Review of Cooperative Truck Platooning Systems
The purpose of this literature review was to provide Transport Canada with background information on cooperative truck platooning systems (CTPS), summarizing the potential benefits, enabling technologies, significant tests and demonstrations, factors affecting safety and fuel consumption, and other considerations. Pertinent information was extracted from technical papers, reports, presentations, conference proceedings, and public websites. Important knowledge gaps, particularly with regard to conditions and constraints unique to Canada (e.g. geography, climate, infrastructure, social-political issues, etc.), were identified.
CTPS employs wireless communication and automation to create a convoy or “platoon” of two or more trucks which follow closely behind one another. Each following truck uses information from its own in-vehicle sensors, plus data received via wireless link from the lead truck, to “cooperatively” measure and adjust its position, based on the speed, direction and acceleration of the preceding truck. The platoon is typically led by a skilled professional driver, with drivers in the following trucks actively involved in the driving task; however, higher levels of automation are possible. The study focuses on heavy truck platoons operating around mixed traffic in non-dedicated lanes of divided highways, with limited consideration given to mixed platoons of cars and trucks, or fully automated truck platoons.
Potential Benefits of CTPS
CTPS may present an opportunity to significantly reduce fuel consumption and emissions, while potentially improving road safety and efficiency. Reducing the spacing between vehicles reduces the aerodynamic drag experienced by all vehicles in a platoon, and maintaining a consistent speed reduces the frequency of acceleration and deceleration, thereby reducing fuel consumption and CO2 emissions. Since long-haul trucks accumulate high annual mileage, most of which is at highway speed, the savings could be substantial. The tests and demonstrations reviewed during the study indicated a range of fuel savings between 4.5 and 21 percent.
The literature revealed that through the use of sensors, vehicle-to-vehicle (V2V) communication, and some automated vehicle control, it may be possible to reduce or eliminate chain collisions, which often result from an inability of drivers to react quickly in emergency situations. In a cooperative truck platoon, the requirement for speed changes or manoeuvres is communicated automatically throughout the platoon in real time such that the platoon operates as a synchronized unit, smoothing traffic flow and improving traffic efficiency. Furthermore, as the gap between vehicles is reduced, traffic density is increased such that roadways are used more efficiently.
CTPS is enabled by the emergence of several complementary technologies, including various advanced driver assistance systems (ADAS), V2V communication, and modern vehicle control methods and human-machine interfaces. Adding V2V communication to adaptive cruise control (ACC), known as cooperative adaptive cruise control, is ultimately what makes CTPS possible.
Technologies used to monitor the field surrounding a vehicle include long-range and short-range Radar, LiDAR, cameras and ultrasonic sensors. Sensors are often combined and integrated to exploit the features of the different technologies, using “sensor fusion” to gain a more accurate picture of the surrounding environment, and to reduce integration complexity. Electronic vehicle control and actuation systems, permitting remote throttle control, steering and braking, are also important technologies for the automation required for CTPS. Modern instrument clusters and dash displays are intuitive and interactive, and may be configured to provide additional information required for platooning.
Various media (frequency bands) have been used for V2V communication in platooning trials, such as ultra-high frequency (UHF), microwave, millimetre wave and infrared. While each of these media and frequency bands has its own advantages and limitations, 5.9 GHz dedicated short range communication (DSRC) has evolved as the “standard” V2V (and vehicle-to-infrastructure) medium.
Studies, Tests and Demonstrations
Major projects have been undertaken in the U.S., Europe and Asia to evaluate the benefits and feasibility of CTPS. The PATH program has been operating in California since 1986, and has conducted several platooning trials including two- and three-truck platoons. The European PROMOTE-CHAUFFEUR project was one of the earlier demonstrations of CTPS with two trucks, using an “electronic towbar” system. A second phase demonstrated the feasibility of a three-truck platoon operating in real world environments. The German KONVOI project investigated the benefits and deployment issues associated with CTPS operating in mixed traffic on autobahns. The European SARTRE project demonstrated a mixed platoon of cars and trucks operated in a public, mixed-traffic environment, where the platoon was led by a manually-driven truck followed by automated vehicles. The Japanese Energy ITS project demonstrated a platoon of three identical 25-tonne single unit trucks, all of which (including the lead vehicle) were controlled automatically while in the platoon. Scania was preparing to start platooning trials between the Swedish cities of Södertälje and Helsingborg, coordinating the daily departure of several trucks, such that they would form a platoon as soon as they reached the motorway. Peloton has proposed a CTPS concept for two class 8 trucks based on the installation of commercial-off-the-shelf components. The proposal includes operation of a platoon network operations centre, where Peloton would coordinate linking opportunities and manage platoon activities to enforce safe platooning conditions.
