, except that no
turn penalty is assigned to both Before and After models. See Pictures 4.9.1 and
Picture 4.9.2 below for assignment results.
60. 52
Figure 4.9.2 TransCAD Model After 4 Trip Assignment Result
For the trip assignment results, Before and After networks intersection delays are
estimated from Synchro models and shown in Table 4.6.3. Delay reduction is not as
much as it was expected with the magnitude in change of nature cycle length. It is about
the same value as the converged value from After 2 model. There is averaged of 4.1
delay reduction per intersection with left-turn elimination applied.
61. 53
Table 4.6.3 Synchro Model Before 3 & After 4 Cycle and Delay
Model
Node ID
Nature
Cycle (s)
Control Delay
(s/veh)
Nature
Cycle (s)
Control Delay
(s/veh)
Delay
Difference (s)
14 100 23.2 85 21 -2.2
35 100 28 85 23.1 -4.9
36 100 28.6 85 26.8 -1.8
47 100 25.4 85 19.6 -5.8
49 100 25.5 85 17.6 -7.9
51 100 26.7 85 20.6 -6.1
54 100 26.9 85 21.2 -5.7
72 100 25.9 85 21.2 -4.7
74 100 29.3 85 26.7 -2.6
75 100 27.3 85 20.5 -6.8
77 100 26.8 85 23.2 -3.6
222 100 25.7 85 20.6 -5.1
501 100 31 85 28.1 -2.9
1000 100 23.6 85 26.3 2.7
1022 100 20.5 85 16.8 -3.7
6000 100 24.6 85 20.3 -4.3
Average - 26.2 - 22.1 -4.1
Before 3 - Without TP After 4 - Without TP
62. 54
4.10 Model Network Performance Evaluation
In this study, the network performance is defined as the network accumulative travel
cost, which is the sum of vehicle-mile-travel (VMT) and travel time (TT). VMT can be
output from TransCAD assignment results. TT is taken of two parts, vehicle-hour-travel
(VHT) and intersection control delay time. VHT can also be output from TransCAD
assignment result, which only calculated from link travel time, not including turn penalty.
Since turn penalty was assumed in models, Synchro averaged signal control delay is
used to calculated network delay.
In table 4.10.1, Volume is the accumulative turn movement volumes in internal zone
network, Control Delay (s) is the averaged intersection control delay of all internal zone
intersections. Thus, network total delay is calculated by the multiplication of
accumulative turn volume and average intersection control delay
63. 55
Table 4.10.1 Internal Zone Network (4x4 nodes) One Hour Network Performance Summary
Peak OD Turn Penalty (s) VMT VHT Volume
Control
Delay (s)
Network
Delay (veh-hr)
Network TT (veh-hr)
= VHT + Delay
VMT change
(%)
TT
Change (%)
Before 1 Global ( 45, 30, 30) 33120 902 79771 26.2 581 1482 N/A N/A
After 1
Global & Reduced
(35, 20, 20)
36395 1054 87843 26.67 651 1705 9.9% 13.1%
After 2
Global & Reduced
(45, 25, 25)
34295 965 82497 20.97 481 1445 3.5% -2.5%
60% of
(Peak OD)
Turn Penalty VMT VHT Volume
Control
Delay (s)
Network
Delay (veh-hr)
Network TT (veh-hr)
= VHT + Delay
VMT %
change
TT %
Change
Before2 Global ( 45, 30, 30) 21243 532 51386 16.86 241 773 N/A N/A
After 3
Global & Reduced
(45, 25, 25)
22068 551 53514 15.13 225 775 3.9% 0.3%
Peak OD Turn Penalty VMT VHT Volume
Control
Delay (s)
Network
Delay (veh-hr)
Network TT (veh-hr)
= VHT + Delay
VMT %
change
TT %
Change
Before 3 None 32789 885 78641 26.2 572 1457 NA N/A
After 4 None 32634 881 78053 22.1 479 1360 -0.5% -6.7%
ComparisonModel Information TransCAD Output Synchro Delay
Comparison
Model Information TransCAD Output Synchro Delay Comparison
Internal Zone Network (4x4 node) One Hour Operation Network Performance
Model Information TransCAD Output Synchro Delay
55
64. 56
4.10.1 Interpretation of Network Performance Parameters
Comparing TransCAD assignment results presented in Table 3.10.1 for different
models, the following observations are concluded:
First, Before models comparison, Before 1 and Before 2. In the peak hour
demand models, Before 1 and Before 2 have nearly the same assignment results
with or without turn penalty applied. Trips were not re-routed much because of
turn penalty 45, 30 and 30 assigned to left-turn, through and right-turn
movements.
