Successive Vehicles Research Articles

Evaluation of Conventional Surrogate Indicators of Safety for Connected and Automated Vehicles in Car Following at Signalized Intersections

The driving behaviors of connected and automated vehicles (CAVs) will differ from those of human-driven vehicles (HDVs) because the CAVs’ driving decisions are controlled by computers. Because of the limited amount of crash data for CAVs, researchers have relied on surrogate measures of safety to assess their safety impacts. However, they often use the same safety indicators for CAVs that were used for HDVs, raising questions about the adequacy of the safety indicators for CAVs. This study aims to investigate the suitability of using conventional safety indicators for CAVs. To achieve this, we evaluated eight safety indicators used for CAVs in the literature: time-to-collision (TTC), post-encroachment time (PET), time-exposed TTC, time-integrated TTC, deceleration rate to avoid a crash (DRAC), crash-potential index, rear-end-collision risk index, and potential index for collision with urgent deceleration (PICUD). For the evaluation, we first simulate CAVs on an approaching lane of signalized intersections using the acceleration-control algorithm. The algorithm replaces the HDV trajectories with CAVs for mixed simulations where HDVs and CAVs coexist. Analyzing the simulation output, we examined the safety indicators for the various car-following scenarios and the CAV proportions. The findings suggest that PET and PICUD can yield different safety implications for CAVs because of their small-gap car-following characteristics. Ignoring such characteristics may lead to interpreting the small-gap car-following situations as simply dangerous traffic interactions for CAVs. The car-following experiments indicate that TTC, PET, and DRAC are insufficient in measuring the safety implications when successive vehicles operate at similar speeds for CAVs.

An extended stochastic car‐following model and its feedback control

Real‐world traffic flow is influenced by various random factors. Gaussian white noise is employed to describe the inherent randomness of traffic flow. A stochastic optimization velocity model is established for dynamic analysis. To enhance the traffic flow stability, the velocity difference of two successive vehicles ahead is considered. The velocity difference feedback control strategy is proposed, utilizing the velocity differences between the vehicle and the two targeted vehicles as control signals. Stability conditions for both the stochastic optimization velocity model and the feedback control model are derived by using the moment stability theory. The study demonstrates that the random factors can destabilize traffic flow, while the proposed feedback control strategy effectively stabilizes traffic flow. Additionally, the influences of random factors on the stability of traffic flow are discussed extensively, aiding in understanding actual traffic congestion mechanisms and offering insights for traffic control strategies.

Elucidating Super-Compressible States in Inner Structure of Vehicular Flow

The article deals with the open questions in the theory of vehicular headway modeling. Specifically, the question of the existence of anomalous constellations in vehicular traffic micro-structure, in which the rate of fluctuations (measured by the stochastic compressibility) exceeds the fluctuation level of systems with non-interacting elements. The solution to this open problem is converted into the mathematical format working with the so-called balance particle systems, where seeking relevant relations is more straightforward and thus significantly easier. Presented research has shown that unit compressibility represents (despite popular opinion) the upper limit only for particle systems, in which there is no attractive interaction between the particles. In the article, the specific system is constructed in which the presence of an attractive force component will cause higher fluctuations than in the Poisson systems of non-interacting elements. This means that traffic constellations with higher compressibility (so-called super-compressible constellations) can be explained either by a discrepancy between the empirical traffic flow and the mathematical model used, or by the presence of attractive forces acting between individual vehicles. Using empirical vehicular data (measured on two parallel freeway lanes under reconstruction), we show that super-compressible states occur even though overtaking is prohibited. This means, therefore, that these super-compressible states arose without a doubt due to the mutual attraction of successive vehicles. In addition, the article shows that the presence of the aforementioned attractive forces appears predominantly in the fast lane, and only in situations where the traffic density is relatively low. At higher densities, the two freeway lanes are markedly synchronized, the opportunity for a sporty style of driving vanishes and the reason for changing lanes disappears. Under these circumstances, the attractive force component vanishes, which finally leads to the transition of the entire traffic system back to standard sub-compressible states.

