DEVELOPMENT AND EVALUATION OF A COORDINATED TRAFFIC … · Traffic Signal Emergency Preemption System EXECUTIVE SUMMARY Most traffic signal controllers have emergency vehicle preemption

DEVELOPMENT AND EVALUATION OF A COORDINATED TRAFFIC SIGNAL

EMERGENCY PREEMPTION SYSTEM

FINAL REPORT

SOUTHEASTERN TRANSPORTATION CENTER

NITHIN AGARWAL, ADAM KIRK

MARCH 2017

US DEPARTMENT OF TRANSPORTATION GRANT DTRT13-G-UTC34

DISCLAIMER

The contents of this report reflect the views of the authors,

who are responsible for the facts and the accuracy of the

information presented herein. This document is disseminated

under the sponsorship of the Department of Transportation,

University Transportation Centers Program, in the interest of

information exchange. The U.S. Government assumes no

liability for the contents or use thereof.

1. Report No. 2. Government Accession No.

3. Recipient’s Catalog No.

4. Title and Subtitle Development and Evaluation of Coordinated Traffic Signal Emergency Preemption System

5. Report Date October 2016

6. Source Organization Code $50,000

7. Author(s) Agarwal, Nithin; Kirk, Adam

8. Source Organization Report No. STC-2015-##-XX

9. Performing Organization Name and Address 10. Work Unit No. (TRAIS)

Southeastern Transportation Center UT Center for Transportation Research 309 Conference Center Building Knoxville TN 37996-4133

11. Contract or Grant No. DTRT13-G-UTC34

12. Sponsoring Agency Name and Address

US Department of Transportation Office of the Secretary of Transportation–Research 1200 New Jersey Avenue, SE Washington, DC 20590

13. Type of Report and Period Covered Final Report: October 2014– March 2017

14. Sponsoring Agency Code USDOT/OST-R/STC

15. Supplementary Notes:

16. Abstract

Most traffic signal controllers have emergency vehicle preemption (EVP) capabilities that utilize detectors and communication technologies to provide the emergency vehicle the right of way. EVP provide safe passage of emergency vehicle through the intersection and appropriate strategies aim to reduce overall network delay. Traffic engineers utilize variety of EVP plans and exit strategies for safe and efficient operation. One of the challenges of implementing EVP under coordinated-actuated signal systems is selecting the best coordination recovery strategy at the end of preemption such that disruptions to the normal traffic signal operations are minimized. Similarly, time-of-day (TOD) traffic operations also produce such disruptions while transitioning between TOD modes and require returning to coordination. However, there are several options available within the controller such as preemption to coordination exit strategy and event synchronization of local cycles that are explored in this study. In addition, preemption can also be trigged at a corridor level instead of triggering at individual intersection level using manual commands in the central management software. This report presents the evaluation results of various EVP recovery and TOD transition strategies in an urban corridor including four coordinated-actuated signals along US 60, Winchester road in Lexington, KY. Since field testing of various preemption and TOD transition strategies is impractical, the study was performed using hardware-in-the-loop simulation, which consisted of a well-calibrated VISSIM microscopic simulation model, four traffic controllers, and four controller interface devices. The study results showed that controllers (e.g., 2070 and ASC/3) have certain features that are advantages for the EVP recovery strategies.

17. Key Words Traffic Signal Systems, Preemption, Traffic Signal Controller,

18. Distribution Statement

Unrestricted; Document is available to the public through the National Technical Information Service; Springfield, VT.

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Unclassified

20. Security Classif. (of this page)

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21. No. of Pages #

22. Price

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Form DOT F 1700.7 (8-72) Reproduction of completed page authorized

Development and Evaluation of a Coordinated i Traffic Signal Emergency Preemption System

TABLE OF CONTENTS

EXECUTIVE SUMMARY ....................................................................................................... 1

DESCRIPTION OF PROBLEM ............................................. Error! Bookmark not defined.

APPROACH AND METHODOLOGY .................................. Error! Bookmark not defined.

FINDINGS; CONCLUSIONS; RECOMMENDATIONS ...... Error! Bookmark not defined.

REFERENCES ........................................................................................................................ 36

APPENDIX ............................................................................. Error! Bookmark not defined.

Development and Evaluation of a Coordinated 1 Traffic Signal Emergency Preemption System

EXECUTIVE SUMMARY

Most traffic signal controllers have emergency vehicle preemption (EVP) capabilities that

use detectors and communication technologies to provide emergency vehicles the right-of-

way. EVP lets emergency vehicles safely pass through intersections and reduces overall

network delay. Traffic engineers use many types of EVP plans and exit strategies to ensure

safe and efficient traffic operations. One of the challenges of implementing EVP under

coordinated-actuated signal systems is selecting a coordination-recovery strategy at the end

of preemption that minimizes disruptions to the normal traffic signal. Similarly, time-of-day

(TOD) traffic operations produce these disruptions when transitioning between TOD modes,

and require the resumption of coordination. This study explores several options within

controllers, such as preemption to coordination exit strategies and event synchronization of

local cycles. Preemption can also be trigged at a corridor level instead at the level of

individual intersections using manual commands in the central management software. This

report presents the evaluation results of various EVP recovery and TOD transition strategies

on Winchester Road (US 60), an urban corridor in Lexington, Kentucky. The study corridor

contained four coordinated-actuated signals. Because field testing various preemption and

TOD transition strategies is impractical, the study was performed using hardware-in-the-loop

simulation, which consisted of a well-calibrated VISSIM microscopic simulation model, four

traffic controllers, and four controller interface devices. The study results showed that

controllers (e.g., 2070 and ASC/3) have certain features that are advantageous for EVP

recovery strategies.


INTRODUCTION

Coordinating traffic signals increases mobility and efficiency within urban signalized

intersections. In addition to traditional timing plans that vary by time of day, advanced

controllers can implement various strategies such as emergency vehicle preemption (EVP),

which lets emergency vehicles to pass through an intersection safely and efficiently. A

principal disadvantage of implementing EVP, however, is the disruption it causes to the

signal system’s normal phasing and timing plan. Within a coordinated system, several

intersections in a network will be affected simultaneously, which can mitigate the benefits of

signal coordination. This study explores several controller configurations and corridor-level

preemption strategies that aim at reduce travel time on mainline and alleviate delays to the

emergency vehicle route.

EVP is typically achieved by equipping emergency vehicles with an electronic emitter that is

read by a detector mounted at the intersection on each approach (Figure 1). The detector

triggers a green signal on the approach the detected vehicle is traveling. Normal signal

operations are suspended to accommodate the emergency vehicle. Once the preemption is

served, the signal enters a transition mode to re-sequence the signal and restore coordinated

system timings. How long the transition mode lasts depends on several variables, but can

take up to four or five cycle lengths. During this transition period, the progression of traffic

within the system can be adversely affected. This increases congestion and may lead to

secondary crash patterns.