The Connected Vehicle Safety Pilot program includes driver clinics across the U.S., and a large-scale model deployment conducted in Ann Arbor, MI, from August 2012 to December 2013. Over 2800 vehicles, including cars, trucks and buses, have been outfitted with V2V devices using 5.9 GHz DSRC. The model deployment will assess the effectiveness of numerous safety applications, and driver clinics will be used to explore driver reactions to the technology and the safety applications. The results will be used by the National Highway Traffic Safety Administration (NHTSA) to decide whether to advance the technology through regulatory proposals, additional research, or a combination of both.
Factors Affecting Safety
CTPS safety is dependent upon several factors, including equipment reliability; vehicle and platoon spacing; platoon length, speed and composition; platooning manoeuvres; the level of automation; surrounding traffic; weather conditions; data security; and human factors. The system design must incorporate a high level of health monitoring (e.g. diagnostics, built-in test), and employ fail-safe modes to mitigate the danger associated with an equipment failure. The driver (if present) must be able to assume control and override the system at any time.
Communication delays and system response times must be considered in determining minimum safe following distances. The use of dedicated lanes could enhance the safety of CTPS, since the behaviour of other vehicles can be reasonably predicted, and speed is much more consistent. Adverse weather can affect the feasibility, effectiveness and safety of CTPS, and there may be conditions when safe platooning is not possible. Data security issues must also be considered, and suitable countermeasures developed.
Factors Affecting Fuel Consumption
The reduction in fuel consumption and CO2 emissions that can be achieved by CTPS is affected by several factors, including vehicle size, type and weight; vehicle and platoon spacing; platoon length and speed; lateral alignment of platoon vehicles, and cross winds; and the duration of effective platooning. Platooning at close following distances can significantly reduce the aerodynamic drag, leading to a reduction of fuel consumption and emissions. The potential fuel savings increase as the gap between vehicles is decreased, to a gap of approximately 8 m for heavy trucks. The most significant fuel savings are experienced by those vehicles between the lead and tail vehicles, so the longer the platoon, the greater the net savings. Similarly, the shorter the gap and the longer the platoon, the greater the traffic density and therefore the road capacity. The length of the platoon is bounded by the V2V communication speed and reliability required in order to maintain string stability. The length must also be limited to avoid bottlenecks at highway entrances and exits. The platoon speed should be optimized to achieve the greatest fuel economy for the individual vehicles. While fuel consumption is affected by vehicle weight (due to rolling resistance), the actual reduction in fuel consumption due to CTPS, expressed in L/100 km, is independent of the vehicle weight. The potential fuel savings are sensitive to the lateral alignment of the vehicles, and crosswinds tend to increase the aerodynamic drag experienced by all vehicles in a platoon. The duration of an established platoon determines the fuel savings that can be achieved due to CTPS. In mixed traffic, cut-ins by non-platoon traffic present the biggest obstacle to maintaining platoon integrity. The acceleration required by all following vehicles to close the gap and re-establish the platoon following a cut-in is inefficient. Since aerodynamic drag varies with air density, and the reduction in the drag coefficient due to CTPS should be similar at all ambient temperatures, the reduction in fuel consumption should be greater at colder temperatures.
In order to conduct CTPS, coordination is required to design and establish the platoon, considering factors such as truck type, weight, performance parameters, installed equipment, current location, destination, etc. Cooperation and financial arrangements between carriers may be required. Provincial regulations will be required to authorize and control platooning, perhaps similar to those developed for long combination vehicles (LCVs). Similarly, equipment specifications, driver training and qualifications, inspecting agency certifications, etc. must also be established. Since data is exchanged between vehicles, privacy issues will need to be addressed. Liability issues must also be addressed since partially-automated systems and a lead driver are assuming some responsibility for the operation of the platoon. Managed lanes or dedicated truck lanes may facilitate the introduction of CTPS with minimal impact to the existing infrastructure. Finally, truck drivers must demonstrate an interest in CTPS for it to become popular. Enhanced driving comfort, safety, and efficiency, as well as reduced fuel consumption, would likely influence driver acceptance.
Comparison with Long Combination Vehicles (LCVs)
LCVs are single vehicles comprised of one tractor and two or three full length trailers, which are well suited for hauling lightweight goods which tend to “cube out”. The fuel consumption and emissions are significantly reduced due to the elimination of one or two tractors, plus the reduction of aerodynamic drag between the trailers due to the close spacing. Restrictions typically include where and when LCVs can operate, as well as the maximum speed and weight. Since an LCV only uses one tractor, it must travel as a complete combination vehicle at all times, usually between terminals designed to accommodate LCVs. Platoons, on the other hand, can be easily formed and dissolved as required. They offer more flexibility because each trailer is physically hitched to a suitably sized tractor, so the tractor-trailer combinations can operate independently. However, there is no reduction of the number of tractors (or drivers), and the minimum gap is greater than that possible with LCVs; therefore, the potential fuel savings are considerably less than that which is possible with LCVs.
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