lec 12 capacity analysis ( transportation engineering dr.lina shbeeb )

Transportation Engineering

Lina Shbeeb

Capacity Analysis

Traffic flow theory in practice

Used in facility planning

The understanding of facility capacity

Used in design

The sizing and geometric features of facilities

Used in performance measure

The level of service offered by a specific facility

Delay

Travel speeds

Travel times

Applications of Traffic Flow Theory

Determining turning lane lengths

Average delay at intersections

Average delay at freeway ramp merging areas

Change in the levels of freeway performance

Simulation (mathematical models (algorithms) to study

interrelationship among the different elements of traffic

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Various Types of Traffic Volumes

Daily Volume

Average Annual Daily Traffic (AADT)

Average Annual Weekday Traffic (AAWT)

Average Daily Traffic (ADT)

Average Weekday Traffic (AWT)

Hourly Volume

Subhourly Volume

AADT and AAWT

Average Annual Daily Traffic (AADT)

Average 24-hour traffic volume at a location over a full 365-day year, which is the total number of vehicles passing the location divided by 365

Average Annual Weekday Traffic (AAWT)

Average 24-hour traffic volume on weekdays over a full year, which is the total weekday traffic volume divided by 260

ADT and AWT

Average Daily Traffic (ADT)

Average 24-hour traffic at a location for any period less than a

year (e.g. six months, a season, a month, a week or even two

days)

Average Weekday Traffic (AWT)

Average 24-hour traffic volume on weekdays for any period less

than a year

DDHV/PHV

Directional Design Hourly volume (DDHV) ~ Peak

Hourly Volume

DDHV (veh/hour) = AADT x K x D

where AADT = Average Annual Daily Traffic (veh/day)

K = Proportion of daily traffic occurring in

peak hour (decimal)

D = Proportion of peak hour traffic traveling

in the peak direction (decimal)

Contd…

DDHV/PHV

For design purposes, K factor is generally chosen as 30 HV and D

factor as the percentage of traffic in predominant direction during

the design hour

General ranges for K and D factors

Facility Type K Factor D Factor

Rural 0.15 – 0.25 0.65 – 0.80

Suburban 0.12 – 0.15 0.55 – 0.65

Urban 0.07 – 0.12 0.50 – 0.60

Sub-Hourly Volume

Sub-hourly Volume ~ 15-min Volume

Suppose, the peak 15-min volume observed = 750 veh

So, the Hourly Volume =

15-min volume x 4

= 750 x 4

= 3000 veh/hour

Peak Hour Factor

Relationship between hourly volume and maximum rate of flow within the hour

For intersection:

PHF = =

where HV = Hourly volume (veh/hour)

V15 = max. 15-min volume within the hour (veh)

Hourly Volume

Max. Rate of Flow

HV

4 x V15

Example of PHF Calculation

Data collected are as follows:

Time Interval Volume (veh)

4:00-4:15 pm 950

4:15-4:30 pm 1100

4:30-4:45 pm 1200

4:45-5:00 pm 1050

Hourly Volume 4300

Example of PHF Calculation

We find from the table,

HV = 4300 veh/hour

V15 = 1200 veh

Therefore,

PHF = = = 0.90

HV

4 x V15

4300

4 x 1200

Peak Hour Factor

For freeway/expressway:

PHF =

where HV = Hourly volume (veh/hour)

V5 = max. 5-min volume within the hour (veh)

HV

12 x V5

Traffic Volume

Number of vehicles passing a given point or a section of a

roadway during a specified time

Data collected by

Manual counting

Electro-mechanical devices

Relationship of Highest Hourly Volume and

ADT on Rural Arterials

Travel Time and Delay Surveys

Want to determine travel time between two points or causes of

delay along a route

Used for

Performance monitoring

Before and after evaluation of traffic management

schemes or new infrastructure

Stream measurements: the moving

observer method Moving Observer

stopwatch, pen and paper

laptop PC

instrumented vehicle

=> Chase car versus floating car techniques

Stationary Observer

number plate survey Field staff or optical character recognition from video

Vehicle electronic tags

Mobile phone

Travel Time

By travelling several times in a car from A to B, a

person notes down the time taken in each run and

then comes up with a statistical average of Travel

Time.