FIGURE 1: Preemption detector installed on span wire

For emergency vehicles traveling along the main corridor in a coordinated system, each

intersection is preempted when the vehicle comes within range of the detector. Generally,

this occurs 1,000 feet upstream of an intersection. Signal controllers require guidance on how

to exit from preemption phase once it is completed. Exit phases and transition modes can be

programmed within the controller. There are several strategies that can be used, and this

study explores several parameters within the controllers and different implementation

strategies.

Purpose and Scope

On most corridors, emergency vehicle paths can be can be predetermined based on

infrastructure (e.g., hospitals, fire stations), and preempt routes on major arterials for inbound

and outbound directions can be established. When emergency vehicles traverse a coordinated

system, it is hypothesized that preempting the entire corridor simultaneously will help clear

queues at intersections so the emergency vehicles can pass without incident. Coordinated

preemption may also help reduce delays and travel time through the network. To evaluate

these scenarios, this research sought to investigate the effects of preempting an entire

corridor for emergency vehicle operations, rather than preempting each intersection upon

arrival of the emergency vehicle. This research also studied the effect of exiting the

preemption phase at different points of a local cycle. Researchers from the Kentucky

Transportation Center (KTC) performed this evaluation with a VISSIM model based on US

60 (Winchester Road) in Lexington, Kentucky. US 60 is a major arterial that leads into the

city and functions as a primary corridor for hospital and fire emergency vehicles due to the


nearby presence of hospitals and fire stations. To realistically emulate operations, the

evaluation utilized hardware-in-the-loop simulation models. We evaluated the traditional

preemption plan and proposed corridor-level preemption strategy using manual command

logic from CENTRACS central management software and Econolite ASC/3 controllers.

This paper is organized in the following manner. The next section briefly explains the

operation of EVP and previous studies of EVP transition methods. Section 3 presents the

procedure used during this study, and Section 4 provides the results of the HILS experiments.

The final section includes conclusions and recommendations for future research.

LITERATURE REVIEW

Many coordinated traffic signal systems are equipped with emergency vehicle preemption

(EVP), which accords preferential treatment to emergency vehicles. During emergencies, a

dedicated green signal indication facilitates the safe and efficient passage of these service

vehicles through intersections. Signal preemption is typically accomplished by equipping

vehicles with an electronic emitter that communicates with a preemption detection device

mounted at the intersection on each approach. The detector triggers a request for a green

indication on the approach the emergency vehicle is traveling. In doing so, normal signal

operations are interrupted to accommodate the emergency vehicle. Once the preemption is

served, the signal typically shifts into transition mode. In this mode, a pre-selected traffic

signal algorithm (i.e., transition strategy) resynchronizes the local intersection reference point

with the coordinated system cycle timings. The transition algorithm may dwell on the

coordinated phase or shorten or lengthen the cycle length to achieve synchronization.

Transition may also take place in the process of entering a coordinated timing plan or

changing between two coordinated timing plans. Transition disrupts traffic operations and

takes up to four or five cycles to resume normal operations. During this transition period, the

progression of traffic within the system can be adversely affected. Congestion increases and

secondary crash patterns may emerge.


In 1992, the National Electrical Manufacturers Association (NEMA) defined traffic signal

preemption as “the transfer of the normal operation of a traffic signal to a special control

mode for the purpose of servicing special vehicles.” NEMA also included preemption in its

TS-2 Standards, establishing the minimum operational requirements and functions that traffic

signal controllers are required to provide. These include the ability to accept commands from

six different types of preemption inputs and the provision of exit transition strategies to re-

attain the normal signal timing plan, among other requirements.

By 1992, traffic signal controller manufacturers had already incorporated different transition

strategies into their products. However, the absence of universal standards and terminology

for transition strategies proved burdensome for practitioners, who were obligated to explore

the idiosyncratic functionalities and capabilities of individual traffic signal controllers.

Manufacturers continue to employ different terminology for transition strategies. This

necessitates describing the specific functionality of the selected transition strategy used in

this research project. A literature review indicated that researchers have obtained noticeably

better results when using the Smooth/Shortway transition strategy. Based on this information,

we decided to focus on the use of the SMOOTH transition logic, which is described in the

Econolite ASC/3 Advanced System Controllers Programming Manual as such:

“Smooth - transition is accomplished by adding or subtracting a maximum of 17% of cycle

length per cycle (REF NTCIP 1202 2.5.2 Integer 3). Econolite allows modifying this factor

by changing the “Dwell / Add Time” when it is non-zero.”

“Preemption Exit Phase(s) – with exit phase(s) enabled, the preemption sequence terminates when all phases are timing.

When no exit phase is enabled and the PMT TO COORD option is not active, the preemptor terminates immediately and exits from the cycling interval directly to normal operation.

When no exit phase is enabled and the PMT TO COORD option is active, the preemptor will terminate and exit from the cycling interval directly to the lowest priority phase(s) that have an open coordination permissive window. This allows the preemptor to exit directly into coordination without requiring a pickup cycle or transition.”

Also:


“Preemption Exits to Coordination Selected Phases – When enabled, allows the preemptor to exit directly into coordination sequence and not require a pickup cycle when no exit phase is programmed and the coordinator is active. When not enabled, allows the preemptor to exit to a FREE condition.”

Furthermore:

“Synchronization Reference - When Reference Time is selected, the sync point is the sync reference time entered by the user, for example midnight. When Last Event is selected, the sync point is referenced to the time of the action plan that initiated the current cycle.”

Very few researchers have explored preemption exit strategies. Our literature review pointed

toward a single paper related to analyzing performance incorporating different exit phases.

Moreover, we found no previous work that examined available synchronization reference

parameters.

As noted previously, the National Electrical Manufacturers Association (NEMA) did not

formally define preemption and establish minimum operational requirements (e.g., transition

strategies) in its TS-2 Standards until 1992. Nevertheless, prior to 1992 several researchers

investigated available transition methods. Shelby et al. (2006) provided a comprehensive

review, including many relevant publications authored between 1973 and 1982. During that

period, the era of electromechanical controllers, Dwell was the only practical form of timing

plan transition. Consequently, our literature review focuses on research performed on

transition strategies that more closely resemble those available today and are commonly used

in North American contexts. The review encompasses research on recovery strategies for

EVP, railroad preemption, and transition strategies analyzed during the implementation of

coordinated timing plans.

Bullock et al. (1998) analyzed recovery from railroad preemption looking at travel time along

an arterial that ran parallel to a train track. Hardware-in-the-loop simulation was used to

investigate if recovering to different patterns would improve travel times. Results indicated

arterial travel times would benefit, but provided no specific methodology to select the best

alternate pattern. They also found that returning to synchronization earlier when subtract


transition was used (i.e., shortening the duration of phases) could lead to additional delays on

phases already affected by a train blockage.