Optimal control strategy for traffic platoon longitudinal coordination around equilibrium state enabled by partially automated vehicles

This paper presents an internal model-Kalman filtering-based optimal hybrid feedforward/feedback control strategy for traffic platoon control coordination enabled by SAE Automation Level 2 or Level 3 vehicles, i.e., partially automated vehicles (PAVs). Based on the Helly linear car-following model, a PAV platoon is established. Taking each vehicle’s characteristic polynomial as the dominant internal model polynomial, an augmented system state model with filtered inputs is formed, and then both the system states and the external disturbance/internal perturbation from previous vehicle can be estimated by a delicately designed Kalman filter. By using a linear quadratic regulation approach, a distributed hybrid optimal feedforward/feedback controller utilizing the local estimated states is constructed to achieve a practically decoupled platoon regulation around an equilibrium state. String stability of the control system thus obtained is further analyzed. Finally, extensive numerical simulations verified the theoretical analysis in decoupling between successive vehicles, in suppressing the influence of various external disturbances, and in maintaining string stability.

Analysis of stability, energy consumption and [formula omitted] emissions in novel discrete-time car-following model with time delay under V2V environment

This article aims to explore the stability, energy consumption and CO2 emissions of a novel discrete car-following model with time delay. Under the assumption that the preceding two successive vehicles can realize the communication through V2V (Vehicle to Vehicle) communication environment. Moreover, the time-delayed average optimal velocity difference control scheme is also considered. Based on the proposed discrete car-following model, error dynamical system is formulated and asymptotic stability is developed. The sufficient condition for the time-delayed average optimal velocity difference control scheme of the discrete car-following model with time delay is obtained, the asymptotic stability is ensured. In addition, numerical simulations are constructed and further illustrated the cooperative driving by V2V communication can improve stability, reduce energy consumption and CO2 emissions of traffic system. Moreover, the time-delayed average optimal velocity control scheme is also conductive to improve stability, reduce energy consumption and CO2 emissions of traffic system.

Adaptive Switched Control for Connected Vehicle Platoon With Unknown Input Delays.

A connected vehicle platoon with unknown input delays is studied in this article. The control objective is to stabilize the connected vehicles, ensuring all vehicles are traveling at the same speed while maintaining a safety spacing. A decentralized control law using only onboard sensors is designed for the connected vehicle platoon. A novel switching-type delay-adaptive predictor is proposed to estimate the unknown input delays. By using the estimated unknown input delays, the control law can guarantee the stability of the successive vehicles. The platoon control adopts a one-vehicle look-ahead topology structure and a constant time headway (CTH) policy, which makes the desired spacing between vehicles vary with time. In this framework, the stability of the connected vehicles can be derived through the analysis of each pair of two successive vehicles in the platoon. Finally, an example is presented to illustrate the applicability of the obtained results.

On-board measurements using two successive vehicles to assess in-cabin concentrations of on-road pollutants

Gaseous and particulate pollutants are produced in large quantities from road vehicles. These traffic-related air pollutants can infiltrate the car-cabins exposing drivers and passengers to toxic substances with serious adverse health effects. The infiltration process depends on many parameters, those from outside of the vehicle (e.g. traffic intensity, meteorology, road type) and those depending on the vehicle itself such as the ventilation and/or the vehicle speed/age. In the present study, we undertook on-road measurements to assess the impact of ventilation and vehicles spacing (proximity effect) on in-cabin to on-road (I/O) concentration ratios of nitrogen oxides (NOx) and ultrafine particles (UFP). The novelty of the present work is that two successive vehicles have been used to circumvent the influence of the emissions of the upstream vehicle on the downstream test vehicle. We show that the distance to the upstream vehicle influences significantly the (I/O) ratios. Overall, the results show that car cabin is less polluted when the ventilation is set to recirculation mode and keeping a distance to the upstream car corresponding to twice its height in non-idle conditions.

