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<article xmlns:xlink="http://www.w3.org/1999/xlink">
  <front>
    <journal-meta />
    <article-meta>
      <title-group>
        <article-title>An Approach to Optimizing CO2 Emissions in Traffic Control via Reinforcement Learning</article-title>
      </title-group>
      <contrib-group>
        <contrib contrib-type="author">
          <string-name>Olexander Ryzhanskyi</string-name>
          <xref ref-type="aff" rid="aff1">1</xref>
        </contrib>
        <contrib contrib-type="author">
          <string-name>Eduard Manziuk</string-name>
          <email>eduard.em.km@gmail.com</email>
          <xref ref-type="aff" rid="aff1">1</xref>
        </contrib>
        <contrib contrib-type="author">
          <string-name>Olexander Barmak</string-name>
          <email>lexander.barmak@gmail.com</email>
          <xref ref-type="aff" rid="aff1">1</xref>
        </contrib>
        <contrib contrib-type="author">
          <string-name>Iurii Krak</string-name>
          <email>yuri.krak@gmail.com</email>
          <xref ref-type="aff" rid="aff0">0</xref>
          <xref ref-type="aff" rid="aff3">3</xref>
        </contrib>
        <contrib contrib-type="author">
          <string-name>Nebojsa Bacanin</string-name>
          <email>nbacanin@singidunum.ac.rs</email>
          <xref ref-type="aff" rid="aff2">2</xref>
        </contrib>
        <aff id="aff0">
          <label>0</label>
          <institution>Glushkov Cybernetics Institute</institution>
          ,
          <addr-line>40 Glushkov ave., Kyiv</addr-line>
          ,
          <country country="UA">Ukraine</country>
        </aff>
        <aff id="aff1">
          <label>1</label>
          <institution>Khmelnytskyi National University</institution>
          ,
          <addr-line>11, Instytuts'ka str., Khmelnytskyi</addr-line>
          ,
          <country country="UA">Ukraine</country>
        </aff>
        <aff id="aff2">
          <label>2</label>
          <institution>Singidunum University</institution>
          ,
          <addr-line>11000 Belgrade</addr-line>
          ,
          <country country="RS">Serbia</country>
        </aff>
        <aff id="aff3">
          <label>3</label>
          <institution>Taras Shevchenko National University of Kyiv</institution>
          ,
          <addr-line>60 Volodymyrska str.</addr-line>
          ,
          <country country="UA">Ukraine</country>
        </aff>
      </contrib-group>
      <abstract>
        <p>Automotive transport plays a key role in ensuring economic development but is accompanied by significant negative impacts on the environment, particularly in areas where vehicles are concentrated. This article presents an approach that uses reinforcement learning and accounts for traffic flow pressure to optimize the travel time of vehicles through road intersections with the aim of reducing CO2 emissions. The proposed method is based on modern approaches to optimizing traffic light operations, but with an emphasis on ecological aspects. Experimental verification on the synthetic scenario SUMO GRID 4x4 demonstrates the efficiency of the developed algorithm. Comparative analysis shows that it outperforms other algorithms, such as MaxPressure and IDQN, in particular, it improves travel time and queue length by 33%, and reduces CO2 emissions by 3233%. The obtained results lay the foundation for further refinement and implementation of the proposed approach in real-world conditions.</p>
      </abstract>
      <kwd-group>
        <kwd>traffic signal control</kwd>
        <kwd>reinforcement learning</kwd>
        <kwd>reward modeling</kwd>
        <kwd>pollutant emissions1</kwd>
      </kwd-group>
    </article-meta>
  </front>
  <body>
    <sec id="sec-1">
      <title>1. Introduction</title>
      <p>Road transport plays an important role in ensuring economic growth and social development.
It is defined as a key component of the transportation system due to its objective advantages,
which are reinforced by significant achievements in the transport infrastructure of the vast
majority of countries. Road transport is also widely used and is a key priority in economic
development. However, such circumstances are accompanied by significant pressure on the</p>
      <p>© 2023 Copyright for this paper by its authors. Use permitted under Creative Commons License Attribution 4.0 International (CC BY 4.0).
environment, especially in places where vehicles are heavily congested. Some of these places
are large cities and transportation interchanges. Hence the problem of transport regulation.