Bullock et al. (1999) analyzed the impact of emergency vehicle traffic signal preemption

across three coordinated intersections on a route in Virginia using hardware-in-the-loop

simulation. For the geometric and operational conditions studied, the impact of emergency

signal preemption on the corridor’s signal coordination was minor, producing a 2.4 percent

increase in the average travel time. The modest impact was attributed to relatively long

spacing between intersections, platoon dispersion, modest traffic demand, emergency vehicle

detection occurring in close range to the intersection, and very long cycle lengths on the

studied corridor. Analysis of different transition strategies was not a focus of this study.

Nelson and Bullock (2000) analyzed the impacts of EVP on four closely spaced coordinated

intersections along a state route in Indiana using hardware-in-the-loop simulation. Different

preemption paths, different transition algorithms, and a varying number of preemption calls

(from one to three) were used for equal simulation periods. They found that a single

preemption call produced a minimal effect on the overall travel time and delay through the

network. Conversely, multiple preemptions at close intervals resulted in more severe impacts.

The smooth transitioning procedure performed the best under most scenarios. The authors

observed that the impact of preemption depends on intersection spacing, transitioning

algorithm, an intersection’s level of saturation, duration of the preemption, and amount of

slack time available in each intersection’s cycle.

Hamilton (2000) analyzed different transition methods using hardware-in-the-loop simulation

to develop a methodology for determining the best transition method when signal timing

plans change by an offset correction of 10, 30, 50, 70, and 90 percent of the cycle length. The

Shortway method exhibited the best performance of the methods tested.

Obenberger and Collura (2001) used hardware-in-the-loop simulation to present a state-of-

the-practice assessment of common transition strategies to exit preemption control available


in four different controller and software product manuals. They offered no recommendations

to either specify or limit the use of transition strategies.

Conducting hardware-in-the-loop testing of Naztec and Eagle traffic signal controllers,

Sunkari et al. (2004) evaluated advanced coordination features, including transition modes.

They found the Shorway transition mode to be the optimal mode on Eagle controllers and

Dwell mode consistently experienced higher cross-street delays. On Naztec controllers, the

use of Short and Long transition modes with 17 percent as the correction mode yielded the

best results.

Shelby et al. (2006) compared available transition methods for several congestion levels to

determine which exhibited the least vehicle delay when transitioning from one timing plan to

another. Simulations indicated the Shortway transition was the most effective in general, and

that under congested conditions the Add transition performed as well or better. The Dwell

transition was generally the most disruptive. Additionally, the research found that the degree

of saturation and offset adjustment were indicators of transition performance. On an arterial,

adjusting the offset quickly — and reestablishing progression — becomes an important

competing objective for matching a transition cycle length to the saturation level.

Accordingly, the Shortway proved to be more effective across a wider range of conditions.

Cohen et al. (2007) used transient profile analysis to investigate signal transition on a real-

world major arterial with variable intersection spacing. Transition was generated with the

implementation of different timing plans. The transient delay and travel time profiles

indicated the Dwell transition elicits a major shock wave in the performance measures,

suggesting this transition method is inappropriate for major-crossing arterials. In a fully

saturated context, the Subtract transition had a very smooth and stable profile.

Park et al. (2008) used hardware-in-the-loop simulation to evaluate EVP and transition

strategies for Northern Virginia Smart Traffic Signal Systems in an urban corridor that

contained four coordinated-actuated traffic signals. Results demonstrated that EVP

significantly impacts coordinated signal systems. One of the experiments during an off-peak


period showed that a single EVP call on the northbound approach caused 21% and 18%

increases in average eastbound and westbound travel times, respectively. The positioning of

the EVP’s local cycle timer was a significant factor in EVP and transition operations. It

affects how long it takes for preemption phases to begin timing as well as delays experienced

during transition periods. Shortway/Smooth outperformed other transition algorithms. The

study also discussed the importance of the selection of exit phases specified to time after

preemption. All cases tested with exit phases outperformed those with no exit phase case. By

exiting to nonpreemption phases, a traffic signal can immediately serve approaches that

likely contain queued vehicles. With no exit phase case, the traffic signal continues serving

the EVP phases and proceeds with the regular phase sequence as it transitions back to

coordination.

Qin and Khan (2012) proposed 1) a real-time control strategy that enables a signal

transitioning from normal operation to service emergency vehicles, and 2) a control strategy

to revert to normal operation. They adopted a two-phase algorithm developed in MATLAB

consisting of a relaxation method and a stepwise search strategy. Results showed that the

real-time control and the optimal control strategies, along with their associated methods,

performed better than commonly used approaches. The control strategies they introduced can

be applied to traffic conditions — up to a slightly oversaturated level — and used with single

or multiple emergency vehicle requests.

Lin et al. (2014) investigated the potential for using advanced features of traffic signal

system software platforms (ATMS.now) to alleviate safety and mobility problems at

highway-railroad at-grade crossings and adjacent arterials. Simulations analyzed the

proposed coordinated pre-preemption strategy, with the objective of maximizing the clearing

of through traffic at several intersections along an arterial before a train’s arrival. Pre-timed

coordinated pre-preemption phases were implemented. This the strategy can effectively

reduce average delay, average stops, and average queue length of the arterials near railroad

crossings. Furthermore, coordinated pre-preemption strategy should be considered when


through traffic volume is higher than 500 vehicles per hour per lane and train block duration

exceeds 100 seconds.

Jordan and Cetin (2015) evaluated the use of vehicle-to-infrastructure (V2I) communication

to send messages to traffic signals for signal preemption, thus allowing emergency response

vehicles to proceed through closely spaced intersections as quickly as possible. Traffic

signals were preempted in a specific order to discharge background traffic before the

emergency vehicle arrived. Kinematic wave theory was used to determine when each

intersection should be preempted. Simulations indicated that this strategy can shorten the

travel time significantly for emergency response vehicles through congested signalized

intersections.

STUDY AREA

Figure 2 is a map of the project corridor, which is located on Winchester Road (US 60) from

the intersection of Sir Barton Way (MP 11.5725) to the I-75 Northbound Ramp (MP 12.135).

