CE 578 Highway Traffic Operations

Lecture 4: HCM Directional Analysis

Objectives

• Understand two-lane highway performance measures

• Relate two-lane highway conditions to input variables

• Calculate adjusted volume• Calculate performance measures

– ATS– PTSF

Two-Lane Highway Performance Measures

• Percent-time-spent-following (PTSF)• Average travel speed (ATS)• Average travel time• Passing supply vs passing demand

HCM Procedure (Chapter 12)

• Precept—Mobility– Consistent high speed– Infrequent passing delays

• Precept—Access – Infrequent passing delays– Freedom to maneuver– Safety– Speed not a primary concern

HCM Procedure Two-Lane Highway Classes

• Different highways and different expectations

• Primary arterials: serve long distance trips, major inter-city travel, daily commuter routes, high travel speed expected

• Secondary roadways: high speed not expected, scenic route, rugged terrain, recreational route, short trips, trip ends

Level of Service Measurement by Highway Classification

• Class I: PTSF and ATS– Passing delay is expected to be at a minimum– High speed is expected

• Class II: PTSF– Higher levels are acceptable but drivers are still

sensitive to this measure– ATS is generally not an issue

• Drivers will not be on the highway long• Or do not expect high speeds

Estimating Two-Lane Highway Level of Service Measures

• Microscopic simulation– TRAR– TWOPAS– Yu and Washburn

• Deterministic procedures– US HCM two-way methodology (not

necessary)– US HCM directional methodology– Polus

US HCM Procedures—General Approach

Input Highway Conditions

Adjustments andCalculations

ComputePerformanceMeasure(s)

US HCM Performance Measures—Base Conditions

• 12 ft lanes• 6 ft shoulders• No no-passing zones• 100% passenger cars• No impediments to through traffic• Level terrain• 50/50 traffic split by direction

Base Condition PTSF Estimation: Graph (50/50 directional split)

US HCM Level-of-Service—Class I Facilities

US HCM Level-of-Service—Class II Facilities

Original HCM Directional Analysis Procedure

• Relative to two-way analysis procedure– Split traffic by direction– Analyze each direction of traffic separately– Volume adjustments are the same for general terrain– ATS

• Equation 20-15• adjustments (use 20-19)

– PTSF • Coefficients for BPTSF (use 20-21)• Equations 20-16 and 20-17• adjustments (use 20-20)

(take care to use analysis direction flow and opposing flow, not two-way flow)

Modified HCM Directional Analysis Procedure

• No change in volume adjustments• No change in ATS calculation• PTSF calculation

– Same BPTSF equation– New PTSF equation– New BPTSF coefficients

• See Table 9 (handout)

– New adjustments replacing fnp

• See Table 10 (handout)

od

dadjdd vv

vfBPTSFPTSF

Exercise—Estimating Performance Measures

• Given (Ideal Conditions): – Terrain = level– % Trucks = 0.0%– % RVs = 0.0%– class I highway with base conditions,– PHF = 0.85– vd = 750 pcph, – vo = 450 pcph, – no no-passing zones, – FFS of 60 mph– 60/40 directional split

• Find – ATSd

– PTSFd and– LOS

Application

• Given– Class I– V = 600 vph– Terrain = Rolling– 60/40 split– 60% no-passing zones– PHF = 0.90– 10% Trucks 5% RVs– FFS = 65 mph

Combining Directional Performance Measures

• ATS– See HCM 2000 Chapter 20 pp 20-12

and 20-29

TT1

VMT1

ATS d TT2

VMT2

ATS o

VMT1 0.25V

PHF Lt VMT2 0.25

Vo

PHF Lt

ATS c

VMT1 VMT2

TT1 TT2

Combining Directional Performance Measures (cont.)

• PTSF– See HCM 2000 Chapter 20 pp 20-12 and 20-

29

PTSFc

TT1 PTSF1 TT2 PTSF2

TT1 TT2

Things to Note from Assignment 5

• How to combine directional performance estimates into a two-way estimate

• How to perform an iteration to maintain consistency between the factors and the adjusted volumes.

• Limitations of HCM methodologies (stay tuned)

Review Assignment 3

• See website for the problems and solutions

Limitations

• Sensitivity to highway section length• Intersection operations• Capacity conditions