Secondly, After models comparison, After 2 and After 4. Comparing After 2 and
After 4, with turn penalty applied, model After 2 has longer total VMT and VHT
than what model After 4 has, which says that After 2 has higher level of trip re-
routing happen compare with After 4.
Since both After 2 and After 4 has the same network (with same application of
left-turn elimination applied), the only difference from the comparison of Before
models (with & without TP) is that, the magnitude of difference in movements
turn penalty. In After 2, movement turn penalty of 45, 25 and 25 are assigned to
left-turn, through and right-turn movements. Left-turn has 20 seconds more of
turn penalty compare with other movements. Note that Before 1 only has 15
seconds difference of left-turn penalty compare with other movements.
In After 2, there may be two main reasons causing trips to re-route. First of all, it
is because of the eliminated left-turn movements. Secondly, trips may also have
been re-routed to avoid making left-turn because of much higher turn penalty LT
65. 57
has. In model After 4, because none turn penalty assigned to any movement, the
only cause of trip-re-routed happened in After 4 is the eliminated left-turn
movements.
Third, compare the Before and After models, Before 1 and After 2, which are
the models with turn penalty assigned. After 2 has much lower intersection signal
control delay after left-turn elimination applied compare with Before 1 delay. This
is an expected positive improvement. However, the overall network performance
remains about the same in terms of travel cost. Travel time does not decrease as
much as the level of decrease in control delay, because trips may have re-routed
too much beside the cost of eliminating left-turn.
Fourth, Comparison of Before and After models, Before 3 and After4, models that
have none turn penalty applied. In model After 4, comparing with Before 3, the
only cause of trips re-routing is the elimination of left-turn movements. After
partial trips re-routed, model After 4 still has nearly the same network VMT a
VHT output from TransCAD assignment results. The Synchro delay estimation
result shows that intersection signal control delay drops from averaged value of
26.2 to 22.1 seconds after left-turn elimination applied. As a result, network
travel time decreases 6.7% because of reduction in intersection control delay. In
this model, application of left-turn elimination does help improving the network
performance. In a network with size of 4 square-mile ( internal zone 4 x 4 nodes,
link length of 0.5 mile), in every hour of operation during peak period, there is
total 97 vehicle-hour of travel time saved, which is 24 vehicle-hours of saving in
every hourly operation during peak period.
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Finally, Comparison of Before and After models with Off-peak demand, Before 2
and After 3 which traffic assignment were done for light traffic demand
conditions. A 60% of the finalized OD-matrix demand is assigned to TransCAD
before and after networks. The same sets of turn penalty were applied as Before
1 and After 2, which after left-turn elimination applied, turn penalty reduced from
(45, 30, 30) to (45, 25, 25) for left-turn, through an right-turn movements. As
discussed in comparison of Before 1 and After 2, there may have been two main
reasons causing trips re-routed in After 3 models, elimination of some left-turn
movements and increased difference between left-turn and other turn movement
penalties. As a result, After 3 has higher network VMT than Before does. Overall,
the reduction in signal control delay was counter acted by the increase in VMT
and network travel cost remaining about the same.
4.10.2 Introduction of Network Performance in Measure of Fuel Consumption
Fuel consumption is resulted from VMT and total Travel Time, which can be estimated
by utilizing the Federal Highway Administration (FHWA) fuel consumption equation
provided in a report prepared by Fredrick Wagner (Wagner, 1980). The fuel
consumption equation is based on vehicle miles traveled (VMT) and the vehicle hours
traveled (VHT).