This segment is heavily traveled, with two traffic signals located at the I-75 and US 60

interchange serving as primary ingress/egress routes to downtown Lexington and multiple

shopping areas west of I-75. When crashes or other incidents occur on I-75, significant traffic

volumes are diverted to this corridor. Average Daily Traffic on this segment of US 60 is

approximately 40,000 vpd. and includes four signalized intersections:

• Winchester Road (US60) at I-75 North Ramp (MP 12.135)

• Winchester Road (US60) at I-75 South Ramp (MP 11.9711)

• Winchester Road (US60) at Elkhorn Road (MP 11.800)

• Winchester Road (US60) at Sir Barton Way (MP 11.5725)


FIGURE 1 US 60 Study Area

The TOD plans for US 60 corridor operate on cycle lengths ranging from 120 seconds to 150

seconds for a typical weekday. The corridor operates on coordinated signal timing during

weekdays between 6:55 a.m. and 9:00 p.m. while operating free overnight. Table 1

summarizes configuration, time periods, and cycle lengths.

TABLE 1 Typical Weekday TOD timing on US 60

Configuration Time Cycle Length a.m. 6:55-9:15 150 a.m. off-peak 9:15-11:30 120 Noon 11:30-13:30 140 p.m. off-peak 13:30-15:45 140 p.m 15:45-18:15 150

FIELD STUDY DESCRIPTION

This study focused on a.m. peak hour duration (7 a.m. – 9 a.m.). The following field data

were collected along the study corridor to calibrate the simulation model:

• GPS travel time

• Queue length


• Turning movement counts

Travel Time Study

Travel time study was conducted to collect the location information of the probe vehicle at

every second which determined instantaneous speed at every second and the total time

required for each of a series of vehicle to travel through the corridor. Travel times were

measured using a laptop computer by logging the location of the probe vehicle with a GPS

receiver mounted on the roof of the vehicle. The GPS logger collected location information

from the GPS receiver at 1-second intervals, which enabled the calculation of vehicle speeds.

The drivers conducting the travel time runs were trained and instructed to use the standard

floating car method, in which the drivers attempt to travel with the flow of traffic. This

method is used to ensure the travel times collected are representative of the travel time of the

average vehicle moving along the corridor.

Queue Length Study

Queue lengths were determined for the main street approach and measured at minor street

approaches. For the main street approach, queue lengths were determined from travel time

run data, which included instantaneous speeds of the probe vehicles. Queue length was

measured as the distance between the stop line of the approaching intersection and the probe

vehicle when the speed of vehicle was less than 3 mph.

Minor-street queue length studies were conducted at critical intersections: Sir Barton Way

and at the two interstate ramps. Queue data were gathered cycle-by-cycle for each of these

minor street approaches. To obtain accurate queue lengths for each approach, they were

measured by a licensed professional surveyor using a reflector-less total station. The

surveyor first took benchmark readings at the stop lines for each study approach. At the

termination of the phase for that approach, the surveyor took the reading of the last car in the

queue, which yielded the queue length on a cycle by cycle basis.


Turning Movement Count

Manual turning movement counts were conducted at all four intersections using JAMAR

count boards. All data collectors were trained and the counts were later verified and balanced

across the corridor. The volume trend was synchronous with the historical dataset, with

heavy directional inflow westbound during the AM peak.

HARDWARE-IN-THE-LOOP SETUP AND MICROSIMULATION

Because we cannot experiment with EVP strategies in the field, we performed

comprehensive evaluations using hardware-in-the-loop system (HILS) which uses the signal

controllers along with a computer program to simulate field traffic movements. Figure 5

illustrates the four principal components of an HILS — simulation software (VISSIM)

replicates traffic conditions; midware software provides two-way data transition (phase

condition data moves from controller to simulation program and traffic flow data flows from

simulation program to the controller); a controller interface device, or CID, (ADAM 5000

TCP) that converts the analog signal from the signal controller to a digital signal that can be

transmitted to the midware software, and vice versa; and the traffic signal controller.

FIGURE 2 Hardware-in-the-loop setup

The Federal Highway Administration’s (FHWA’s) Traffic Analysis Toolbox Volume III:

Guidelines for Applying Traffic Microsimulation Modeling Software provided guidelines for

developing the project VISSIM models (1). The following process was adopted to develop

the simulation model:

MIDWARE VISSIM ADAM

5000 TCP

Traffic Signal Controller


• Determine scope of study

• Collect data on a typical day representative of average field conditions

o Data collection included field volumes, signal timings, and calibration

parameters in the VISSIM model

• Established and replicated base model to produce alternative models

Software

VISSIM software was chosen for simulation. VISSIM is a microscopic, behavior-based

traffic simulation tool developed by PTV (2). VISSIM was selected due of its ability to

simulate vehicles and pedestrians. Additionally, VISSIM utilizes external signal controllers

that are set up in the HILS. VISSIM provided the required measures of effectiveness (MOEs)

identified as benchmark by the study focus group. Figure 7 is a screenshot of the VISSIM

GUI.

Data Collection

We obtained high-resolution scaled image files of the study area from the Kentucky GIS

database. Storage bays and tapers where applicable were measured in the field. Turning

movement counts were performed at each signalized intersection in the study area. The GPS

travel time study conducted for the study duration was used for model calibration. Signal

timing files acquired from LFUCG’s traffic department were loaded into signal controllers in

the HILS system using CENTRACS. CENTRACS is an advanced traffic management suite

developed by Econolite that is used to upload, download, and monitor signal timings to

individual controllers as well as to develop manual command logic for corridor-level

preemption.


Figure 7: PTV VISSIM GUI

Base Model Development

For the base model development, we used high-resolution image of the study area as

background. The geometry (number of lanes, lane widths, urban link behavior type, and link

lengths) of the study area were modeled to scale. Once the geometric network was created,

the desired speed was coded as the posted speed (35 mph +/– 5 mph for US 27 and 55mph

+/– 5 mph for US 60). Reduced speed zones in VISSIM were coded for turn lanes (15 mph

+/– 3 mph), which forces the vehicles to reduce speeds during turning movements as per our

observations. Next, the intersection controls were coded. No timing input in VISSIM was

required because HILS communicates directly with signal controllers,. However, individual

intersections were coded with external signal controller parameters that included a

VTDatex.dll program file, a STDSC_GUI.dll dialog DLL file, and VTDatex.wtt files.

Detectors at each intersection were coded for individual lanes and linked to their

corresponding signal controllers. Right turns on red were coded at all signalized

intersections. One unsignalized intersection on US 60 (Thunderstick Drive) was coded with

stop signs as default parameters for gap acceptance.

In a simulation model, when vehicles cross one another’s path, such as a permitted left turn

movement, it creates a conflict zone. When this occurred, we used VISSIM conflict areas

tool to assign higher priority to specific movement. For example, in a permitted left turn


situation, a through vehicle has higher priority that a left turning vehicle. Pedestrians were

coded with higher priority at crosswalks when they had the right-of-way. Vehicles were

coded to enter the system at 15-minute intervals. We selected a default vehicle composition

of 98% cars and 2% heavy goods vehicles (HGVs) were coded.