Piezoelectric-based energy harvesting from bridge vibrations subjected to moving successive vehicles by functionally graded cantilever beams – Theoretical and experimental investigations

This study aims to harvest electrical energy from the vibrations of bridges. For this purpose, a functionally graded (FG) cantilever beam installed at the mid-point of a bridge was utilised. First, the motion equations for the bridge and FG harvester were acquired based on Hamilton's principle. The bridge was considered as a simply-supported Timoshenko beam, whilst the Euler-Bernoulli beam assumption was used for the FG energy harvester. The material properties were assumed to be functionally graded in the thickness direction according to the power law distribution for both the substrate and the piezoelectric layer of the harvester. The obtained equations were solved using the Runge-Kutta method. In addition, a finite element (FE) simulation was carried out in COMSOL on the bridge-FG harvester system, and a test rig was constructed to perform some empirical tests. Finally, the voltage and power obtained from the theoretical model were compared with the FE results and measured data. An excellent agreement was observed among the results. This subject indicates that the proposed model was accurate enough to predict the behaviour of the bridge and FG harvester. The maximum power harvested by the FG harvester was measured to be about210 µWat the optimum value for the resistance.

A Novel Spatio-Temporal Synchronization Method of Roadside Asynchronous MMW Radar-Camera for Sensor Fusion

Roadside sensors, such as camera and millimeter-wave (MMW) radar, provide traffic information beyond the visual range of intelligent vehicles in cooperative vehicle-infrastructure systems. Unlike onboard equipment, roadside sensors are affiliated with different systems and lack synchronization in both space and time. In this paper, we propose a novel spatio-temporal synchronization method of asynchronous roadside MMW radar–camera for sensor fusion, which utilizes features of the scenario to extract lane line corner points to pre-calibrate the camera. Based on the consistent time flow rate of the separate sensors, multiple virtual detection lines are set up to match the time headway of successive vehicles and conduct objective matching to track data. Finally, a synchronization optimization model is formulated and a constrained nonlinear minimization solver is applied to tune the parameters. Measure data from Donghai Bridge in Shanghai is applied to verify the feasibility and effectiveness of the method. The results determine that there are 33 frames (33*40 ms) of temporal deviation between the camera and the radar in this case. After the synchronization, the average spatial deviation is reduced from 2.47 m to 0.42 m in the X-direction and 64.06 m to 2.34 m in the Y-direction, respectively. This study provides an economical and effective way to solve the problem of spatio-temporal synchronization of roadside sensors.

Impacts of Cooperative Adaptive Cruise Control Links on Driving Comfort under Vehicle-to-Vehicle Communication

Although automated vehicles could release drivers from the driving task, there are still passengers sitting in the vehicle. It is required that the driving comfort of passengers should be guaranteed. Cooperative adaptive cruise control (CACC) vehicle is of the one important type of automated vehicles using vehicle-to-vehicle (V2V) communications with various communication links. Different V2V communication links might have different driving comfort. Then, this paper focuses on exploring which link type for CACC vehicles is better from the perspective of improving driving comfort. To deal with this, car-following models of manual-driven vehicles (MDV) and CACC vehicles were first described. Then, simulations were performed using these car-following models, in which various CACC feedback link types, CACC penetration rates, and flow scenarios were taken into consideration. Simulations outputted microcosmic trajectory data of vehicles, based on which the driving comfort was evaluated using the comfort index described by the International Organization for Standardization (ISO) 2631-1. From the driving comfort perspective, simulation results suggest that CACC should monitor the immediately preceding vehicle and the third vehicle ahead when CACC penetration rates are less than approximately 50%. Additionally, if CACC penetration rates exceed 50%, the better choice is that CACC receives feedback links from two immediately successive vehicles ahead.

