1.3 Capacity and level of service

Capacity and level of service are crucial concepts in transportation engineering. They determine how well roads and transit systems handle traffic flow and user experience. Understanding these metrics helps planners design efficient, user-friendly transportation networks.

Capacity refers to the maximum traffic a facility can handle, while level of service measures user satisfaction. These concepts guide infrastructure improvements, traffic management strategies, and long-term planning decisions to balance efficiency and quality of service for all road users.

Capacity concepts

Capacity definition

• Capacity refers to the maximum number of vehicles or persons that can pass through a given point or section of a transportation facility during a specified time period under prevailing conditions
• Typically expressed in vehicles per hour (vph) or persons per hour (pph) depending on the type of facility and mode of transportation
• Capacity is a fundamental concept in transportation engineering and planning, as it determines the ability of a facility to accommodate traffic demand and influences the level of service experienced by users

Factors affecting capacity

• Roadway geometry, including the number and width of lanes, shoulder widths, and alignment (horizontal and vertical curves)
• Traffic composition, such as the proportion of heavy vehicles (trucks and buses) in the traffic stream, which have different performance characteristics compared to passenger cars
• Driver behavior and vehicle characteristics, including desired speed, headway (spacing between vehicles), and acceleration/deceleration capabilities
• Environmental conditions, such as weather (rain, snow, fog) and lighting (day vs. night), which can affect driver perception and vehicle performance
• Traffic control devices and strategies, such as traffic signals, stop signs, and yield signs, which regulate the flow of traffic and impact capacity

Capacity analysis methods

• Empirical methods based on field observations and measurements of traffic flow, density, and speed under various conditions (e.g., Highway Capacity Manual)
• Analytical methods using mathematical models and equations to estimate capacity based on key input parameters (e.g., Greenshields model, Pipes model)
• Simulation methods employing computer software to model and analyze complex traffic scenarios and interactions (e.g., VISSIM, CORSIM, AIMSUN)
• Capacity analysis is essential for identifying bottlenecks, evaluating the performance of existing facilities, and designing new infrastructure to meet current and future traffic demands

Level of service (LOS)

LOS definition and purpose

• Level of service (LOS) is a qualitative measure that describes the operating conditions within a traffic stream and their perception by motorists and passengers
• Represents the quality of service provided by a transportation facility from the users' perspective, considering factors such as speed, travel time, freedom to maneuver, comfort, and convenience
• Used to assess the performance of existing facilities, identify deficiencies, and establish targets for future improvements in transportation planning and design

LOS categories and criteria

• LOS is typically categorized into six levels, ranging from A (best) to F (worst), each representing a range of operating conditions based on specific criteria
  • LOS A: Free-flow conditions with high speeds and low densities
  • LOS B: Reasonably free flow with slightly reduced speeds and maneuverability
  • LOS C: Stable flow, but speeds and maneuverability are more restricted
  • LOS D: Approaching unstable flow with significantly reduced speeds and limited maneuverability
  • LOS E: Unstable flow with low speeds, high densities, and little or no maneuverability (capacity)
  • LOS F: Forced or breakdown flow with stop-and-go conditions and queue formation (over capacity)
• Criteria for determining LOS vary depending on the type of facility and the parameter being measured (e.g., density for freeways, delay for intersections)

Measuring and calculating LOS

• Field measurements of traffic flow parameters, such as volume, speed, and density, are used to determine the actual LOS of a facility
• Analytical methods, such as the Highway Capacity Manual (HCM) procedures, provide equations and tables to estimate LOS based on key input variables (e.g., volume-to-capacity ratio, control delay)
• Simulation models can also be used to evaluate LOS under various scenarios and conditions, particularly for complex facilities and networks

LOS thresholds and standards

• Transportation agencies establish LOS thresholds and standards to define acceptable operating conditions for different types of facilities and areas (urban vs. rural)
• LOS C or D is often used as the minimum acceptable level for design and planning purposes, balancing capacity, cost, and user satisfaction
• Higher LOS standards may be applied to critical or strategic facilities, such as evacuation routes or major freight corridors, to ensure reliable and efficient operations

Capacity and LOS relationship

Capacity as a determinant of LOS

• Capacity is a key factor influencing the level of service, as it sets the upper limit for the amount of traffic that can be accommodated by a facility
• When traffic demand approaches or exceeds capacity, the LOS deteriorates, resulting in congestion, delays, and reduced operating speeds
• Improving capacity through infrastructure upgrades or operational strategies can help maintain or enhance the LOS, especially in high-demand corridors or during peak periods