Next, we coded routes into the model. We used static routing features because the turning

volume counts were available for 15-minute intervals for the entire study period. Our final

step was to set up the simulation parameter, including the seeding time and simulation time.

Seeding time is the warm-up time during which the simulation lets all vehicles enter the

network and interact with one another. FHWA’s Traffic Analysis Toolbox recommends the

seeding period should be equal to or greater than twice the estimated travel time required at

free flow conditions to travel the entire network. Based on distance and posted speed, it

would take approximately 2.2 minutes to travel the network, which equates to a minimum

seeding period of seven minutes. To be conservative, we chose a seeding period of 15

minutes.

Emergency vehicles were not modeled in the simulation program due to the complex nature

of driver behavior modeling, which fell beyond the scope of this study. An external program

was developed that triggered the preempt call and was configurable to a specific time and

duration.

Error Checking

During error checking, we fixed coding errors in VISSIM and time errors between simulation

and controllers. CENTRACS server and VISSIM workstation were set at the same time (to

the second) to prevent a time mismatch or time lag for TOD operation. All other coded data

(geometry, speeds, signal timing data, traffic volumes) were reviewed, and we observed

animations for peak-hour duration. Some parameters in the model were adjusted based on the

engineering judgment to accurately reflect field conditions. Section 5.1.9 lists these

adjustments.


Calibration

Calibration of the simulation models was based on four data sets gathered from the field,

including:

• Volumes

• Average travel time • Average travel Speed • Controller time

Volume Calibration

Volume data from the field were input in VISSIM base model in 15-minute intervals at all

the corridor’s entry points. Routing decisions were established using the turning movement

data collected from all intersections in the field. Data collection points were set up in the

VISSIM model at upstream approaches of all intersections to verify the flow rate matched

field data. The study advisory committee established an acceptable error threshold of 15%

because the study area included high volumes along with extended sections beyond the study

area. As the following table indicates, traffic flow for simulated models aligned closely with

the field volumes and were within the threshold limits. The overall error was -0.5%.


TABLE 2: Volume Calibration Results

US 60 Volume Comparison

Approach AM Peak

FIELD VISSIM % change

US 60 WB 3841 3297 14.20% US 60 EB 2272 2316 -1.90%

SIR_BARTON_NB 1074 1278 -19.00%

SIR_BARTON_SB 555 489 -11.89%

NBRAMP_NB 1749 1980 -13.20%

ELKHORN_SB 566 509 10.10% ELKHORN_NB 317 286 9.80%

SBRAMP_SB 1437 1599 -11.30%

Total 11811 11754 -0.48%

Speed & Travel Time Calibration

Vehicle speeds in VISSIM are determined by a stochastic distribution of desired vehicular

speeds. This essentially sets the free flow speed for each vehicle in the network. Desired

speed is typically set within +/ – 5 mph of the desired or observed speed limit to provide

speed variance in the corridor. For the study corridors, we adjusted speed spreads to match

the field values. Table 3 shows the travel time and speed calibration results. The travel time

and speeds for US 60 EB were within the 15% error threshold set forth by the study advisory

committee.

TABLE 3: Speed and Travel Time Calibration

US 60

Calibration Parameters EB Field VISSIM

TT - TOD 189.7 185.47

Avg Speed 30.36 31.05


Controller Time

VISSIM can simulate at a higher speed than real time. However, the controllers cannot speed

up any of their functions and must operate at real time. Consequently, the simulation

parameters — including resolution and speeds of VISSIM models — were calibrated so the

VISSIM would simulate in real-time speed. Simulation time was also matched with the

CENTRACS server time and the workstation time. The Workstation time was used as the

basis of timing comparisons. VISSIM simulation parameters were set at 10 time steps per

simulation second resolution while the simulation speed was set at 1 simulation second per

second along with active retrospective synchronization, which syncs the simulation to

controller timings. The model conducted simulated runs for 15 hours. As Table 4

demonstrates, all the timings were in sync, and the controllers did not have any time drifts.

TABLE 4 Controller Time Calibration

Check Points

VISSIM Time

Controller Time

Centracs Server Time

Workstation time % Error

Start 60 5:46:00 5:46:00 18:17:50 N/A 1 54391 20:51:30 20:51:30 9:23:21 0:00:00 2 59400 22:14:59 22:14:59 10:46:50 0:00:00

Adjustments

We visually reviewed the model and executed several modeling adjustments to better

replicate field conditions. The model was reviewed and no visual errors were found, but since

the intersections were so closely spaced we edited the following parameters, to avoid

excessive congestion caused by late merge behavior:

• Look back distances for connectors at Leader Avenue northbound were

increased from the default of 400 ft to 800 ft to allow vehicles to change lanes

appropriately.

• To minimize the obstruction to the traffic flow, we lowered time-before-

diffusion parameter from the default of 60 seconds to 10 seconds


• We selected consider next turning decision for more efficient turning

movements.

• Cooperative lane change was selected to improve the general lane change

behavior for more realistic modeling.

METHODOLOGY

The study’s two principal objectives were to 1) evaluate the effectiveness of corridor-level

EVP techniques as opposed to standard preemption by individual intersection, and 2) explore

the effectiveness of various parameters and functions available within advanced traffic

controllers. In total four EVP parameters were evaluated:

• Preemption Level

• Local Cycle Time of Preemption

• Preemption to Coord Function

• Sync Reference Point

Preemption Level

We evaluated two levels of preemption: 1) intersection-level preemption and 2) corridor-

level based pre-emption. Preempt calls were placed at the local intersection for the

intersection-level preemption scenario. Downstream preemption calls were offset between

intersections based on the travel time between intersections using the 85th percentile speed of

corridor traffic (44mph). We initiated Corridor-level preemption with manual command logic

and action sets developed in the CENTRACS Advanced Traffic Management System, which

managed the HILS. This logic triggered the preempt call on all intersection controllers once

the first controller received a preempt signal. Corridor-level preemption was timed to clear

downstream queues before an emergency vehicle arrived.


Location of EVP Call Exit Point in Local Cycle Timer

Due to the potential for random arrivals of emergency vehicles, preemption calls were placed

at multiple times within the local intersection cycle. They were reported as the exit point of

the preemption call. The EVP call exit point was timed to match the following local cycle

points:

• Beginning of phase 2

• Half way through phase 2

• 75% after beginning of phase 2

• Halfway through phase 4

Transition Methods

The 2070 ASC/3 controller has the following transition methods available: Max, Dwell and

Smooth. To reduce the number of scenarios, we only examined Smooth. Instead of

evaluating exit phase strategies under logic processor with all possible transition methods, we

selected the Smooth method because it is generally superior to other transition methods.

