Capacity and Level of Service for Highway Segments (I)

Capacity and Level of Service for Highway Segments (I) 1

Learn how to use the HCM procedures to determine the level of service (LOS) Become familiar with highway design capacity terminology Apply the equations and procedures for estimating speed and density to determine LOS 2

Highway Capacity Manual 2010 The Bible of traffic engineers Planning Evaluation Design 3

9.1 Freeways A freeway is a divided highway with full Access Control and two or more lanes in each direction Signalized or stop-controlled, at-grade intersections and direct access to adjacent land use are not permitted Freeway is not just fast but full access control Opposing Traffic is separated by a raised barrier, an at-grade median, or a raised traffic island The capacity of a freeway is the maximum sustainable hourly flow rate in one direction, based on the peak 15-minute rate of flow, expressed in passenger cars per hour Basic segment Influence area (reduced capacity) The points at which a basic freeway segment begins and ends depends on the current LOS 4

capacity 6

Freeways Influence areas (capacity reduction) 7

Highway Capacity Terminology and Definitions Base Conditions: Criteria that must be satisfied for a basic freeway segment to operate at its maximum capacity (i.e.: good visibility, no heavy vehicles) Free-Flow Speed: A constant value beginning at the y- intercept of each curve and extending to the breakpoint, where the mean speed diminishes Flow Rates Beyond the Breakpoint: For demand volumes beyond the breakpoint, flow rate declines in value until it reaches the capacity of the segment Computing Free-Flow Speed: Can be determined either by field measurements or by formula estimation 8

Highway Capacity Terminology and Definitions Base Conditions: Criteria that must be satisfied for a basic freeway segment to operate at its maximum capacity (i.e.: good visibility, no heavy vehicles) Free-Flow Speed: A constant value beginning at the y- intercept of each curve and extending to the breakpoint, where the mean speed diminishes Flow Rates Beyond the Breakpoint: For demand volumes beyond the breakpoint, flow rate declines in value until it reaches the capacity of the segment Computing Free-Flow Speed: Can be determined either by field measurements or by formula estimation 9

Highway Capacity Terminology and Definitions Free-Flow Speed: A constant value beginning at the y-intercept of each curve and extending to the breakpoint, where the mean speed diminishes It is typically considered the speed limit When traffic is low on the freeway, speed limit is the only restriction When traffic is high, you begin to feel the interactions with other vehicles 10

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Pipe s model H. Rakha et al. A Comparison of the Greenshields, Pipes, and Van Aerde Car-Following and Traffic Stream Models 12

Highway Capacity Terminology and Definitions Base Conditions: Criteria that must be satisfied for a basic freeway segment to operate at its maximum capacity (i.e.: good visibility, no heavy vehicles) Free-Flow Speed: A constant value beginning at the y- intercept of each curve and extending to the breakpoint, where the mean speed diminishes Flow Rates Beyond the Breakpoint: For demand volumes beyond the breakpoint, flow rate declines in value until it reaches the capacity of the segment Computing Free-Flow Speed: Can be determined either by field measurements or by formula estimation 13

Flow Rates Beyond Break Point Equation 2015 Cengage Learnng Engineering. All Rights Reserved. 14

Highway Capacity Terminology and Definitions Base Conditions: Criteria that must be satisfied for a basic freeway segment to operate at its maximum capacity (i.e.: good visibility, no heavy vehicles) Free-Flow Speed: A constant value beginning at the y- intercept of each curve and extending to the breakpoint, where the mean speed diminishes Flow Rates Beyond the Breakpoint: For demand volumes beyond the breakpoint, flow rate declines in value until it reaches the capacity of the segment Computing Free-Flow Speed: Can be determined either by field measurements or by formula estimation 15

Free Flow Speed Computation Equation History data + regression analysis (curve fitting) 16

Free Flow Speed Computation Equation From Highway Capacity Manual, 2000 17

Level of Service for Freeway Segments LOS is a qualitative measure that describes the operating conditions within a given freeway segment using density (pc/mi) as the quantitative variable and an alphabetic grading system, ranging from A to F Conveys how the density values are perceived by drivers and passengers when traveling in the traffic stream at various demand flow rates (pc/h/ln) k = q p u q p = flow rate u= Average passenger car speed mi/h (km/h) k= Density (pc/mi/ln or pc/km/ln) Note: we use different types of LOS on different types of roads (e.g., freeway vs. arterials 18

Density Ranges for each LOS 19

LOS A Levels of Service Free-flow operation LOS B Reasonably free flow Ability to maneuver is only slightly restricted Effects of minor incidents still easily absorbed

From Highway Capacity Manual, 2000 LOS C Levels of Service Speeds at or near FFS Freedom to maneuver is noticeably restricted Queues may form behind any significant blockage. LOS D Speeds decline slightly with increasing flows Density increases more quickly Freedom to maneuver is more noticeably limited Minor incidents create queuing

From Highway Capacity Manual, 2000 Levels of Service LOS E Operation near or at capacity No usable gaps in the traffic stream Operations extremely volatile Any disruption causes queuing LOS F Breakdown in flow Queues form behind breakdown points Demand > capacity

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Freeway Capacity Estimation Most of equations are empirical and based on regression analysis Not all have the sound math proofs We can NEVER 100% understand driver behaviors as long as human beings are involved We do not have the luxury of extensively collecting data everywhere Simple traffic models will not reflect many factors, such as truck ratio, even though they are well calibrated (e.g., 1000 cars vs. 1000 trucks) Equations must be empirically fine tuned to address these issues 24

Definitions Passenger car equivalents (PCE) Trucks and RVs behave differently Baseline is a freeway with all passenger cars Traffic is expressed in passenger cars per lane per hour (pc/ln/hr or pcplph) Driver population Non-commuters suck more at driving They may affect capacity Capacity Corresponds to LOS E and v/c = 1.0

Calculating the Flow Rate for a Basic Freeway Section q q p = PHF N f p f HV ) q q 26

Terrain Segments General terrain refers to a series of single grades that aren t too steep or too long Freeway segments are considered to be on level terrain if the combination of grades and horizontal alignment permits heavy vehicles to maintain the same speed as passenger cars. Grades are generally short and not greater than 2% If the grades and horizontal alignment causes heavy vehicles to reduce to values substantially below those of passenger cars but not to crawl speed*, the segment is considered to be on rolling terrain If heavy vehicles operate at crawl speed then the segments are in mountainous terrain *Crawl Speed: Maximum sustained speed that trucks can maintain on an extended upgrade of a given percent 27

Computation of PCE Example (Composite grade) Tables 9.4 and 9.5 are given in the next slide 28 Only suitable for less than 4% grade of all subsections and the total length is shorter than 4,000 feet

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When any subsection s grade is more than 4% or total length is shorter than 4,000 feet We calculate the truck/bus speed when they exit the road segment Assume the truck/bus travel at the same grade Using the figure below to estimate the equivalent grade Initial speed Final speed Total Travel distance These are the empirical curves to describe truck/bus s speed changes at different grade The underlying theory is still vehicle kinematics waves but fine tuned with real-world data Because speed changes too much from subsection to the next 30

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