LOS as a measure of capacity utilization

• LOS provides a way to assess how well a facility is utilizing its available capacity and how much spare capacity exists to accommodate future growth
• Facilities operating at LOS E or F indicate that the capacity is being fully utilized or exceeded, leading to congested and unstable traffic conditions
• Monitoring LOS over time can help identify trends in capacity utilization and the need for capacity expansion or management strategies

Balancing capacity and LOS goals

• Transportation planners and engineers must balance the goals of providing sufficient capacity to meet demand and maintaining an acceptable LOS for users
• Increasing capacity can improve LOS but may also induce additional travel demand, leading to a cycle of congestion and expansion (induced demand)
• Strategies that focus on managing demand (e.g., congestion pricing, transit improvements) or optimizing existing capacity (e.g., signal coordination, reversible lanes) can be more sustainable and cost-effective than capacity expansion alone

Capacity and LOS for different facilities

Freeways and highways

• Freeways and highways are high-speed, access-controlled facilities designed to carry large volumes of traffic over long distances
• Capacity is influenced by factors such as the number and width of lanes, terrain, driver population, and the presence of on- and off-ramps
• LOS is typically based on density (passenger cars per mile per lane), with thresholds defined for each level (e.g., LOS A: ≤ 11 pc/mi/ln, LOS E: > 45 pc/mi/ln)

Arterial roads and intersections

• Arterial roads are major urban streets that carry high volumes of traffic and provide access to adjacent land uses
• Capacity is affected by factors such as the number and width of lanes, the presence of turning lanes, and the spacing and timing of traffic signals
• LOS for arterials is often based on average travel speed, while intersection LOS is determined by control delay (seconds per vehicle)

Public transit systems

• Public transit systems include buses, light rail, heavy rail, and commuter rail, each with different capacity and LOS characteristics
• Capacity is influenced by factors such as vehicle size and configuration, headway (time between vehicles), and dwell time at stops or stations
• Transit LOS is typically based on service frequency, hours of service, passenger loading, and reliability measures (e.g., on-time performance)

Pedestrian and bicycle facilities

• Pedestrian facilities include sidewalks, crosswalks, and paths, while bicycle facilities include bike lanes, cycle tracks, and shared-use paths
• Capacity is affected by factors such as the width of the facility, the presence of obstacles or conflicts, and the mix of user types and speeds
• LOS for pedestrian and bicycle facilities is often based on measures such as space per person, flow rate, speed, and delay at intersections

Intelligent transportation systems and capacity

ITS technologies for capacity management

• Intelligent transportation systems (ITS) encompass a range of advanced technologies and strategies designed to improve the safety, efficiency, and sustainability of transportation systems
• Examples of ITS technologies for capacity management include adaptive traffic signal control, ramp metering, variable speed limits, and dynamic lane assignment
• These technologies can help optimize the use of existing capacity by responding to real-time traffic conditions and balancing demand across the network

Real-time capacity monitoring and optimization

• ITS technologies enable real-time monitoring of traffic flow, density, and speed, providing valuable data for assessing capacity utilization and identifying bottlenecks
• Advanced traffic management systems (ATMS) can use this data to dynamically adjust signal timings, ramp meter rates, and speed limits to maximize throughput and minimize congestion
• Traveler information systems can also use real-time data to provide users with updates on traffic conditions, alternate routes, and expected travel times, helping them make informed decisions and avoid congested areas

Dynamic capacity allocation strategies

• Dynamic capacity allocation involves the flexible and adaptive use of transportation infrastructure to match supply with demand in real-time
• Examples include reversible lanes (changing the direction of travel based on peak flow), high-occupancy vehicle (HOV) lanes (prioritizing carpools and transit), and congestion pricing (varying toll rates based on demand)
• These strategies can help manage capacity more efficiently by incentivizing desired behaviors (e.g., carpooling, off-peak travel) and allocating scarce road space to the most critical or high-value users

Capacity and LOS analysis tools

Highway Capacity Manual (HCM)

• The Highway Capacity Manual (HCM) is a widely-used reference guide published by the Transportation Research Board (TRB) that provides methods for evaluating the capacity and level of service of various transportation facilities
• Includes procedures for analyzing freeways, highways, urban streets, intersections, pedestrian and bicycle facilities, and transit systems
• The HCM is regularly updated to incorporate new research, technologies, and best practices, with the most recent edition (HCM 2016) including enhanced methodologies and a greater focus on multimodal analysis