Preempt to Coord Parameter

Preempt to Coord is a unique parameter in the Economlite firmware that, when active, allows

the preemptor to exit directly into the coordination sequence without requiring a transition

period to begin coordinated operations. We selected this parameter to determine its effect on

travel time of the network.

Sync Reference

Sync Reference time is used to establish the baseline time coordinated operations are based

on. Offsets within a coordinated signal system are referenced to the Sync Reference Time.

Default parameters for this value is the Zero Sync Reference, which refers to the daily clock

reset at 12:00:00 a.m. ASC/3 firmware lets users establish an Event Sync Reference, which

uses the time of a plan action as the reference point. In this study, initiation of the preempt


call was used as the Event Sync Reference point and was combined with corridor-level

preemption plans to enable automatic switching of plans. This theoretically minimizes

transition periods with the preemption.

Scenarios

Using all parameters and configurations discussed above, we evaluated a total of 40

scenarios. Table 5 summarizes these scenarios.

Table 4: Evaluation Scenario Summary

YES NO 0:00:00 EVENT YES NO 2 START 2 MID 2 - 75% 4 - 50% Inter. Corridor

1 X N/A2 X X3 X X4 X X X X X5 X X X X X6 X X X X X7 X X X X X8 X X X X X9 X X X X X

10 X X X X X11 X X X X X12 X X X X X13 X X X X X14 X X X X X15 X X X X X16 X X X X X17 X X X X X18 X X X X X19 X X X X X20 X X X X X21 X X X X X22 X X X X X23 X X X X X24 X X X X X25 X X X X X26 X X X X X27 X X X X X28 X X X X X29 X X X X X30 X X X X X31 X X X X X32 X X X X X33 X X X X X34 X X X X X35 X X X X X36 X X X X X37 X X X X X38 X X X X X39 X X X X X40 X X X X X

PMT TO CORD

SCENARIO

PREEMPTION ENABLED SYNC REFERENCE LOCAL CYCLE EXIT POINT PMT LEVEL


RESULTS

Each scenario identified above was evaluated using VISSIM HILS discussed earlier in the

report. Each scenario was evaluated based on various system wide measures of effectiveness

including 1) average network delay, 2) average westbound (inbound) travel time and 3)

average eastbound (outbound) travel time. Table 6 shows summary of all runs for all 40

scenarios evaluated.

In addition to network measures instantaneous speed plots were developed for each

intersection which shows second by second vehicle trends allowing for review of vehicle

speed impact before and after preemption events, an example of the speed plots is shown in

Figure 8 below. Time is shown on the x-axis with vehicle speed on the y-axis. Traffic signal

cycle trends can be identified with each increase and decrease in speed. The preemption

event is identified by the Yellow Line.


Figure 7: Instantaneous Speed Plot

US 60 at Sir Barton

US 60 at Elkhorn

US 60 at I-75 SB Ramps

US 60 at I-75 NB Ramps


Table 6: Simulation Output Summary

YES

NO

0:00

:00

EVEN

TYE

SN

O2

STAR

T2

MID

2 - 7

5%4

- 50%

Inte

r.Co

rrid

orAv

g O

f Tr

avel

Ti

me

Avg

Of

Del

ay

Avg

Of

Trav

el

Tim

e

Avg

Of

Del

ay

1X

N/A

42.2

229

105.

716

650

.12

XX

42.6

217

93.8

169

52.8

3X

X38

.922

410

1.5

168

51.4

4X

XX

XX

41.8

219

95.9

171

55.9

5X

XX

XX

41.0

228

104.

716

347

.46

XX

XX

X39

.322

298

.716

750

.57

XX

XX

X41

.722

299

.116

649

.28

XX

XX

X36

.921

793

.417

054

.09

XX

XX

X40

.722

299

.316

649

.910

XX

XX

X44

.822

510

2.3

175

58.8

11X

XX

XX

41.7

222

99.3

172

55.7

12X

XX

XX

43.8

222

98.6

175

59.4

13X

XX

XX

36.7

221

97.6

163

46.9

14X

XX

XX

40.5

217

93.6

161

45.3

15X

XX

XX

42.0

221

97.7

164

48.8

16X

XX

XX

41.3

224

100.

416

750

.817

XX

XX

X41

.821

591

.816

548

.618

XX

XX

X40

.721

692

.816

851

.119

XX

XX

X43

.822

310

0.3

167

50.7

20X

XX

XX

38.7

222

99.1

165

48.5

21X

XX

XX

39.4

219

96.1

166

50.2

22X

XX

XX

41.3

222

98.9

169

52.5

23X

XX

XX

42.9

227

103.

516

851

.324

XX

XX

X44

.822

810

4.7

165

49.3

25X

XX

XX

38.8

231

108.

017

255

.326

XX

XX

X40

.921

490

.816

751

.327

XX

XX

X40

.722

510

2.3

166

49.5

28X

XX

XX

41.2

231

108.

316

953

.529

XX

XX

X41

.522

710

3.6

166

49.9

30X

XX

XX

39.4

230

107.

417

660

.131

XX

XX

X43

.321

086

.817

256

.432

XX

XX

X41

.122

910

5.8

174

57.1

33X

XX

XX

41.5

232

108.

716

852

.434

XX

XX

X39

.422

299

.017

054

.335

XX

XX

X36

.821

995

.917

356

.436

XX

XX

X37

.922

810

5.2

163

46.6

37X

XX

XX

45.7

230

106.

816

750

.738

XX

XX

X39

.922

510

2.1

174

57.2

39X

XX

XX

42.2

215

91.8

167

50.6

40X

XX

XX

37.7

226

103.

016

751

.1

PMT

TO C

ORD

SCEN

ARIO

PREE

MPT

ION

EN

ABLE

DSY

NC

REFE

REN

CELO

CAL

CYCL

E EX

IT P

OIN

TPM

T LE

VEL

Aver

age

Net

wor

k D

elay

Inbo

und

Out

boun

d


Intersection Vs. Corridor Level Preemption

Table 6 summarizes network MOEs for intersection level and corridor level preemption. All

scenarios had the following setup: preemption enabled, cycle length of 120 sec, zero sync

reference time, preempt to cord enabled and the local cycle exit time varied. The average

network delay among all three phase exit points is 41.5 seconds for the intersection based

preemption and 42.9 seconds for the corridor based preemption methods. When evaluating

the inbound and outbound travel times, it is shown that the corridor based preemption

provided a significant higher inbound travel time (229.5 seconds) compared to the

intersection based preemption (221.6 seconds). However, outbound travel time remained

relatively constant between the two scenarios, (166.5 and 167.2 for intersection and corridor

preemption, respectively). When evaluating individual runs however, it is evident that the

placement of the call within the cycle was the larger determinant of changes in delay. Under

the corridor preemption scenario, a 10 percent increase in system delay was evident when the

preemption ended within the 75 percent point of phase 2, while the start and midpoint

preemptions showed minimal impact on either intersection based on corridor based

preemption.