There is a wide range of problems for the solution of which information technologies of traffic
improvement are [1]. Accordingly, the development of systems for the formation of
climateneutral and smart cities is of great importance given the current challenges associated with
climate change and urban growth. Such systems are determined by the need for the following
reasons:
•
•
•
•
•</p>
      <p>They help to achieve climate neutrality, which is a strategic objective under the
European Green Pact [2]. Cities make a significant contribution to greenhouse gas
emissions, and the implementation of systems aimed at optimizing resources and
reducing environmental impact helps to solve this problem.</p>
      <p>Smart cities improve the quality of life for citizens. By optimizing traffic flows and
reducing air and noise pollution, they contribute to the health and well-being of the
population.</p>
      <p>Such systems help to reduce the urban ecological footprint and support sustainable
development. They aim to reduce resource consumption and develop more efficient
strategies for managing urban resources.</p>
      <p>The development of such systems promotes political coherence and citizen
participation in decision-making. This is important for ensuring the effectiveness of
strategies and achieving climate neutrality.</p>
      <p>Smart cities are being integrated into European and global strategies, contributing to
the achievement of global climate goals and providing synergies with other initiatives.</p>
      <p>To summarize, intelligent systems are a key element of digital transformation and
innovation, enabling cities to use modern technologies more effectively to achieve climate
neutrality and support sustainable development.</p>
      <p>One approach to developing intelligent systems is to use reinforcement learning, which
can be applied to a similar class of tasks [3]. This approach to artificial intelligence allows
systems to learn from the data they receive and gain experience to make optimal decisions in
real time. One of the key challenges is the efficient management of urban resources and
infrastructure to ensure sustainability and efficiency. Reinforcement learning can analyze and
optimize the operation of traffic lights, transportation systems, and other aspects aimed at
reducing emissions and improving energy efficiency. Particularly important is the ability to
train automated transport management systems, which helps to improve traffic flow and
reduce traffic congestion. This has an impact on CO2 emissions and improves air quality in
cities, which in turn affects public health and overall quality of life.</p>
      <p>Thus, the main contribution of the paper is the proposed approach using Reinforcement
Learning to finding the optimal mode of vehicles passing through a traffic light-controlled
crossroads according to the criterion of reducing CO2 emissions.</p>
      <p>The main contributions of the research include:
•</p>
      <p>A new approach to traffic signal control at road intersections using reinforcement
learning that takes into account the environmental impact of traffic, in particular CO2
emissions is proposed.
•
•
•
•</p>
      <p>The MPLightCO2 algorithm is developed, which is an extension of the existing
MPLight approach with additional consideration of CO2 emissions from vehicles
queuing to enter and exit the crossroads. This makes it possible to optimize crossroads
traffic modes in order to reduce environmental impact.</p>
      <p>It is proposed to take into account the "traffic flow pressure" metric to determine the
efficiency of vehicle distribution in the crossroads network and improve throughput.
Experimental verification and comparative analysis of the developed MPLightCO2
algorithm with other approaches, such as MaxPressure, MPLight, and IDQN, were
carried out on the synthetic test scenario SUMO GRID 4x4.</p>
      <p>The results showed that MPLightCO2 outperforms existing approaches in terms of
travel time, average queue length, and CO2 emissions, demonstrating increased
efficiency in both optimizing traffic flow and reducing its environmental impact,
which allowed reducing queue length by 75-76% and reducing CO2 emissions by
3233%.</p>
      <p>The article is structured as follows. In the Related Works section, we review current
approaches to solving similar problems and formulate the purpose of the paper. The Methods
and Materials section describes the crossroads control system, provides a formalization of its
elements, presents an approach using traffic pressure, describes the DQN agent, the
implementation of deep Q-learning, characterizes the SUMO GRID 4x4 synthetic test and
approaches to assessing the quality of the solutions obtained. The Results and Discussion
section analyzes the results of experimental testing on SUMO GRID 4x4, the quality indicators
of the models, and compares them with other algorithms. The Conclusions and Future Work
section summarizes the results of the study, outlines limitations and directions for further
work.</p>
    </sec>
    <sec id="sec-2">
      <title>2. Related Works and Basic Concepts of Approximate Dynamic</title>
