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A Review on Energy, Environmental, and Sustainability Implications of Connected and Automated Vehicles
Morteza Taiebat, Austin L. Brown, Hannah R. Safford, Shen Qu, Ming Xu
TL;DR
CAVs could reshape transportation, but their environmental effects remain uncertain across vehicle, transportation, urban, and society levels. This review synthesizes evidence across these levels and finds promising benefits at lower levels that greater utilization and changing travel patterns may offset.
Problem
The environmental effects of CAVs across behavioral, ownership, and system levels remain uncertain, although these effects determine whether CAVs are shared or privately owned.
Method
The paper reviews peer-reviewed studies, reports, and consensus quantitative results using a four-level framework spanning vehicles, transportation systems, urban systems, and society.
Results
Long-term environmental impacts seem positive at vehicle, transportation-system, and urban-system levels, but greater vehicle utilization and VMT at the society level may offset benefits.
Takeaways & Limitations
Future research should focus on synergies among automation, electrification, right-sizing, and shared mobility rather than isolated mechanisms.
Takeaways & Limitations
Most studies on CAV utilization assume a low shared-automated-vehicle adoption rate of around 10%, limiting broader system-level inference.
Abstract
from arXiv · showhide
Connected and automated vehicles (CAVs) are poised to reshape transportation and mobility by replacing humans as the driver and service provider. While the primary stated motivation for vehicle automation is to improve safety and convenience of road mobility, this transformation also provides a valuable opportunity to improve vehicle energy efficiency and reduce emissions in the transportation sector. Progress in vehicle efficiency and functionality, however, does not necessarily translate to net positive environmental outcomes. Here we examine the interactions between CAV technology and the environment at four levels of increasing complexity: vehicle, transportation system, urban system, and society. We find that environmental impacts come from CAV-facilitated transformations at all four levels, rather than from CAV technology directly. We anticipate net positive environmental impacts at the vehicle, transportation system, and urban system levels, but expect greater vehicle utilization and shifts in travel patterns at the society level to offset some of these benefits. Focusing on the vehicle-level improvements associated with CAV technology is likely to yield excessively optimistic estimates of environmental benefits. Future research and policy efforts should strive to clarify the extent and possible synergetic effects from a systems level in order to envisage and address concerns regarding the short- and long-term sustainable adoption of CAV technology.
1. INTRODUCTION
Transportation is a major and growing source of global and national greenhouse-gas emissions, with road travel responsible for the largest share within transportation. This review examines how connected and automated vehicles may affect energy, environmental, and socioeconomic outcomes amid substantial uncertainty about their real-world operation and adoption.
- Motivation: Over 7 gigatons of carbon dioxide equivalent (GtCO2 equiv) greenhouse gas (GHG) emissions came directly from transportation worldwide in 2010, representing 23% of total global energy-related GHG emissions.Annual transportation GHG emissions were increasing faster than emissions from any other sector.
- Motivation: 28.5% of total national energy-related GHG emissions came from the U.S. transportation sector in 2016, making it the largest national source.Transportation-sector CO2 emissions were 1893 million metric tons or MMt, compared with 1803 MMt from electric power, from October 2015 through September 2016.
- Motivation: Road-based travel is responsible for the largest share of transportation CO2 emissions, GHG emissions, and energy use, with passenger cars, light-duty trucks, and freight trucks emitting 41.6%, 18.0%, and 22.9%, respectively, of U.S. transportation-sector GHG emissions in 2016.Strategic development of technologies such as connected and automated vehicles could therefore help curb transportation-sector environmental impacts.
- Background: Connectivity enables vehicles to exchange information through V2V, V2I, and other cooperative communications networks, while automation encompasses broader classes of vehicle automation and connectivity can enable automation.The review uses “CAV technology” for vehicle technology with both connectivity and automation characteristics.
- Problem and contribution: CAV technology will significantly change transportation’s environmental profile, but the literature reports large uncertainty partly because real-world data on CAV operations are scarce.The review considers downstream emissions and wastes, upstream resource and energy demands, and socioeconomic aspects associated with energy and the environment.
2. LEVELS OF INTERACTIONS BETWEEN CAVS AND THE ENVIRONMENT
CAVs interact with the environment across four increasingly complex levels: vehicle, transportation system, urban system, and society. Although higher-level interactions have farther-reaching implications, they are harder to quantify and more uncertain.
- Four levels of interaction: CAV-environment interactions span the vehicle, transportation system, urban system, and society levels, with complexity increasing across them.Interactions may arise directly from CAV technology or from CAV-facilitated effects.