Table 5: Preemption Level: Intersection level Vs. Corridor level (Preempt to Coord)

Conversely, when intersection and corridor based preemption is evaluated with preempt to

coord disabled, a different pattern emerges (Table 8). Under this scenario the corridor

preemption is shown to have a reduced delay for the entire network of 40.1 seconds,

compared to 42.6 seconds for the intersection based coordination. When evaluated by

direction the corridor preemption still produces a higher travel time in the inbound direction

(229.5 vs 222.6), but lower for outbound vehicles (168.6 vs 171.2). The difference in

network delay may also be attributed to lower delay on the minor street approaches. It is

noted that under this configuration, preemption during the 75 percent point of phase 2

actually had the lowest network delay of all scenarios evaluated at 37.9 seconds.

SCENARIO YES NO 0:00:00 EVENT YES NO 2 START 2 MID 2 - 75% 4 - 50% Inter. CorridorAverage Network

Delay

Avg Of Travel Time

Avg Of Delay

Avg Of Travel Time

Avg Of Delay

5 X X X X X 41.0 228 104.7 163 47.411 X X X X X 41.7 222 99.3 172 55.717 X X X X X 41.8 215 91.8 165 48.629 X X X X X 41.5 227 103.6 166 49.933 X X X X X 41.5 232 108.7 168 52.437 X X X X X 45.7 230 106.8 167 50.7

PREEMPTION ENABLED LOCAL CYCLE EXIT POINT PMT LEVEL InboundSYNC REFERENCE PMT TO CORD Outbound


Table 7: Preemption Level: Intersection level Vs. Corridor level (No Preempt to Coord)

Figure 7 shows an instantaneous speed plot for scenarios 17 and 37, which show impacts of

intersection and corridor preemption at the 75 percent point of phase 2 with preemption to

coord disabled. individual vehicle instantaneous speed for the network for scenario #3 (left

graph) and #5 (right graph) which had the lowest and highest travel time respectively. The

point of preemption is shown with vertical yellow (Intersection based preemption) and green

lines (corridor based preemption). The blue dots represent individual instantaneous speeds

for individual vehicles and the red line indicates the average speeds for all vehicles for that

duration. We can see that after the preemption in scenario 5, the average vehicle speeds dips

at northbound ramp where as in scenario 3, it was consistent.


Delay

Avg Of Travel Time

Avg Of Delay

Avg Of Travel Time

Avg Of Delay


OutboundPREEMPTION ENABLED SYNC REFERENCE PMT TO CORD LOCAL CYCLE EXIT POINT PMT LEVEL Inbound


Figure 3 Instantaneous vehicle speed plot Intersection vs Corridor Preemption

(Preempt to Coord Disabled)

Figure 8 shows the same set of plots for with the preempt to coord enabled. The most

obvious difference between the graphs is identified at Sir Barton Way, the top intersection

within the plot. One cycle after the preemption, a discernible difference in speed is noticed

with a minimal reduction in speed, which is most pronounced within Figure 8, and the graph

on the right. The elimination of speed reduction at this point in the graph indicates a

minimization of stopping on the corridor, including side streets. The combination of

corridor-wide preemption and preempt to coord enabled, minimized dwell time, reducing

stops and delays on the sidestreet which is demonstrated within the graph at this intersection.


In effect, the preemption phase served to flush the system of congestion, returning to normal

operation several cycles after the preemption event.

Figure 8 Instantaneous vehicle speed plot Intersection vs Corridor Preemption

(Preempt to Coord Enabled)

Sync Reference Point

As discussed above, the option exists to shift the sync reference point, from a default

reference point of midnight or 0:00:00 hrs to an event based reference point coinciding with

the time of the cycle change or in this case, the emergency preemption event. The use of the

event sync reference provides the ability to identify a reference point that is more compatible


with the planned action, especially for unknown events such as emergency preemption. As

such it may provide short transitions and minimize dwell times to new cycles lengths. Table

6 shows the output for simulations evaluating the default sync reference time of 0:00:00 and

event based syncs for different local cycle exit points for intersection level preemption with

Preempt to coord disabled. Average network delay for the 3 Event based sync alternatives

show an average network delay of 38.7 seconds while the default sync had an average

network delay of 41.2 seconds. Average travel times for both the inbound and outbound

directions were also lower with the event based sync reference point at 218.5 and 165.4,

compared to 219.7 and 169.9 for the default sync reference. The fact that this configuration

provides both the lowest overall network delay, which accounts for side street delays, as well

as the lowest travel times for inbound and outbound movements on the primary roadway

indicates a significant improvement over the default values.

Examining results with preempt to coord enabled (Table 6) the results are not as significant.

Both event based sync and the default time based sync have identical network delay averages

of 40.7 seconds with only minimal improvement in inbound travel time for the event based

sync (220.8 seconds vs 222.0 seconds). Both sync reference points provide the identical

outbound travel times of 165.4 when preempt to coord is enabled.

TABLE 6 Sync Ref: Zero sync Vs. Event sync - NO PMT to Coord

TABLE 6 Sync Ref: Zero sync Vs. Event sync - with PMT to Coord


Delay

Avg Of Travel Time

Avg Of Delay

Avg Of Travel Time

Avg Of Delay


Inbound OutboundPREEMPTION ENABLED SYNC REFERENCE PMT TO CORD LOCAL CYCLE EXIT POINT PMT LEVEL


Delay

Avg Of Travel Time

Avg Of Delay

Avg Of Travel Time

Avg Of Delay


PREEMPTION ENABLED SYNC REFERENCE PMT TO CORD LOCAL CYCLE EXIT POINT PMT LEVEL Inbound Outbound


Figure 9 shows the instantaneous speed plot , relating to scenarios 21 and 23 identified in

Table 6 above. These scenarios relate to preempt calls exiting during 75 percent of phase 2

with no preempt to coord based on zero sync and event sync reference points. The top graph

provided for Sir Barton Road shows that in the event based simulation, significantly reduce

delays immediately after the preemption event with a longer stopped period and low speeds

within the initial cycle following the preemption. Subsequent queues for the event based

sync are shown to have higher average speeds and less stops than the zero sync simulation.

Figure 9 Instantaneous vehicle speed plot Zero Sync vs Event Sync (No Preempt to

Coord)


Local Cycle Exit Points

Table 8 shows the simulation output evaluating the preemption call exit points within the

local cycle. When we compare the travel time for inbound direction which is the preemption

route, the lowest travel time of 209.7 occurred when the preemption exited at beginning of

phase 2 using event sync reference and when corridor level preemption was implemented. On

the other hand, when corridor level preemption was adopted, preempt to coord function was

enabled, at zero sync reference and when the preemption call exited at mid-point of phase 2,

the travel time was the longest (231.9 sec). When averaged among all scenarios (Table 9),

exiting preemption at the beginning of phase 2 provides the overall lowest network delay.