Statistical Modeling of Time Headway on Urban Roads: A Case Study in Baghdad

Time headway is an important feature of traffic engineering, determined by measuring the time elapsed between successive vehicles crossing a point on the road. This paper proposes a time headway modeling of urban roads for each lane of the selected sites based on headway distribution. The study was conducted in the capital of Baghdad, Iraq. The Mohamed Al-Qassim and Qanat Al-Jaish highways were chosen to collect field headway data. Data were obtained using image processing and Python for video analysis. Each road consisted of six lanes divided into three lanes in each direction. The three lanes were studied in the forward movement of both highways. Seven flow rates were considered for the headway data to evaluate the flow state for each lane. The EasyFit 5.5 software was used to select an appropriate headway distribution for each flow rate, using a method depending on the Kolmogorov Smirnov test for each lane. The results demonstrated that the proper model per lane was different. The selected distributions were used to develop a model through nonparametric regression using Theil's slope estimator method.

Enhanced drive‐by bridge modal identification via dual Kalman filter and singular spectrum analysis

The drive-by bridge health monitoring is to assess the bridge condition using the acceleration responses measured on the body or axle of instrumented vehicles. The vehicle responses are greatly affected by the road surface roughness that makes the bridge dynamic information blurred. Instead of direct using vehicle responses for the bridge monitoring, the dynamic response of contact point (CP) between the vehicle and bridge is further explored to enhance the drive-by bridge modal identification. A novel three-step framework is proposed to extract the components related to the bridge vibration from vehicle responses. The first step is to identify the input forces of two successive vehicles by solving the combined state-input estimation problem using dual Kalman filter. The CP responses of two contact points are calculated using the input forces and vehicle parameters. In the second step, the subtraction technique is applied to the identified CP responses of the two instrumented vehicles and the effect of the road surface roughness can be significantly reduced. Finally, an automatic singular spectrum analysis technique (auto SSA) is incorporated to decompose the response residual. Then the mono-component modes related to the bridge response are extracted from the response residual for drive-by bridge modal identification and/or the nonstationary characteristic identification of vehicle–bridge interaction (VBI) system. Results of numerical and experimental study demonstrate that the method can significantly suppress the vehicle response component and reduce the effect of road surface roughness to enhance the bridge modal identification.

On-Board Measurements Using Two Successive Vehicles to Assess In-Cabin Concentrations of On-Road Pollutants

On-Board Measurements Using Two Successive Vehicles to Assess In-Cabin Concentrations of On-Road Pollutants

Nonlinear adaptive cruise control of vehicular platoons

The paper deals with the design of nonlinear adaptive cruise controllers for vehicular platoons operating on an open road or a ring road. The constructed feedback controllers are nonlinear functions of the distance between successive vehicles and their speeds. It is shown that the proposed novel controllers guarantee safety (collision avoidance), speed positivity, bounded acceleration, speed limits by explicitly characterising the set of allowable inputs and by leveraging barrier function techniques. Moreover, we guarantee global asymptotic stability of the platoon to the desired configuration, as well as string stability. Certain macroscopic properties are also investigated. The main challenges stem from the fact that all previous objectives must be satisfied simultaneously and the fact that the control systems studied do not evolve in a finite-dimensional linear space but rather on specific sets. The efficiency of the nonlinear adaptive cruise controllers is demonstrated by means of numerical examples.

Tradeoffs between safe/comfortable headways versus mobility-enhancing headways in an automated driving environment: preliminary insights using a driving simulator experiment