- Vehicle level: At the vehicle level, connectivity and automation physically alter vehicle design and operation.These are described as the most direct and well-studied interactions.
- Transportation system level: At the transportation system level, CAV technology can drastically change how vehicles interact with each other in the driving environment.
- Urban system level: At the urban system level, CAV-based transportation interacts with infrastructure such as roads, the power grid, and buildings, altering resource and energy use and emissions and waste generation.
- Cross-level implications: Higher-level interactions have farther-reaching implications but are more difficult to quantify and associated with greater uncertainty.Many important high-level questions are beyond quantitative or predictive modeling and must be addressed qualitatively.
3. ENVIRONMENTAL IMPACTS OF CAV AT EACH SYSTEM LEVEL · 3.1. Vehicle Level.
At the vehicle level, CAVs are generally more energy efficient and generate fewer emissions than conventional vehicles through improvements in operation, electrification, design, and platooning. However, auxiliary energy demands, higher speeds, manufacturing impacts, and uncertain real-world conditions may offset these benefits.
- 3.1. Vehicle Level.: Vehicle-level studies generally find that individual CAVs improve energy efficiency and reduce emissions through operation, electrification, design, and platooning.These direct effects can also manifest across fleets.
- 3.1.1. Vehicle Operation.: CAV operation can improve fuel economy and reduce energy consumption and tailpipe emissions by optimizing driving cycles, routing, idling, and cold starts.Self-parking features reduce energy intensity by approximately 4%.
- 3.1.1. Vehicle Operation.: 5−7% fuel-use reductions are estimated for partial automation with connectivity, while cooperative vehicle communication has produced up to 13% fuel savings and 12% CO2 reductions.A single CAV can also produce up to 40% reductions in total traffic fuel consumption by dampening stop-and-go patterns.
- 3.1.1. Vehicle Operation.: CAV operation can also increase energy use through sensing, computing, connectivity, and higher speeds, with above-optimal speeds decreasing overall fuel economy by 5−22%.The extent to which these increases offset efficiency gains remains unclear.
- 3.1.2. Electrification.: CAV automation can complement electrification by optimizing routes and driving cycles, maximizing regenerative-braking recovery, extending battery life, and matching vehicle range to trips.Electric CAVs may also improve the economics of electrification and transport decarbonization, while shared automated electric vehicles magnify benefits.
- 3.1.3. Vehicle Design.: Vehicle design can reduce environmental impacts through light-weighting, right-sizing, smaller safety systems, and downsized powertrains.Each 10% reduction in vehicle weight yields on average a direct fuel economy improvement of 6−8%, while optimal vehicle right-sizing yielded fuel savings of 45% in one scenario.
- 3.1.3. Vehicle Design.: ICT equipment can increase CAV manufacturing emissions and operational energy use, and may reduce high-speed fuel efficiency by worsening vehicle aerodynamics.The magnitude of aerodynamic effects remains uncertain because empirical data are lacking.
3.2. Transportation System Level.
At the transportation-system level, CAVs affect environmental outcomes through travel costs, mobility services, congestion, and roadway capacity. Shared mobility and automation can improve utilization and emissions, but cheaper and more convenient travel may increase vehicle travel, energy use, and emissions, with outcomes depending on sharing and market penetration.
- CAV transportation-system impacts arise through changing travel costs, mobility services, congestion, and roadway effective capacity, making net effects difficult to predict across market-penetration levels.
- Lower CAV travel costs can expand transportation access and equity, but rebound effects may increase energy consumption and environmental impacts, offsetting vehicle-level efficiency benefits.
- Shared mobility combined with automation can reduce emissions through higher vehicle utilization, while one shared autonomous vehicle could replace 5 to 14 private vehicles.Automated EV sharing was associated with higher profitability and lower emissions per passenger-mile than conventional car-sharing services.
- CAVs can complement mass transit by solving first- and last-mile access, but inexpensive shared CAV travel may reduce transit ridership and disproportionately harm transit-dependent low-income populations.
- Increased convenience and unoccupied travel can raise system-wide VMT, energy use, and emissions; private-CAV household relocation was estimated to increase total VMT by around 30%.If rides are never shared, a SAV-only fleet was found to generate 8.7% more VMT than a private-vehicle-only fleet; impacts depend on trip-sharing frequency and SAV penetration.
3.3. Urban System Level.
At the urban-system level, CAVs could reduce energy use and parking needs while enabling vehicle electrification and grid integration, but new ICT infrastructure and uncertain land-use and charging effects complicate the environmental balance.
