🚗Transportation Systems Engineering Unit 4 – Highway Capacity & Level of Service Analysis

Key Concepts and Definitions

• Highway capacity represents the maximum hourly rate at which vehicles or persons can reasonably be expected to traverse a point or uniform segment of a lane or roadway during a given time period under prevailing roadway, traffic, and control conditions
• Level of service (LOS) is a qualitative measure used to describe the operational conditions within a traffic stream and their perception by motorists and passengers
• Free-flow speed is the theoretical speed of traffic when density is zero, usually measured in ideal or low-volume conditions
• Density is the number of vehicles occupying a given length of lane or roadway, averaged over time and usually expressed as vehicles per mile or vehicles per mile per lane
• Flow rate is the equivalent hourly rate at which vehicles pass over a given point or section of a lane or roadway during a given time interval of less than one hour
  • Typically expressed in vehicles per hour (vph) or passenger cars per hour (pcph)
• Peak hour factor (PHF) represents the variation in traffic flow within an hour, calculated as the hourly volume divided by the peak 15-minute flow rate within the hour
• Passenger car equivalent (PCE) is a measure used to convert mixed traffic stream with different vehicle types into an equivalent traffic stream composed exclusively of passenger cars

Highway Capacity Fundamentals

• The capacity of a highway is influenced by several factors, including the number and width of lanes, shoulder width, horizontal and vertical alignment, and traffic composition
• The maximum flow rate that can be accommodated under prevailing conditions is referred to as the capacity of the facility
• Capacity is typically expressed in terms of passenger cars per hour per lane (pcphpl) for each roadway component (segments, weaving areas, ramps)
• The capacity of a facility is not a constant value but varies with changes in prevailing conditions
  • Factors such as lane width, lateral clearance, heavy vehicles, and driver population can affect capacity
• The capacity of a facility is reached when the maximum flow rate is achieved, and any additional demand will result in the formation of queues and a reduction in speed
• As demand approaches capacity, the traffic flow becomes unstable, and small disruptions can cause significant operational problems
• The relationship between speed, flow, and density is fundamental to understanding highway capacity and level of service
  • As flow increases, density increases, and speed decreases until the maximum flow rate (capacity) is reached

Factors Affecting Highway Capacity

• Roadway conditions, such as the number and width of lanes, shoulder width, and horizontal and vertical alignment, significantly impact highway capacity
  • Narrow lanes, lack of shoulders, and steep grades can reduce capacity
• Traffic composition, particularly the presence of heavy vehicles (trucks, buses, recreational vehicles), affects capacity due to their larger size and slower acceleration/deceleration rates
  • Heavy vehicles are typically converted to passenger car equivalents (PCEs) for capacity analysis
• Driver characteristics, such as familiarity with the route, reaction time, and aggressiveness, can influence capacity
  • Commuters familiar with a route may have higher capacities compared to recreational drivers
• Environmental factors, such as weather conditions (rain, snow, fog) and lighting (day vs. night), can impact capacity by affecting driver behavior and vehicle performance
• Traffic control devices, including traffic signals, stop signs, and yield signs, can limit capacity by interrupting traffic flow
• Lane changing and merging behavior near on-ramps, off-ramps, and weaving areas can create turbulence in the traffic stream and reduce capacity
• The presence of pedestrians and bicyclists, particularly in urban areas, can affect capacity by requiring additional caution from drivers and potentially reducing available lane width

Level of Service (LOS) Classification

• Level of service (LOS) is a qualitative measure that characterizes the operating conditions within a traffic stream and their perception by motorists and passengers
• LOS is typically designated by letters A through F, with A representing the best operating conditions and F the worst
  • LOS A: free-flow conditions with low traffic volumes and high speeds
  • LOS B: reasonably free flow with slightly reduced speeds due to traffic volume
  • LOS C: stable flow but speeds and maneuverability are more restricted
  • LOS D: approaching unstable flow with tolerable operating speeds but considerably affected by changes in operating conditions
  • LOS E: unstable flow with volumes at or near capacity, characterized by stop-and-go conditions
  • LOS F: forced or breakdown flow with low speeds, high densities, and queue formation
• The LOS criteria for different highway facilities (freeways, multilane highways, two-lane highways) are based on specific measures of effectiveness (MOEs) such as density, speed, and percent time-spent-following
• Highway agencies often use LOS as a performance measure to assess the quality of service provided by a facility and to determine the need for improvements
• LOS can be determined for individual highway segments, weaving areas, ramp junctions, and intersections, as well as for entire facilities or corridors
• The Highway Capacity Manual (HCM) provides detailed procedures and methodologies for determining LOS based on various input parameters and MOEs

Traffic Flow Analysis Techniques

• Traffic flow analysis involves the study of the movement of individual vehicles and their interactions within a traffic stream
• Macroscopic flow models consider traffic as a continuous flow, focusing on aggregate measures such as flow rate, density, and average speed
  • The fundamental relationship between flow ($q$), density ($k$), and speed ($u$) is given by: $q = k \times u$
• Microscopic flow models examine the behavior of individual vehicles, considering factors such as car-following, lane-changing, and gap acceptance
  • Examples of microscopic traffic simulation software include VISSIM, AIMSUN, and PARAMICS
• Mesoscopic flow models combine elements of both macroscopic and microscopic models, providing a balance between computational efficiency and detailed representation of traffic behavior
• Queuing theory is used to analyze the formation and dissipation of queues at bottlenecks, such as on-ramps, off-ramps, and intersections
  • Queuing models help determine the expected queue lengths, waiting times, and delays experienced by vehicles
• Shock wave analysis examines the propagation of disturbances (such as abrupt changes in density or speed) through a traffic stream
  • Shock waves can be used to study the formation and dissipation of congestion and to estimate queue lengths and delays
• Traffic data collection techniques, such as loop detectors, video cameras, and probe vehicles, provide the necessary input data for traffic flow analysis
  • Data collected includes traffic volumes, speeds, densities, and travel times