This is likely due to the decrease dwell time and minimized impact on the overall system.

However, when corridor level preemption is used, lower network delays are shown to occur

during the latter portion of phase 2 or during phase 4, showing an improved transition

method for off-cycle interruptions. It is noted that travel times for the inbound and outbound

directions are actually shortest when the preemption exit point is at the midpoint of phase 2.

This is likely due to the result of added time for the phase, in conjunction with minimized

disruption through transition.

TABLE 8: Simulation Result by Local Cycle Exit Point

SCENARIO YES NO 0:00:00 EVENT YES NO 2 START 2 MID 2 - 75% 4 - 50%Average Network

Delay

Avg Of Travel Time

Avg Of Delay

Avg Of Travel Time

Avg Of Delay

10 X X X X 41.7 222.0 99.1 165.5 49.216 X X X X 36.7 220.8 97.6 163.4 46.922 X X X X 43.8 223.1 100.3 167.0 50.728 X X X X 38.8 231.0 108.0 171.8 55.3

9 X X X X 39.3 221.8 98.7 166.6 50.515 X X X X 43.8 221.5 98.6 175.5 59.421 X X X X 40.7 215.9 92.8 167.6 51.127 X X X X 44.8 227.8 104.7 165.0 49.312 X X X X 40.7 222.3 99.3 165.8 49.918 X X X X 42.0 220.9 97.7 164.3 48.824 X X X X 39.4 219.3 96.1 166.0 50.230 X X X X 40.7 225.4 102.3 165.7 49.511 X X X X 36.9 216.6 93.4 170.1 54.017 X X X X 40.5 216.7 93.6 161.3 45.323 X X X X 38.7 222.2 99.1 164.8 48.529 X X X X 40.9 214.1 90.8 167.4 51.3

LOCAL CYCLE EXIT POINT Inbound OutboundPREEMPTION ENABLED SYNC REFERENCE PMT TO CORD


TABLE 9: Average Simulation Results by Local Cycle Exit Point

Table 10 summarizes the network delay for all scenarios. When comparing the sync

reference points, event sync reference had lower average delay of 40 sec/ vehicle when

compared to delay for zero syn reference of 41.7 sec. When the preempt to coord was

disabled, the travel time was lower with 40.8 seconds when compared to when it was enabled

with 41.4 seconds. When the preempt call exited at beginning of phase 2, the travel time was

the lowest with 40.7 seconds. When the preemption level was compared between intersection

and corridor level, there was no significant difference between the two although the corridor

level preemption had higher maximum travel time.

TABLE 70 Summary for Network Delay Results for all scenarios

Network Delay (sec/veh)

Parameter Min Max Average

Zero sync reference 36.7 45.7 41.7

Event sync reference 36.8 43.3 40

Preempt to coord: Yes 36.7 45.7 41.4

Preempt to coord: No 36.9 44.8 40.8

Beginning of phase 2 36.9 43.3 40.7

Mid of Phase 2 36.7 44.8 41.5

Phase 2 (75%) 37.9 45.7 41.3

Preemption level: Intersection 36.7 44.8 41.1

Preemption level: Corridor 36.8 45.7 41.2

CONCLUSION

This research sought to identify controller configurations to minimize transition impacts

resulting from emergency vehicle preemptions. The study analyzed a single urban corridor

Exit PointAverage Network

DelayInbound TT Outbound TT

Start F2 39.65 220.66 167.00Mid F2 40.74 219.98 166.1375% F2 40.66 220.12 166.34Mid F4 41.31 224.58 167.49


which serves as a major commuting corridor and retail development corridor within

Lexington Kentucky. The corridor was evaluated using microsimulation software in

conjunction with a Hardware in the Loop System allowing evaluation of standard advanced

traffic controller configurations. Three basic configurations were evaluated, 1) Sync

Reference Point, 2) Preemption to Coordination (Preempt to Coord) Functionality and 3)

Corridor and Intersection Level Preemption Action Sets.

The results demonstrated that the corridor level preemption scheme developed by this

research, which implements action sets through an ATMS to preempt the entire corridor in

order to empty downstream queues and minimize transition, provides the lowest overall

travel time for the preempt direction when used in combination with event based sync

options and preempt to coord functionality. However, this also provides one of the longer

average network delays as priority is given to inbound direction.

The use of the event based sync reference point was shown to provide lower average network

delays and higher inbound and outbound speeds through most scenarios evaluated. Of all 3

configurations evaluations T-test analysis identified only the event based reference as having

a statistically significant impact on lowering delays and travel times.

Preempt to coordination functionality was shown to have mixed results, having an overall

average network delay and inbound and outbound travel times similar to results when the

functionality was disabled. However, under certain configurations, preempt to coord was

shown to assist in providing superior results such as the lowest reported travel time runs.

Finally, the preempt exit points were also shown to be critical in minimizing network delay

and minimizing arterial travel times. As expected, minimal disruption and subsequent lower

network delays were evident when the preempt call exited during the beginning of phase 2.

However, inbound and outbound arterial travel times were minimized when the preempt call

exited at the midpoint.


This research shows that the preempt call configuration settings have the potential to

significantly impact the network delay and travel times of arterial coordinated traffic signal

system associated with emergency vehicle preemption. Operational changes are the result of

minimized disruption of the transition period to coordinated operations after the preemption

event. Several factors were shown to positively influence this transition which may be used

to implement or adjust emergency vehicle systems, but may also be used to improve

efficiency of coordinated plan change implementations. Overall the most significant function

was the use of event sync reference points, over zero reference points used at most

installations. The use of event based coordination provided reduced travel times and

minimized delays resulting from more efficient transition steps. In addition, the timing of the

preempt call exit was shown to significantly impact overall corridor performance. While

travel time savings for the corridor or for individual emergency vehicles, are not significant

enough to warrant delayed preemption calls, it is possible that additional planning go into

scheduled coordinated plan change times to better target the beginning of mainline phase

operations at primary intersections on a corridor, as opposed to selecting arbitrary start end

times. Finally, corridor level preemptions were shown to be effective in reducing travel time

and providing improved speeds for preempt vehicles, however, this does come at the cost of

increased delays and impacts on opposing directions. As such corridor preemption plans

may be considered sparingly in areas of high congestion.


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Documents

DEVELOPMENT AND EVALUATION OF A COORDINATED TRAFFIC … · Traffic Signal Emergency Preemption System EXECUTIVE SUMMARY Most traffic signal controllers have emergency vehicle preemption