A B

Delay

Delay = Actual travel time - Expected travel time

Stopped time delay

Travel time delay

Calculation of Delay

Travel time delay Actual travel time data collected by traveling in a car through

the stretch of the road under study.

Expected travel time calculated from the distance and average speed.

Travel time delay calculated from the difference between actual travel time and expected travel time.

Stopped time data for the vehicular speed of 0 to 5 mph.

Introduction

Shock wave - a rapid change in traffic conditions (speed,

density, and flow)

A moving boundary between two different traffic states

Forward, backward, and stationary shock waves

Travel Time

By travelling several times in a car from A to B, a

person notes down the time taken in each run and

then comes up with a statistical average of Travel

Time.

A B

Delay

Delay = Actual travel time - Expected travel time

Stopped time delay

Travel time delay

Calculation of Delay

Travel time delay Actual travel time data collected by traveling in a car through

the stretch of the road under study.

Expected travel time calculated from the distance and average speed.

Travel time delay calculated from the difference between actual travel time and expected travel time.

Stopped time data for the vehicular speed of 0 to 5 mph.

Level of Service (LOS)

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Concept – a qualitative measure describing operational conditions within a traffic stream and their perception by drivers and/or passengers

Levels represent range of operating conditions defined by measures of effectiveness (MOE)

LOS A (Freeway)

Free flow conditions

Vehicles are unimpeded in

their ability to maneuver

within the traffic stream

Incidents and breakdowns

are easily absorbed

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LOS B

Flow reasonably free Ability to maneuver is

slightly restricted General level of physical and

psychological comfort provided to drivers is high

Effects of incidents and breakdowns are easily absorbed

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LOS C

Flow at or near FFS

Freedom to maneuver is noticeably restricted

Lane changes more difficult

Minor incidents will be absorbed, but will cause deterioration in service

Queues may form behind significant blockage

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LOS D

Speeds begin to decline with increasing flow

Freedom to maneuver is noticeably limited

Drivers experience physical and psychological discomfort

Even minor incidents cause queuing, traffic stream cannot absorb disruptions

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LOS E Capacity

Operations are volatile, virtually no usable gaps

Vehicles are closely spaced

Disruptions such as lane changes can cause a disruption wave that propagates throughout the upstream traffic flow

Cannot dissipate even minor disruptions, incidents will cause breakdown

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LOS F

Breakdown or forced flow

Occurs when: Traffic incidents cause a

temporary reduction in capacity

At points of recurring congestion, such as merge or weaving segments

In forecast situations, projected flow (demand) exceeds estimated capacity

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Design Level of Service

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This is the desired quality of traffic conditions from a driver’s perspective (used to determine number of lanes)

Design LOS is higher for higher functional classes Design LOS is higher for rural areas LOS is higher for level/rolling than mountainous terrain Other factors include: adjacent land use type and development

intensity, environmental factors, and aesthetic and historic values Design all elements to same LOS (use HCM to analyze)

Design Level of Service (LOS)

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Capacity – Defined

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Capacity: Maximum hourly rate of vehicles or

persons that can reasonably be expected to pass a point, or

traverse a uniform section of lane or roadway,

during a specified time period under prevailing conditions

(traffic and roadway)

Different for different facilities (freeway, multilane, 2-

lane rural, signals)

Why would it be different?