- Urban infrastructure: CAV deployment may make existing urban infrastructure obsolete while requiring new infrastructure, leaving its net environmental impacts largely unknown.V2I and improved safety capabilities could change infrastructure needs, but the overall consequences remain insufficiently understood.
- Existing infrastructure: Reducing road lighting by 30% could save 16.5 TWh of energy, 11 MMTs of CO2eq, and around $1.65 billion annually.The estimate applies to highway lighting excluding traffic signals.
- Existing infrastructure: Adaptive lighting could preserve safety while reducing energy use by turning lights on for approaching CAVs and dimming or switching them off on empty roads.V2I capabilities facilitate intelligent lighting systems, while passenger safety concerns may limit eliminating road lighting altogether.
- New infrastructure requirements: CAV operations require frequent communication and data processing, creating demand for large-scale ICT infrastructure whose life cycle is energy intensive and environmentally consequential.Examples include datacenters supporting information exchange for pickup locations, routing, and safe arrival.
- Urban power systems: CAV fleets can promote electrification, while automated and wireless charging can improve energy management, vehicle-grid integration, and renewable electricity uptake.Dynamic charging on automated electric highways was estimated to decrease fossil-fuel energy use by more than 25% and emissions by up to 27%.
- Urban land use: CAVs can reduce parking demand, but their effects on urban form are ambiguous because denser development may coexist with suburbanism and urban sprawl.CAVs were found to reduce needed parking space by an average of 67%, while SAVs could reduce parking land by 4.5% in Atlanta at penetration as low as 5%.
3.4. Society Level.
Society-level environmental implications of CAVs are potentially the largest but remain highly uncertain. Increased vehicle utilization, induced travel demand, modal shifts, and broader social changes could offset efficiency-related environmental benefits.
- 3.4. Society Level.: Society-level environmental implications are potentially the largest, but their magnitude and direction remain highly uncertain because CAVs are not yet commercially available.Public opinion and consumer choice will influence market penetration, complicating assessment of societal effects.
- 3.4. Society Level.: More than 80% of surveyed Texan families would increase vehicle utilization under a CAV paradigm.Other survey evidence suggests people may be interested in riding with CAVs without being willing to buy one.
- 3.4. Society Level.: CAV convenience, accessibility, lower travel costs, and expanded mobility for underserved populations may induce higher travel demand and longer or additional trips.Potential beneficiaries include elderly, young, unlicensed, and medically or physically restricted individuals who currently have unmet travel needs.
- 3.4. Society Level.: Rebound effects from increased travel activity could offset vehicle-level efficiency gains and create discrepancies between predicted and realized net environmental impacts.The rebound effect connects system levels by representing environmental benefits offset through increased use of an efficient technology.
- 3.4. Society Level.: 25−35% of air-travel demand for trips of 500 miles or more could be displaced by CAVs, with environmental impacts potentially mitigated through larger shared vehicles.Road travel may replace aviation or rail, which generally have lower marginal energy use and emissions per passenger-mile than low- or single-occupancy vehicles.
- 3.4. Society Level.: 15.5 million U.S. workers are employed in occupations that could be affected by automated vehicles, with unemployment potentially altering consumption patterns and harming health.Affected occupations include labor-intensive transportation services such as freight trucking, public transit, and taxi driving.
3.5. Summary of Environmental Impacts of CAVs.
CAVs could produce positive environmental impacts at vehicle, transportation-system, and urban-system levels, but uncertainty increases with broader scope and society-level travel responses may offset benefits. Realizing these benefits depends on deployment, technology synergies, and policies that control travel demand and congestion.
- Vehicle level: Vehicle-level fuel savings range from 2% to 25%, occasionally reaching 40%, through energy-efficient CAV design.Integrating CAV technology with vehicle electrification can improve the economics and attractiveness of transportation decarbonization.
- Transportation system level: Transportation-system benefits arise from optimized fleet operations, improved traffic behavior, efficient vehicle utilization, and shared mobility services.Shared mobility and CAV technology have significant mutual reinforcing effects.
- Urban system level: Urban-system impacts include energy savings from less necessary street lighting and traffic signals, but sprawl, longer commutes, datacenter communications, and infrastructure demands may offset benefits.Urban mechanisms might not deliver significant net environmental benefits without high CAV penetration.
- Society level: Society-level lower travel costs and induced demand are likely to increase vehicle utilization and VMT, potentially offsetting benefits at narrower system levels.Many studies assume current travel patterns, vehicle ownership, and utilization rather than behavioral changes from increased CAV penetration.