    </sec>
    <sec id="sec-3">
      <title>Programming</title>
      <p>A review of recent publications on the topic of the study showed that the modern
reinforcement learning approach is actively used to solve such problems. Below is an
overview of these publications. Modern development trends in the field of artificial
intelligence are actively used to implement effective strategies for optimizing traffic flows in
cities. The main goal is to reduce environmental impact through the development and
application of various methods and technologies. Artificial intelligence plays a key role in this
context, helping to create intelligent systems that ensure efficient traffic management. In
particular, deep learning algorithms are used to develop smart traffic light control systems
aimed at dynamic adaptation to changes in traffic flow in real time [4][6]. This not only
minimizes stops and saves fuel, but also has a positive impact on emissions. It is important to
study approaches that would meet all the requirements of AI reliability [7][8][9]. Much of the
papers is aimed at optimizing traffic flows to increase the capacity of transportation routes.</p>
      <p>Another approach is to predict and manage transportation demand, which is becoming
another aspect where machine learning methods are used to accurately analyze passenger
flow data and predict its changes at different times of the day [10][12]. This allows optimizing
the allocation of resources, reducing the number of empty flights and thus contributing to the
reduction of CO2 emissions.</p>
      <p>The use of traffic monitoring and analysis systems based on data from sensors and cameras
allows artificial intelligence algorithms to identify patterns and predict possible traffic
congestion [13]. This opens up opportunities for taking effective measures to avoid
congestion and, therefore, reduce the negative impact on the environment. The use of route
optimization algorithms is also important in the context of reducing the environmental impact
of transport [14] [15]. These algorithms take into account various aspects, such as minimizing
the use of traffic lights and separating environmentally friendly routes.</p>
      <p>The introduction of electric vehicles and autonomous cars is a key step in ensuring
environmentally sustainable transportation. Research on the safety [16] and reliability of
communication equipment is also important [17]. Artificial intelligence is used to optimize
their movement and develop charging station infrastructure [18] [19]. An integrated approach
to optimizing traffic flows in cities allows achieving traffic efficiency, reducing emissions and
promoting sustainable urban transport. Such approaches can improve the conditions of
movement of vehicles along the roads, reducing their delay, improving speed conditions,
which ultimately has a positive impact on transport emissions, improving the environment.
Methods for constructing neural networks are being developed and refined [20][22]. In
pursuit of this goal, research is being conducted using modern reinforcement learning
algorithms to optimize the performance of signal controllers in real time [23] [24]. In this
approach, the state of the crossroads is determined by the parameters of vehicles (lane, speed,
waiting time, queue position) and the actual signal (traffic permission). The main task of
reinforcement learning, which is used in the form of an agent, is to optimize a strategy that
adapts states to the signals. This approach has shown a potential reduction in vehicle delays
of up to 73% compared to a fixed response time [25]. A method of multi-agent reinforcement
learning known as cooperative dual Q-learning is used to solve the complexity of traffic signal
synchronization in large-scale traffic control systems [26]. It uses independent dual
Qlearning methods and an upper confidence bound policy to avoid overestimation problems
that can occur in traditional algorithms. A new reward distribution mechanism and a local
state distribution method are introduced to ensure stable and robust learning. Experiments on
traffic flow scenarios show that the proposed system outperforms state-of-the-art
decentralized algorithms on various traffic metrics.</p>
      <p>Current research is overwhelmingly focused on the use of intelligent systems to optimize
traffic flows and reduce congestion. Strategies should also actively seek to improve the
environmental performance of transportation. Research focuses on traffic optimization rather
than on the full range of environmental aspects and practical measures to reduce the
environmental impact of transport. To achieve environmentally sustainable transport, it is
important to consider not only traffic efficiency, but also improvements in air quality and
overall environmental sustainability.</p>
      <p>Thus, the aim of the study is to develop and test the effectiveness of an approach that uses
reinforcement learning and traffic pressure to optimize vehicle travel times through road
intersections. Particular emphasis is placed on traffic signal control to reduce CO2 emissions.