PurposeThe anticipated benefits of connected and autonomous vehicles (CAVs) include safety and mobility enhancement. Small headways between successive vehicles, on one hand, can cause increased capacity and throughput and thereby improve overall mobility. On the other hand, small headways can cause vehicle occupant discomfort and unsafety. Therefore, in a CAV environment, it is important to determine appropriate headways that offer a good balance between mobility and user safety/comfort.Design/methodology/approachIn addressing this research question, this study carried out a pilot experiment using a driving simulator equipped with a Level-3 automated driving system, to measure the threshold headways. The Method of Constant Stimuli (MCS) procedure was modified to enable the estimation of two comfort thresholds. The participants (drivers) were placed in three categories (“Cautious,” “Neutral” and “Confident”) and 250 driving tests were carried out for each category. Probit analysis was then used to estimate the threshold headways that differentiate drivers' discomfort and their intention to re-engage the driving tasks.FindingsThe results indicate that “Cautious” drivers tend to be more sensitive to the decrease in headways, and therefore exhibit greater propensity to deactivate the automated driving mode under a longer headway relative to other driver groups. Also, there seems to exist no driver discomfort when the CAV maintains headway up to 5%–9% shorter than the headways they typically adopt. Further reduction in headways tends to cause discomfort to drivers and trigger take over control maneuver.Research limitations/implicationsIn future studies, the number of observations could be increased further.Practical implicationsThe study findings can help guide specification of user-friendly headways specified in the algorithms used for CAV control, by vehicle manufacturers and technology companies. By measuring and learning from a human driver's perception, AV manufacturers can produce personalized AVs to suit the user's preferences regarding headway. Also, the identified headway thresholds could be applied by practitioners and researchers to update highway lane capacities and passenger-car-equivalents in the autonomous mobility era.Originality/valueThe study represents a pioneering effort and preliminary pilot driving simulator experiment to assess the tradeoffs between comfortable headways versus mobility-enhancing headways in an automated driving environment.

A Combined Simulation Approach to Evaluate Overtaking Behaviour on Two-Lane Two-Way Rural Roads

A significant percentage of road fatalities and injuries occur in the nonmotorway rural road network. One of the main causes of accidents on these roads is represented by overtaking, as, by its nature, it involves a risk of a head-on collision with oncoming traffic. The paper describes a combined simulation approach (driving simulator and traffic microsimulation) designed to examine the influence of different traffic conditions on passing manoeuvres on two-lane two-way rural roads. The main focus was the evaluation of the end of the passing manoeuvre because it reflects the risk of a head-on collision. In addition, the study aimed to assess the usefulness of the proposed combined approach in the ability to proactively and quickly diagnose traffic safety problems and consequently to evaluate appropriate solutions. The data collected with an interactive driving simulator on a sample of 54 participants have been used to adjust some input data of the traffic microsimulation software. A specific situation consisting of a stationary heavy vehicle obstructing the entire lane was repeated in both experiments. The analyses focused on time-to-collision (TTC), defined as the remaining gap between the passing vehicle and the oncoming vehicle at the end of the passing manoeuvre. The results showed that the type of manoeuvre performed is significantly influenced by the traffic condition. Furthermore, the manoeuvre is influenced by the gap between two successive vehicles in the opposite lanes. Focusing on the end of the manoeuvre, it was found how a traffic increase leads to a significant reduction of the TTC values. Furthermore, the comparative analysis conducted between the data recorded following the combined approach and those obtained using exclusively the input data of the microsimulation software supports the usefulness of the proposed methodology for conducting road safety analyses, especially in complex traffic environments where drivers’ behaviour plays a decisive role.

Outage in Motorway Multi-Lane VANETs With Hardcore Headway Distance Using Synthetic Traces

In this paper we analyze synthetic mobility traces generated for three-lane unidirectional motorway traffic to find that the locations of vehicles along a lane are better modeled by a hardcore point process instead of the widely-accepted Poisson point process (PPP). In order to capture the repulsion between successive vehicles while maintaining a level of analytical tractability, we make a simple extension to PPP: We model the inter-vehicle distance along a lane equal to the sum of a constant hardcore distance and an exponentially distributed random variable. We calculate the J-function and the Ripley's K-function for this hardcore point process. We fit its parameters to the available traces, and we illustrate that the higher the average speed along a lane, the more prominent the hardcore component becomes. In addition, we consider a transmitter-receiver link on the same lane, and we generate simple formulae for the moments of interference under reduced Palm measure for that lane, and without conditioning for other lanes. We illustrate that under Rayleigh fading a shifted-gamma approximation for the distribution of interference per lane provides a very good fit to the simulated outage probability using the synthetic traces, while the fit using the PPP is poor.