Total Fuel Consumed = 0.0425*VMT + 0.6*VHT
Note that, VHT in this equation is equal to network TT, which composes of VHT from
TransCAD model assignment and Delay from Synchro estimation.
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One improvement of a network performance is cause of saving in fuel consumption.
From equation presented above, fuel saving can be achieved by decrease in VMT and
network TT.
The other direct measure of improvement is saving in human labor. Drivers and
Passenger hours can be saved if network total TT decrease. However, this saving is
difficult to measure which involve ridership and labor value variation in population.
Saving in TT can be measured in unit of vehicle-hour, however, not in person hour in
this study.
Table 3.10.2 summarizes the After models improvements in TT changes and Fuel
Consumptions comparing with corresponding Before network. After 2 model has saving
of 37 vehicle-hours in every hour of operation, however, in the mean time there is an
increase of 28 gallons in fuel consumption. After 3 shows increases in both TT and fuel
consumption comparing with Before 2 results.
After 4 has very positive results of both saving in TT and Fuel consumption. There is a
total of 97 vehicle-hours decrease in network TT and 65 gallons of saving in fuel
consumption with every hour of operation during peak period in model After 4
comparing with model Before 3.
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Table 4.10.2 Internal Zone Network (4x4 node) One Hour Performance Measure in Fuel Consumption
Peak OD Turn Penalty (s) VMT
Network TT
(veh-hr)
TT Difference
(veh-hr)
by VMT by TT Total Diffrence
Change
(%)
Before 1 Global ( 45, 30, 30) 33120 1482 N/A 1408 889 2297 N/A N/A
After 1
Global & Reduced
(35, 20, 20)
36395 1705 223 1547 1023 2570 273 11.9%
After 2
Global & Reduced
(45, 25, 25)
34295 1445 -37 1458 867 2325 28 1.2%
60% of
(Peak OD)
Turn Penalty VMT
Network TT
(veh-hr)
TT Difference
(veh-hr)
by VMT by TT Total Diffrence
Change
(%)
Before2 Global ( 45, 30, 30) 21243 773 N/A 903 464 1367 N/A N/A
After 3
Global & Reduced
(45, 25, 25)
22068 775 3 938 465 1403 37 2.7%
Peak OD Turn Penalty VMT
Network TT
(veh-hr)
TT Difference
(veh-hr)
by VMT by TT Total Diffrence
Change
(%)
Before 3 None 32789 1457 N/A 1394 874 2268 N/A N/A
After 4 None 32634 1360 -97 1387 816 2203 -65 -2.9%
Network Performance
Network Performance
Network Performance
Internal Zone Network (4x4 node ) One Hour Operation Saving In TT and Fuel Consumption
Model Information
Model Information
Model Information
Fuel Consumption (Gallon)
Fuel Consumption (Gallon)
Fuel Consumption (Gallon)
60
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4.11 Discussion of Model Assumption and Validation
Before any conclusion to be draw from this case study, it is necessary to discuss the
assumptions that have been made in this modeling analysis and their validation. Some
assumptions that have been discussed in other sections may not be repeated here. A
list of assumptions is stated as follow:
1. GPS devices are widely used by drivers.
2. GPS can find out the new shortest path once a left-turn movement is prohibited.
3. In the shortest path Calculation, GPS can detect signal control delay difference
from intersection to intersection, and the difference of delay by turn movement at an
intersection. ( This assumption applied to all models with Turn Penalty assigned).
4. Drivers use GPS to find shortest paths constantly and re-route until user
equilibrium achieved. (Applied to all models with Turn Penalty Assigned).
5. Drivers are not so much sensitive in little change in travel time. Re-routing only
happens when a left-turn movement is prohibited and it has to be done. Shortest
path can be found with help of GPS for re-routing. (Applied to models without Turn
Penalty assigned).