The research includes validation of the proposed approach on a synthetic grid4x4 test and
further analysis of the results.</p>
    </sec>
    <sec id="sec-4">
      <title>3. Methods and Materials</title>
      <p>To systematize the traffic flow at a crossroads, we define typical scenarios, which we will call
"states". At a signalized crossroads, there are incoming and outgoing roads, each of which may
include one or more lanes. For each crossroads, we define a set of states ST, where each
specific state st ∈ ST is associated with a specific direction of traffic.</p>
      <p>The states are considered as conflicting if they cannot be activated simultaneously due to
traffic crossroads. At each stage, the signaling controller is responsible for establishing a
specific combination of non-conflicting states in order to optimize the long-term objective
function. For reinforcement learning-based controllers, the signalized crossroads environment
can be modeled using the following description.</p>
      <p>The state space is formed by the mapping of incoming traffic and active states. It is
particularly important to consider the differences in research approaches, where some take
into account high-resolution traffic detection technologies such as real-time observations of
vehicle counts, waiting times, and average speeds, while others are limited to less informative
data such as visibility of queue lengths or waiting times for the first vehicle. In terms of the
envisioned sensing radius, some take a broad approach that covers all entrance roads, but a
more realistic approach is to use a fixed sensing radius rs. This may depend on the
technological capability of the detection to provide reliable results, and take into account local
features such as terrain, visibility limitations, or the presence of obstacles such as buildings or
trees. In the context of alarm management, at each time step, the controller determines a set
of non-conflicting states that are allowed to move, which is indicated by the green light. If
there is a difference between the selected states and the active states, a mandatory yellow
state is automatically entered for a fixed period of time. The assignment of yellow states is a
constraint on the sequence of environmental control, not part of the action space.</p>
      <p>The transition function is determined by the development of traffic after the signal is
activated. This dynamics can be modeled according to a specific traffic model in the simulated
environment or taken from real traffic progress data as part of a real-world implementation.
The reward function typically uses the reduction in queue length as the sum of the respective
scores of all incoming lanes and is expressed as an integral reward. This is effective in
reducing congestion, but does not always normalize the benefits of signal optimization over
the travel time of a particular route. Therefore, other reward functions are used, such as total
delays, crossroads delays, crossroads waiting time, traffic volume, and others.</p>
      <p>However, for proper systematization of traffic at the crossroads, it is necessary to take into
account the action space. At each time step, the controller selects a set of non-conflicting
states that receive permission to move, which is indicated by turning on the green light. If the
selected states differ from the current active states, a mandatory yellow state is automatically
entered for a predefined period of time. The assignment of yellow states is a constraint
imposed on the sequence of environmental control, not part of the action space.</p>
      <p>The consistency of the defined action space and the choice of optimal states determines the
efficiency and safety of traffic at the crossroads. Even taking into account the different
approaches to traffic detection, it is important to consider that parameters such as the
prescribed sensing radius can affect the accuracy and reliability of the data obtained. Let's
formalize the elements of the control system at the crossroads. To do this, let's define the main
parameters taking into account the states.</p>
      <p>Let us denote the set of states as  = {  1,   2, . . . ,    },   is the specific state associated
with the direction of traffic;  ∈  - the total number of states at the crossroads, determined
by the number of specific directions of traffic that are selected for modeling or need to be
taken into account.</p>
      <p>The number of states can be determined by a ratio that takes into account the number of
possible options for each direction of traffic on each road. If there are  input roads, each of
which has    possible directions of movement. Then the total number of states  is
calculated by formula:</p>
      <p>
        = ∏ =1   . (
        <xref ref-type="bibr" rid="ref1">1</xref>
        )
      </p>
      <p>This formula presents the product of the number of possible directions of movement on
each input road. Thus, the total number of unique states in the state space is determined  ∗.</p>
      <p>Let us define the action domain  = {  1,   2, . . . ,    }, where    is the specific
action of the controller at each time step;  ∈  is the total number of possible controller
actions. The transition function is defined as follows  :  ×  →  , where  (   ,    )
represents the new state to which the system will move by performing an action    in state
   .</p>
      <p>For the reward function, we establish a ratio  :  ×  →  that determines the
amount of reward for choosing a specific action in a specific state.</p>
      <p>A restriction is set if the selected states differ from the active states, and a mandatory
yellow state is automatically entered for a certain period of time.</p>
      <p>Yellow state restrictions can be represented in the form of the following ratios.
We denote sets:
   is the set of active states;
is the set of selected states;
is the set of yellow states to be entered as mandatory for a certain period of time.
Then the constraint can be expressed by the following formula:
  
= (  
∖   
) ∪ (  
∩   
).</p>
      <p>This formula defines the set of yellow states as the union of those selected states that are
not yet active (  
∖   
) and the crossroads of selected and active states (  
∩</p>
      <p>
        A mandatory yellow state can be introduced with an additional parameter, for example
Tmylw , which defines the duration of the mandatory yellow state. In this way, it is possible can
define the time interval during which the yellow state will be active after the selected states
have changed. For example:
  
( ) = {
  
( ) ∖   
∅
( ) ,  ≤   
,  &gt;   
,
(
        <xref ref-type="bibr" rid="ref2">2</xref>
        )
(
        <xref ref-type="bibr" rid="ref3">3</xref>
        )
(
        <xref ref-type="bibr" rid="ref4">4</xref>
        )
(
        <xref ref-type="bibr" rid="ref5">5</xref>
        )
      </p>
      <p>).