Observer-Based Time-Variant Spacing Policy for a Platoon of Non-Holonomic Mobile Robots.

This paper presents a navigation strategy for a platoon of n non-holonomic mobile robots with a time-varying spacing policy between each pair of successive robots at the platoon, such that a safe trailing distance is maintained at any speed, avoiding the robots getting too close to each other. It is intended that all the vehicles in the formation follow the trajectory described by the leader robot, which is generated by bounded input velocities. To establish a chain formation among the vehicles, it is required that, for each pair of successive vehicles, the ()-th one follows the trajectory executed by the former i-th one, with a delay of units of time. An observer is proposed to estimate the trajectory, velocities, and positions of the i-th vehicle, delayed units of time, consequently generating the desired path for the ()-th vehicle, avoiding numerical approximations of the velocities, rendering robustness against noise and corrupted or missing data as well as to external disturbances. Besides the time-varying gap, a constant-time gap is used to get a secure trailing distance between each two successive robots. The presented platoon formation strategy is analyzed and proven by using Lyapunov theory, concluding asymptotic convergence for the posture tracking between the ()-th robot and the virtual reference provided by the observer that corresponds to the i-th robot. The strategy is evaluated by numerical simulations and real-time experiments.

Empirical approach for identifying potential rear-end collisions using trajectory data

This paper proposes a novel approach for examining rear-end collisions between successive vehicles in a traffic stream. In this approach, a new safety measure of the follower driver's attentiveness is proposed, referred to herein as instantaneous heeding time (IHT), reflecting the subject follower's heeding nature concerning its leader. A safety framework that integrates the IHT with the distance gap and the instantaneous follower's speed is presented. The applicability of the framework is demonstrated using an Indian-traffic trajectory database (developed in this study) and the homogeneous traffic database of the next generation simulation (NGSIM) project developed in the United States (U.S.). Five study sections in India and two study sections in the U.S. are analyzed for three traffic-flow levels. For Indian traffic, the results show that motorized two-wheelers (MTW) have degraded road safety due to the unrestrained lateral crisscross movements. Due to the presence of MTW, the Indian-traffic stream operates in a disorderly fashion, thereby increasing the probability of rear-end collisions with other vehicle classes. Further, the importance of implementing cautioning measures for drivers that reduce the probability of collisions is demonstrated. Besides, the NGSIM application results confirmed the proposed framework's applicability to both Indian and homogeneous traffic conditions. In practice, the proposed framework can be used in real-time to monitor the driver's aggressive instincts.

Introducing a New Apparatus for Designating Two-Lane Highway Passing and No-Passing Zones

Two-lane highways constitute a large proportion of the U.S. highways. An essential component needed in the design of safe two-lane highways is the passing sight distance (PSD). Otherwise, insufficient PSDs lead to passing-related crashes and, therefore, no-passing zones ought to be marked. This research involves the development of a new apparatus of the two-vehicle method, which is used for measuring the PSD in the field. That is to replace the defunct apparatus used by the Wyoming Department of Transportation (WYDOT). To the best of the authors’ knowledge, the introduced apparatus is the most up-to-date system and addresses shortcomings of previous research. The two-vehicle method involves two successive vehicles spaced at a gap, equivalent to PSD, and both vehicles travel at the speed limit. The driver of the rear vehicle operates a switch when the lead vehicle becomes invisible because of sight obstructions, such as vegetation, signaling the beginning point of the no-passing zone. Similarly, the switch is operated when the lead vehicle returns to view to designate the endpoint of the no-passing zone. The apparatus is composed of vehicle-to-vehicle radio communication devices, global positioning system devices, the switch and computers with graphical user interfaces to record and display the data. Testing was conducted on two two-lane highway segments. As per the results, overall discrepancies between WYDOT’s no-passing zone markings and those designated by the apparatus, developed, ranged from 1% to 7%. This research lays the foundation for a future study involving the development of a cutting-edge prototype.