Traffic simulation models

• Traffic simulation models are computer programs that replicate the behavior and interactions of vehicles, pedestrians, and other road users in a virtual environment
• Can be used to analyze capacity and LOS under various scenarios, such as different road geometries, traffic volumes, control strategies, and ITS implementations
• Examples of traffic simulation software include VISSIM, CORSIM, AIMSUN, and TransModeler, each with different capabilities and applications depending on the scale and complexity of the analysis

Data requirements and collection methods

• Capacity and LOS analyses require various types of data, such as traffic volumes, speeds, densities, turning movements, and signal timings
• Data can be collected through manual methods (e.g., field observations, traffic counts) or automated technologies (e.g., loop detectors, video cameras, Bluetooth sensors)
• The quality and accuracy of the data are critical for the reliability of the analysis results, and appropriate data collection and validation procedures should be followed
• Emerging data sources, such as connected vehicle data and mobile phone data, offer new opportunities for more comprehensive and real-time capacity and LOS assessments

Capacity and LOS in transportation planning

Capacity and LOS performance measures

• Capacity and LOS are key performance measures used in transportation planning to assess the current and future performance of the transportation system
• Other related measures include volume-to-capacity (V/C) ratio, delay, queue length, and travel time reliability
• These measures are used to identify deficiencies, prioritize improvements, and evaluate the effectiveness of transportation plans and projects

Incorporating capacity and LOS in project evaluation

• Capacity and LOS considerations are integral to the evaluation and selection of transportation projects, as they directly impact the mobility, accessibility, and quality of service for users
• Project alternatives are often compared based on their ability to provide sufficient capacity and maintain an acceptable LOS under future traffic conditions
• Benefit-cost analysis (BCA) and multi-criteria analysis (MCA) are common methods for evaluating projects, with capacity and LOS improvements quantified in terms of reduced delay, increased throughput, or enhanced reliability

Capacity expansion vs. operational improvements

• Transportation planners must weigh the trade-offs between capacity expansion (e.g., adding lanes, building new facilities) and operational improvements (e.g., signal optimization, demand management) when addressing capacity and LOS issues
• Capacity expansion can provide long-term benefits but is often costly, time-consuming, and may have negative environmental and social impacts
• Operational improvements can be more cost-effective and quicker to implement but may have limited effectiveness in addressing long-term capacity needs
• A balanced approach that considers both capacity expansion and operational improvements, along with multimodal solutions and land use strategies, is often recommended for sustainable transportation planning

Emerging trends and challenges

Connected and autonomous vehicles impact

• Connected and autonomous vehicles (CAVs) are expected to have significant impacts on capacity and LOS, both positive and negative
• Positive impacts may include reduced headways (due to platooning), smoother traffic flow (due to coordinated acceleration and deceleration), and fewer accidents (due to improved safety features)
• Negative impacts may include increased vehicle miles traveled (due to induced demand and empty vehicle repositioning), reduced capacity for non-CAVs (due to mixing with human-driven vehicles), and new infrastructure requirements (e.g., dedicated lanes, communication systems)
• The net impact of CAVs on capacity and LOS will depend on factors such as market penetration, regulatory frameworks, and public acceptance, and ongoing research is needed to better understand and plan for these impacts

Shared mobility and ride-hailing services

• Shared mobility services, such as car-sharing, bike-sharing, and ride-hailing (e.g., Uber, Lyft), are changing the way people travel and impacting capacity and LOS in urban areas
• These services can reduce the need for personal vehicle ownership and parking, potentially freeing up road space for other uses or modes
• However, they can also contribute to increased vehicle miles traveled (VMT) and congestion, particularly if they compete with public transit or encourage single-occupancy trips
• Integrating shared mobility services with public transit and promoting high-occupancy shared rides can help mitigate negative impacts and improve overall system capacity and LOS

Sustainable transportation and capacity trade-offs

• Sustainable transportation goals, such as reducing greenhouse gas emissions, improving air quality, and promoting active transportation, can sometimes conflict with traditional capacity and LOS objectives
• For example, dedicating road space for bike lanes or transit-only lanes may reduce capacity for general-purpose traffic, leading to lower LOS for motorists
• However, these measures can also improve the capacity and LOS for non-motorized and transit users, leading to a more balanced and equitable transportation system
• Transportation planners must navigate these trade-offs and consider the broader societal benefits of sustainable transportation when making capacity and LOS decisions
• Innovative solutions, such as congestion pricing, complete streets, and transit-oriented development, can help reconcile capacity and sustainability goals by managing demand, optimizing existing infrastructure, and supporting multimodal travel