actions;</p>
      <p>(   ,    ) – a reward function that determines how effective the choice of a
 is the coefficient that takes into account the limitations of yellow states and the
specific set of actions is in each state;
number of conflict states;
|</p>
      <p>∩    | is the number of conflict states in the current state and selected actions.</p>
      <p>This formula takes into account the dynamics of the system, the impact of the selected
actions on the state, the reward for these actions, and the limitations on the number of
conflict states.</p>
      <p>It is also possible to take into account the effect of pressure on the movement of vehicles,
and possible changes in the system over time  . Taking these parameters into account, the
description of the system state will take the following form:
where  is the time;
  
( ) and</p>
      <p>( ) represent the sets of selected and active states at a given time  .</p>
      <p>Accordingly, the crossroads control system will look like this:
{
).</p>
      <p>The dependencies can be adapted and extended to fit the specific details of the crossroads
management system and to take into account various conditions and constraints.</p>
      <p>The required system parameters can be represented as follows:
where:    +1 is the new state of the system at the next time;</p>
      <p>(   ,    ) – transition function, which determines how the system moves from a
state    to a new state under the influence of the selected actions   ;

 is the coefficient that takes into account the impact of the reward on the selected
(   ,    ) + 
⋅ [
(   ,    ) −  ⋅   ⋅ (| 
 ∩    |) −  ⋅ 
(   ,    )],
where   is the dynamic coefficient that takes into account changes in the system over time;
 is the coefficient that takes into account the effect of pressure on the movement of
transport;</p>
      <p>(   ,    ) is the function of pressure on the movement of vehicles, which may
include factors such as traffic density, speed, and other factors.</p>
      <p>Let's take a closer look at the traffic pressure function. One possible approach is to take
into account traffic density and vehicle speed. The pressure function   ( 
 ,    ) can be
expressed as follows:
actions.
the crossroads.
where:  is the coefficient that determines the weight of the impact of traffic density and
vehicle speed on pressure;
   (</p>
      <p>,    ) is the traffic density, which can be measured by the number of
vehicles in a certain state and with selected actions;
 
 (</p>
      <p>,    ) is the average speed of vehicles in a certain state and with selected
The pressure function can be customized according to the specific characteristics of the
crossroads and the optimization goal. This formula allows taking into account both traffic
density and vehicle speed as factors that affect the pressure on the movement of vehicles at
Traffic density 
  (</p>
      <p>,    ) can be determined in a variety of ways, depending on the
data availability and specifications of the crossroads control system.</p>
      <p>
        Some solutions for determining traffic density:

(   ,    ) =  ⋅ 
  ( 
 ,    ) ⋅  
 ( 
 ,    ),
(
        <xref ref-type="bibr" rid="ref6">6</xref>
        )
(
        <xref ref-type="bibr" rid="ref7">7</xref>
        )
(
        <xref ref-type="bibr" rid="ref8">8</xref>
        )
•
•
•
•
      </p>
      <p>Installing counters on the entrance roads to count the number of vehicles entering the
crossroads. Traffic density is defined as the number of vehicles per unit of time on
each input road.</p>
      <p>Using modern transportation sensor technologies, such as cameras or sensors, to
automatically determine traffic density. Video stream or sensor data is analyzed to
determine the number of vehicles and their movement on the roads.</p>
      <p>Using information from transportation agents or transportation monitoring systems
that can provide traffic density data.</p>
      <p>Traffic modeling, which uses mathematical models to simulate traffic flows and
determine traffic density based on parameters such as speed, number of lanes, and
others.</p>
      <p>Depending on the conditions and availability of resources, it is possible to choose one or a
combination of these methods to determine the traffic density at a particular time and the
state of the crossroads.</p>
      <p>The general approach to determining the schedule density is as follows:
   ( 
 ,    ) =
where:   
time;
crossroads.