- Technology synergies: Combining vehicle automation, electrification, right-sizing, and shared mobility could cut global energy use by more than 70% and urban passenger CO2 emissions by more than 80% by 2050.The projected effects of these combined technologies exceed those of any isolated mechanism.
- Sustainability conditions: Sustainable CAV adoption requires energy-efficient transportation, emissions reduction, local-air-pollution mitigation, public-health safeguards, and strategic control of travel demand and congestion.The review also finds that environmental-impact uncertainty increases as the scope of interaction broadens.
4.1. CAV Design and Testing.
CAV environmental performance depends strongly on vehicle-design choices, yet the literature lacks clear guidance on optimizing those choices. Research should combine life-cycle assessment with real-world testing and prototype data while protecting privacy and intellectual property.
- CAV Design and Testing: Vehicle-design evolution is a major uncertainty in CAV energy consumption and emissions, and the literature lacks clear optimization and decision-making protocols.Conventional life-cycle assessment can characterize first-order impacts and inform more sustainable early CAV designs.
- CAV Design and Testing: CAV design research should examine different real-world scenarios and societal acceptance levels while avoiding consumer-privacy and intellectual-property compromises.Scenario-specific evaluation can support design decisions without infringing on consumer privacy or compromising intellectual property.
- CAV Design and Testing: Early commercial CAV designs require proving-ground and test-facility evaluation to determine whether theoretical energy-efficiency improvements are achieved in practice.On-board diagnostics data from current prototypes can help identify best practices and designs for real-world development and deployment.
4.2. CAV-Specific Models and Tools.
CAV-specific models and research tools should integrate system-level attributes and environmental effects across varying market penetrations to improve projections of future travel trends. Because net environmental impacts depend strongly on shared versus private ownership, consumer preferences, ownership, and ride-sharing behavior require greater analysis.
- CAV-Specific Models and Tools.: Integrated assessment models should incorporate environmental effects of system-level CAV attributes across market penetrations to improve projections of future travel trends.CAV impacts interact with land use, travel demand, demographics, economic factors, fueling infrastructure, and local policies, making current transportation-demand predictions unreliable.
- CAV-Specific Models and Tools.: Net environmental impacts depend strongly on whether CAVs are shared or privately owned, yet consumer preferences, vehicle ownership, and ride-sharing evolution remain understudied.Pooling and shared mobility services alleviate most adverse environmental effects of CAV technology, while social norms may discourage sharing with strangers.
4.3. Behavioral Studies.
Behavioral studies should investigate consumer adoption of CAVs, including whether and under what circumstances people accept and use them. Because CAV technology is novel, surveys may be less useful than real-world tests and creative approaches such as virtual or augmented reality.
- 4.3. Behavioral Studies.: Consumer adoption of CAVs requires further investigation through real-world data from surveys and tests.The passage identifies surveys and tests as sources of real-world behavioral data.
- 4.3. Behavioral Studies.: The novelty of CAV technology may limit surveys because respondents often cannot provide informed responses.Most respondents may lack sufficient familiarity with CAVs to answer surveys knowledgeably.
- 4.3. Behavioral Studies.: Novel approaches should examine when people accept CAVs and how they use them, potentially using virtual and augmented reality.The passage proposes creative techniques such as virtual and augmented reality for studying acceptance and use.
4.4. Policy Needs and Opportunities.
Effective CAV policy must address environmental impacts beyond per-vehicle emissions, account for travel-demand and use-case effects, and adapt as technology and markets evolve. A robust understanding of sustainability impacts requires considering technological, behavioral, market, regulatory, and policy factors together.
- Policy gaps: Current CAV policy emphasizes safety, equity, and mobility while paying scant attention to environmental implications.The passage cites U.S. self-driving legislation principles that do not mention environmental concerns.
- Policy gaps: CAV policy should assess broader environmental impacts because induced travel demand could offset or eliminate per-vehicle efficiency and emissions improvements.As CAVs are expected to be more efficient and produce lower emissions than conventional vehicles, per-vehicle regulation alone may be insufficient.
- Adaptive policy frameworks: Policymakers should establish adaptable CAV policy frameworks that evolve with changing markets and technology.Current policies may fail to discourage use cases with significant external costs or incentivize the most beneficial ones.
- Adaptive policy frameworks: Large, personally owned, inefficient CAVs could impose significant system costs by cruising empty and underpaying for infrastructure impacts.The passage presents this as a possible use case whose real-world emergence remains uncertain.
- Systems-level assessment: Assessing CAV sustainability requires integrating technology evolution, behavioral responses, market penetration, and regulatory and policy considerations.Including all relevant factors is described as critical for maximizing environmental benefits and minimizing adverse consequences.