6. In After 2 model, turn penalty was assumed to be 20 seconds higher for left-turn
than for other turn movements.
7. Assumption of UE happens in real life (since all TransCAD assignment was done
with UE for all models).
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Model results are affected by the validation of each assumption stated above.
Conclusion can be drawn from model results with assumptions stated above. However,
if assumption is invalid, any conclusion base on that can be meaningless.
In real life, GPS can only detect the excessive delay and then re-routed. GPS data has
delay itself. There is possibility that GPS device could obtain more accurate and instant
delay information from source such as loop detection data on road. This may happen in
near future. In this case, model Before 1 and After 2 may catch the performance results
with assumption that "perfect delay information available". Before 2 and After 3 is the
same study with light traffic demand condition.
Currently, a closer to reality assumption is that drivers would re-route when they have to
(when left-turn prohibited) and when congestion happen (excessive delay) that GPS can
detect and find a better path. Model Before 3 and After 4 is the study with such
assumption.
4.12 Conclusion from Case Study
Two main conclusions can be drawn basing on the modeling network performance
results and assumptions stated above:
First, Before 3 and After 4 result says that left-turn elimination does help improve the
overall network performance in terms of vehicle-hours and fuel consumption savings. In
such case, only a low level of trip re-routed happened for the reason of some
eliminating left turn movements. A grid network provide flexibility in re-routing that the
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network VMT does not necessary increase with such portion of re-routing happened.
Left-turn elimination may work when the level of trip re-routing is moderate.
Secondly, "Perfect information" in delay may not help improving network performance.
See comparison of Before 1 and Before 3 results. "Perfect Delay information" may also
promote a higher level of trip re-routing happen which cause higher total turn
movements volumes at intersection and higher network VMT. Higher level of
convergence in turn movement turn penalty used may be further studied and
determined to validate this conclusion.
One thing important to mention is the detail scope of the network modeling. In this case
study, the network is not modeled to include many minor street links, which that could
have been used for alternative route choices. In other words, the network performance
is expected to be better after application of left-turn elimination if there are more
alternatives for trip re-routing.
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Chapter 5 Recommendations
The main objective of this study is to assess the impact of application of left-turn
elimination to a network performance. The question of if application of left-turn
elimination would help improving a network performance may be answered with a
specific network and under a set of specific assumption. Beside such a purpose, the
other goal of the study is to provide information about methodology of such case
analysis process and recommendations for future study.
First of all, planning and operation can be done with interaction in terms of turn
movement demand, delay and travel time for a small network. The limitation of this
study is that TransCAD and Synchro do not take data transfer from one to the other. As
a result, a lots of data entry work has to be done in both models even for only a small
network, and the level of convergence is limited in between turn penalty and turn
movement delay. A new tool may be needed in the future study if a bigger network and
higher level of model accuracy is desired.
The second comment is about intersection delay (or delay by turn movement)
estimation accuracy and a thought of an innovative way of estimating delay. The
estimated intersection control delay is used as Turn Penalty in TransCAD assignment,
as well as a parameter to calculate total travel time. However, both of HCM2010 and
Synchro Control Delay estimations may not have such desired accuracy in estimating
the absolute value of movement delay.
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Question to be asked is that, what other way can we do to estimate delay? A new
thought is that, delay could be estimated by a calibrated model as function of traffic
parameters from real world data. Traffic parameters can be similar as the ones used in
HCM 2010 equation. Real world delay data statistic under a certain signal operation
setting could be output from detection loop data on road. It could also be obtained from
GPS data.
The third, very importantly, this network analysis is only done to one single case of
application of left-turn elimination. More study of other ways of applying left-turn
elimination in different circumstances is encouraged.
Lastly, one theory has been proven in many cases is that perfect information may not
help in achieving system optimization. The purpose of left-turn elimination is to achieve
system optimization by improving network performance. However, with assumption of
perfect information (applying turn penalty), this purpose may be countered. However,
the accurate estimation of delay is still very important as its part of travel time equation
74. 66
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