is the represents the Q volume of vehicles entering the crossroads per unit of
is the represents the Q total length of all entrance roads leading to the
The total volume of vehicles Vlmtotal is determined by taking into account the number of
traffic flows and their characteristics on each input road. Let    the traffic flow from
direction  to direction . Then the total vehicle volume is defined as:
vehicles traveling from the respective directions.</p>
      <p>The total length of all input roads   
if it is different for different roads. Let be 
length is determined by the formula:
is determined by adding the width of each road,
  the width of the input road. Then the total</p>
      <p>In order to take into account the factors for determining the volume of vehicles, we write
down the total volume of vehicles</p>
      <p>, taking into account the factors related to the
speed of vehicles</p>
      <p>on each input road between crossroads  and  . We also introduce the
travel time factor Timeij , which determines the time required to travel the distance between
roads  and  and take into account the traffic density between the roads in terms of the
number of vehicles per unit time per kilometer 
_   .</p>
      <p>
        Then the formula for determining the total volume of vehicles will look like this:
(
        <xref ref-type="bibr" rid="ref9">9</xref>
        )
(
        <xref ref-type="bibr" rid="ref10">10</xref>
        )
(
        <xref ref-type="bibr" rid="ref11">11</xref>
        )
  
= ∑ ∑(


_ 
  ⋅    ⋅ 
  ⋅  
 ⋅ 
  ).
      </p>
      <p>Thus, we take into account not only the length of each road, but also its width, which is an
important parameter in determining the volume of vehicles and traffic density at the
crossroads.</p>
      <sec id="sec-4-1">
        <title>3.1. Pressure-based Coordination</title>
        <p>distribution of vehicles.
capacity.</p>
        <p>In order to optimize the traffic flow in the field of pressure management, the concept of flow
pressure is becoming a staple. The main focus is on improving the efficiency of the traffic flow
in general. The crossroads load is defined as the difference between the length of the queues
of vehicles approaching the crossroads and those leaving it. This reflects an imbalance in the</p>
        <p>The main task is to minimize this pressure in order to achieve equilibrium in the
distribution of vehicles along the network of directions and, as a result, increase the network</p>
        <p>The maximum pressure control strategy aims to optimize stability by not only stabilizing
traffic but also maximizing flow using local data from each crossroads.</p>
        <p>The main aspect of this strategy is to optimize traffic signal performance by reducing the
pressure in each state. In real-world maximum pressure control, a greedy approach is used to
achieve a locally optimal decision.</p>
        <p>Algorithm 1: Controlling the maximum pressure for each crossroads.
1. Pressure initialization and estimation. For each state at the crossroads, the pressure
 (   ) is calculated, taking into account various aspects of the traffic flow, such as
density, speed, and vehicle interaction.
2. Weight determination of the next state. Given the need to balance the various aspects
of traffic, determine the next state    +1 as the argument that maximizes pressure
reduction while taking into account environmental considerations.
3. Adaptive pressure control. Taking into account the dynamics of the movement, the
pressure calculation parameters are adaptively changed to ensure an effective
response to changing road conditions.
4. Synchronization with other road intersections. Optimized state selection, taking into
account common and interacting factors with other road intersections, to achieve
harmonious traffic in the system.
5. Additional function of emergency states. Additional functions, such as emergency
management or improved mobility of road users, are tested to ensure that a wide
range of circumstances are taken into account.</p>
      </sec>
      <sec id="sec-4-2">
        <title>3.2. DQN Agent</title>
        <p>Agents in the reinforcement learning method seek to maximize their overall reward within
the objectives of the maximum pressure control method. This increase in reward is
proportional to the overall network throughput, subject to certain constraints.</p>
        <p>Each agent is constrained to a certain subset of the overall system state. For example, for a
typical crossroads that manages traffic flows, the agent's observation covers the active state
and the pressure associated with the flows. In the case of fewer flows, the observation vector
may contain zeros to maintain consistency.</p>
        <p>The agent selects the state at any given time, determining the traffic light configuration.
This approach allows for greater adaptability by allowing the agent to choose the optimal
state to activate.</p>
        <p>The reward for the agent is determined by the reduced pressure at the crossing. This
pressure takes into account the difference in CO2 emissions from vehicles waiting to enter and
exit the crossroads.</p>
      </sec>
      <sec id="sec-4-3">
        <title>3.3. Implementation of Deep Q-learning</title>
        <p>Based on the chosen basic model, we apply the DQN method to solve problems related to
various scenarios of traffic lights control at road intersections. The DQN implementation
takes as input the state characteristics of different traffic flows and calculates the Q-value for
each possible action, i.e., traffic state, based on the following Bellman equation:
 is the discount factor;
 (   ,    ) = 
(   ,    ) +  
 (   +1,    +1),
where:  (   ,    ) is the represents the Q-value for state    and action   ;

(   ,    ) is the reward for performing an action    in the state   ;
 (   +1,    +1) is the maximum Q-value for the next state    +1 and all
possible actions   +1.