Capacity Analysis for Different Road Types

• Freeways are divided highways with full control of access and no intersections, typically designed for high-speed, long-distance travel
  • Capacity analysis for freeways considers factors such as the number of lanes, lane width, lateral clearance, and the presence of heavy vehicles
  • The HCM provides methods for analyzing basic freeway segments, weaving areas, and ramp junctions
• Multilane highways are similar to freeways but may have at-grade intersections and occasional traffic signals
  • Capacity analysis for multilane highways accounts for the impact of traffic signals, access points, and turning movements on traffic flow
• Two-lane highways are undivided roadways with one lane in each direction, often found in rural areas
  • Capacity analysis for two-lane highways considers factors such as passing opportunities, directional split, and the presence of heavy vehicles
  • The HCM provides methods for analyzing two-lane highway segments and passing lanes
• Urban streets are characterized by frequent intersections, traffic signals, and high levels of pedestrian and bicycle activity
  • Capacity analysis for urban streets focuses on the performance of individual intersections and the coordination of traffic signals along a corridor
  • The HCM provides methods for analyzing signalized and unsignalized intersections, as well as urban street segments
• Interchange ramp terminals are critical points where traffic enters and exits a freeway or multilane highway
  • Capacity analysis for interchange ramp terminals considers factors such as ramp configuration, traffic control, and the interaction between ramp and mainline traffic
• Roundabouts are circular intersections where traffic flows counterclockwise around a central island
  • Capacity analysis for roundabouts accounts for factors such as the number of circulating lanes, entry lanes, and the geometric design of the roundabout

Performance Measures and Calculations

• Density (passenger cars per mile per lane) is a key performance measure for freeways and multilane highways, calculated as the flow rate divided by the average speed
  • $Density = \frac{Flow\ Rate}{Average\ Speed}$
• Average travel speed (miles per hour) is used to assess the performance of two-lane highways and urban streets, calculated as the segment length divided by the average travel time
  • $Average\ Travel\ Speed = \frac{Segment\ Length}{Average\ Travel\ Time}$
• Percent time-spent-following (PTSF) is a measure of the freedom to maneuver and the comfort and convenience of travel on two-lane highways, representing the average percentage of travel time that vehicles must travel in platoons behind slower vehicles due to the inability to pass
• Control delay (seconds per vehicle) is a measure of the additional travel time experienced by vehicles at intersections due to traffic control devices, calculated as the difference between the actual travel time and the free-flow travel time
  • $Control\ Delay = Actual\ Travel\ Time - Free\text{-}Flow\ Travel\ Time$
• Volume-to-capacity (v/c) ratio is a measure of the degree of saturation of a facility, calculated as the flow rate divided by the capacity
  • $v/c\ Ratio = \frac{Flow\ Rate}{Capacity}$
  • A v/c ratio greater than 1.0 indicates that the demand exceeds the capacity, resulting in congestion and queue formation
• Queue length (vehicles or feet) is the number of vehicles waiting in a queue at a particular location, such as an intersection or a ramp
  • Queue length can be estimated using queuing theory or observed directly in the field
• Level of service (LOS) is determined based on the calculated performance measures and the corresponding thresholds provided in the HCM for each facility type
  • For example, the LOS for a basic freeway segment is determined based on the density, with thresholds ranging from 11 pc/mi/ln for LOS A to >45 pc/mi/ln for LOS F

Real-World Applications and Case Studies

• Highway capacity analysis is used to assess the performance of existing facilities and to evaluate the impact of proposed improvements or changes in traffic conditions
  • For example, a state department of transportation may use capacity analysis to determine the need for additional lanes on a congested freeway segment
• Traffic impact studies for new developments (residential, commercial, or industrial) often require a capacity analysis to determine the impact of the generated traffic on nearby intersections and roadways
  • The results of the analysis can be used to identify necessary improvements, such as adding turn lanes or modifying traffic signal timings
• Capacity analysis is an essential tool for planning and designing highway facilities, helping engineers determine the appropriate number of lanes, intersection configurations, and traffic control strategies
  • For instance, when designing a new interchange, capacity analysis can help determine the required number of ramp lanes and the optimal ramp metering rates
• In urban areas, capacity analysis is used to optimize traffic signal timings and coordinate signals along corridors to minimize delays and improve traffic flow
  • Software such as Synchro and TRANSYT can be used to perform capacity analysis and signal optimization for urban street networks
• Managed lanes, such as high-occupancy vehicle (HOV) lanes and express toll lanes, are often implemented to improve the capacity and performance of congested highway corridors
  • Capacity analysis can be used to determine the effectiveness of managed lanes and to set appropriate occupancy requirements or toll rates
• In work zones, capacity analysis is used to determine the impact of lane closures and to develop traffic control plans that minimize delays and ensure the safety of workers and motorists
  • The HCM provides specific guidance for analyzing the capacity and level of service of work zones based on factors such as the number of closed lanes, the length of the work zone, and the presence of heavy vehicles
• Capacity analysis can also be used to evaluate the performance of alternative intersection designs, such as diverging diamond interchanges (DDI) and continuous flow intersections (CFI)
  • These innovative designs can improve capacity and safety compared to traditional intersections, but require careful analysis to ensure proper operation