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 Deterministic procedures –US HCM two-way methodology (not necessary) –US HCM directional methodology

US HCM Procedures—General Approach Input Highway Conditions Adjustments and Calculations Compute Performance Measure(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 adjustments (use 20-19) –PTSF Coefficients for BPTSF (use 20-21) Equations and 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 f np See Table 10 (handout)

Exercise—Estimating Performance Measures Given: –class I highway with base conditions, –v d = 750 pcph, –v o = 450 pcph, –no no-passing zones, –FFS of 60 mph –50/50 directional split Find –ATS d –PTSF d 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 and 20-29

Combining Directional Performance Measures (cont.) PTSF –See HCM 2000 Chapter 20 pp and

Discrepancies before and after modifications HCM 2000 two-way vs HCM 2000 directional (combined) –ATS ATS c = 56 mph; ATS two-way = 56.5 mph –PTSF PTSF c = 81.8; PTSF two-way = 62.7 –Is this significant?

Things to Note from Assignment 5 How to combine directional performance estimates into a two-way estimate How to implement modified HCM procedure to maintain consistency with two-way analysis 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