about future rewards and maximum Q-values.</p>
      </sec>
      <sec id="sec-4-4">
        <title>3.4. Synthetic test SUMO scenario GRID 4x4</title>
        <p>This equation estimates the Q-value for the current state and action using information
•
•
•
•
•
•
•
The GRID 4x4 scenario in SUMO (Simulation of Urban MObility) is a synthetic test case used
to simulate vehicle movements at road intersections in an urban environment. This scenario is
used to test and evaluate traffic signal control algorithms, road safety, and transportation
efficiency.</p>
        <p>In the GRID 4x4 scenario, the crossroads consists of 4x4 road segments, creating a network
of 16 road intersections. A large number of road intersections allow studying the interaction
of traffic flows, conflicts, and optimal management strategies.</p>
        <p>The main characteristics of the GRID 4x4 test scenario include:
location of 4x4 road intersections;
the number of roads is 16;
a large number of road intersections to study the interaction of traffic flows.
Identification of vehicle types, such as cars, trucks, buses, etc. Each type may have its
own characteristics in terms of fuel consumption and CO2 emissions.</p>
        <p>For each type of vehicle, it is necessary to specify characteristics such as average fuel
consumption and CO2 emission factor per unit of fuel consumed.</p>
        <p>Simulate the movement of vehicles in an urban environment, recording their routes,
speeds, and fuel consumption while driving.</p>
        <p>Based on the recorded data, we calculate CO2 emissions using the entered vehicle
characteristics.</p>
        <p>Such a test scenario allows researching and testing traffic control algorithms at road
intersections in an urban environment.</p>
      </sec>
      <sec id="sec-4-5">
        <title>3.5. Evaluation of the quality of solutions</title>
        <p>To determine the environmental impact of transport, in particular CO2 emissions, we use the
developed models with a traffic simulator. This makes it possible to model traffic in an urban
environment and take into account its environmental impact.</p>
        <p>To determine the environmental impact of CO2 emissions in the simulation, we will take
into account the following parameters:</p>
        <p>The calculation of CO2 emissions is usually based on fuel consumption and vehicle-specific
CO2 emission data. To determine the CO2 emissions, we defined the types of vehicles and
specify their characteristics, such as average fuel consumption and CO2 emission factor per
kilometer. During the simulation, we determine the fuel consumption for each vehicle and the
CO2 emissions.</p>
        <p>The fuel consumption is defined as follows:</p>
        <p>Each vehicle belongs to a certain category or type 
that consumes a certain amount of
fuel .</p>
        <p>
          CO2 emissions:
(
          <xref ref-type="bibr" rid="ref13">13</xref>
          )
(14)
Emissions are thus determined based on the fuel consumed 
by each vehicle unit and
    2 the CO2 emission factor per unit of fuel consumed.
        </p>
      </sec>
    </sec>
    <sec id="sec-5">
      <title>4. Results and Discussion</title>
      <p>To evaluate the effectiveness of our approach, we chose the SUMO GRID 4x4 scenario. This
scenario is characterized by a 4x4 road grid where each crossroads has the same settings and
parameters.</p>
      <p>In the SUMO GRID 4x4 scenario, traffic flows in a network consisting of a 4x4 grid of road
intersections. Each crossroads has the same settings and parameters, making it ideal for
testing the effectiveness of our approach.</p>
      <p>Throughout the experiments, we analyze various aspects such as the environmental impact
of transportation, fuel consumption, and traffic signal efficiency. These aspects are
determined by the scenario parameters and the performance of our approach to optimizing
traffic at road intersections.</p>
      <p>To test the performance of the developed algorithm, we used a traffic scenario similar to
the synthetic 4 × 4 symmetric network shown in Figure 1, the 4 × 4 Grid. The study conducted
a comparative analysis of the four algorithms:
•
•
•
•</p>
      <p>Maximum pressure control in which a combination of states with maximum joint
pressure is enabled.</p>
      <p>MPLight algorithm, which uses traffic light control approaches to optimize traffic
flow [4].</p>
      <p>Idependent Deep Q-Network, i.e. independent DQN agents. For each intersection, a
separate DQN agent is used, each of which uses the same convolutional neural
network to aggregate information from different lanes. The hyperparameters are left
by default in the Preferred RL library, except for the target network update interval,
which was adapted to the environment.</p>
      <p>An extension of MPLight that takes into account the environmental impact,
specifically CO2 emissions from vehicles queuing to enter and exit the crossroads.</p>
      <p>The algorithms were evaluated and compared across various environmental metrics,
providing conclusions on the effectiveness and sustainability of their implementation. The
diagrams below show the dynamics of queue length changes according to different numbers
of training episodes.</p>
      <p>A comparative analysis of the impact of different traffic signal control algorithms on traffic
efficiency and environmental impacts yielded the following results. The MPLight and
MPLightCO2 methods proved to be the most effective, improving travel time and waiting time
by about 33% compared to the MaxPressure algorithm. IDQN also showed improvement, but
less significant, increasing travel time and waiting time by about 34%. MPLight and
MPLightCO2 were effective in reducing CO2 emissions by about 32-33%, making them
environmentally friendly compared to MaxPressure and IDQN.</p>
      <p>Based on the results for travel time, MPLightCO2 performed the best, with a shorter
average travel time compared to the other models. The average queue length in number of
vehicle for MPLightCO2 is also the shortest, indicating more efficient traffic management and
reduced congestion. MPLightCO2 showed the lowest CO2 emissions of all the models,
indicating its greater environmental efficiency.</p>
      <p>MPLightCO2 performs better in terms of travel time, average queue length, and CO2
emissions than both MPLight and MaxPressure. Compared to the baseline MaxPressure
model, MPLightCO2 shows an improvement in travel time of 5.77%, in average queue length
of approximately 29.51%, and in CO2 emissions of approximately 7.43%.</p>
      <p>MPLight, MPLightCO2, and IDQN showed similar improvements in queue length and
maximum queue length, reducing them by about 75-76% and 70-71%, respectively, compared
to MaxPressure. Hence, MPLight and MPLightCO2 algorithms seem to be more effective from
both a traffic improvement and environmental perspective than MaxPressure and IDQN in the
studied scenario.</p>
      <p>MPLight and MPLightCO2 performed significantly better than MaxPressure and IDQN in
terms of travel time and waiting time. This may indicate the importance of considering not
only traffic but also environmental aspects in crossroads management. Taking into account
CO2 emissions in the MPLightCO2 algorithm led to a significant reduction in the
environmental impact of traffic. This is an important aspect in the context of urban
sustainability. The reduction in queue length and maximum queue length in the MPLight and
MPLightCO2 algorithms indicates their ability to effectively regulate traffic flow and provide
better crossing capacity at road intersections. It is important to consider how adaptive the
MPLight and MPLightCO2 algorithms are to different traffic conditions. Consider optimizing
the parameters to improve adaptability in different scenarios.</p>
    </sec>
    <sec id="sec-6">
      <title>5. Conclusions and Future Work</title>
      <p>The study presents an approach that uses reinforcement learning and traffic pressure to
optimize vehicle travel time through a crossroads to reduce CO2 emissions. The proposed
method is based on the modern approaches MPLight, PressLight, but with an emphasis on the
priority of the CO2 emission metric as a key component of decision-making.</p>
      <p>The developed algorithm has been experimentally tested on the synthetic test scenario
SUMO GRID 4x4, which simulates the movement of vehicles at road intersections in an urban
environment. The comparative analysis showed that the MPLight and MPLightCO2
algorithms, which take into account the impact on the environmental situation, proved to be
more effective than MaxPressure and IDQN. They demonstrated an improvement in travel
time and waiting time of up to 33%, a reduction in queue length by 75-76%, and a reduction in
CO2 emissions by 32-33%.</p>
      <p>MPLightCO2 showed the best results among the algorithms compared, with the shortest
travel time, shortest average queue length, and lowest CO2 emissions, indicating its high
performance from both a traffic improvement and environmental perspective.</p>
      <p>The results obtained are preliminary and more testing on different models and
configurations, as well as verification in real-world conditions, is needed. However, the
proposed approach has shown satisfactory control results in a large-scale road network,
which gives grounds for its further improvement and implementation.</p>
    </sec>
    <sec id="sec-7">
      <title>6. Acknowledgements</title>
      <p>
        The study was executed as a component of the Horizon Europe Framework Program,
receiving support from the initiative aimed at aligning Ukrainian cities with the mission for
Climate-neutral and smart cities (HORIZON-MISS-2023-CIT-02).
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