Ideal Capacity

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Freeways: Capacity (Free-

Flow Speed)

2,400 pcphpl (70 mph)




Multilane Suburban/Rural


2,100 (55 mph)

2,000 (50 mph)

1,900 (45 mph)

2-lane rural – 2,800 pcph

Signal – 1,900 pcphgpl

Principles for Acceptable Degree of

Congestion:

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1. Demand <= capacity, even for short time

2. 75-85% of capacity at signals

3. Dissipate from queue @ 1500-1800 vph

4. Afford some choice of speed, related to trip length

5. Freedom from tension, esp long trips, < 42 veh/mi.

6. Practical limits - users expect lower LOS in expensive

situations (urban, mountainous)

Multilane Highways

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Chapter 21 of the Highway Capacity Manual

For rural and suburban multilane highways

Assumptions (Ideal Conditions, all other conditions

reduce capacity):

Only passenger cars

No direct access points

A divided highway

FFS > 60 mph

Represents highest level of multilane rural and suburban

highways

Multilane Highways

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Intended for analysis of uninterrupted-flow highway

segments

Signal spacing > 2.0 miles

No on-street parking

No significant bus stops

No significant pedestrian activities

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Source: HCM, 2000

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Source: HCM, 2000

Step 1: Gather data

Step 2: Calculate capacity

(Supply)

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Source: HCM, 2000

42 Source: HCM, 2000

Lane Width

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Base Conditions: 12 foot lanes

Source: HCM, 2000

Lane Width (Example)


How much does use of 10-foot lanes decrease

free flow speed?

Flw = 6.6 mph

Lateral Clearance

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Distance to fixed objects

Assumes

>= 6 feet from right edge of travel lanes to obstruction

>= 6 feet from left edge of travel lane to object in median

Source: HCM, 2000

Lateral Clearance

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TLC = LCR + LCL

TLC = total lateral clearance in feet

LCR = lateral clearance from right edge of travel lane

LCL= lateral clearance from left edge of travel lane

Source: HCM, 2000

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Example: Calculate lateral clearance adjustment for a 4-lane

divided highway with milepost markers located 4 feet to the

right of the travel lane.

TLC = LCR + LCL = 6 + 4 = 10

Flc = 0.4 mph

Source: HCM, 2000

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fm: Accounts for friction between opposing directions of

traffic in adjacent lanes for undivided

No adjustment for divided, fm = 1

Source: HCM, 2000

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Fa accounts for interruption due to access points along

the facility

Example: if there are 20 access points per mile, what is

the reduction in free flow speed?

Fa = 5.0 mph

Estimate Free flow Speed

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BFFS = free flow under ideal conditions

FFS = free flow adjusted for actual conditions

From previous examples:

FFS = 60 mph – 6.6 mph - 0.4 mph – 0 – 5.0 mph =

48 mph ( reduction of 12 mph)


Step 3: Estimate

demand

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Calculate Flow Rate

Heavy Vehicle Adjustment

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Heavy vehicles affect traffic Slower, larger fhv increases number of passenger vehicles to account for presence of heavy trucks

f(hv) General Grade Definitions:

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Level: combination of alignment (horizontal and vertical) that

allows heavy vehicles to maintain same speed as pass. cars

(includes short grades 2% or less)

Rolling: combination that causes heavy vehicles to reduce

speed substantially below P.C. (but not crawl speed for any

length)

Mountainous: Heavy vehicles at crawl speed for significant

length or frequent intervals

Use specific grade approach if grade less than 3% is more than

½ mile or grade more than 3% is more than ¼ mile)

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Example: for 10% heavy trucks on rolling

terrain, what is Fhv?

For rolling terrain, ET = 2.5

Fhv = _________1_______ = 0.87

1 + 0.1 (2.5 – 1)

Driver Population Factor (fp)

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Non-familiar users affect capacity

fp = 1, familiar users

1 > fp >=0.85, unfamiliar users


Step 4: Determine

LOS

Demand Vs.

Supply

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Calculate vp

Example: base volume is 2,500 veh/hour

PHF = 0.9, N = 2

fhv from previous, fhv = 0.87

Non-familiar users, fp = 0.85

vp = _____2,500 vph _____ = 1878 pc/ph/pl

0.9 x 2 x 0.87 x 0.85

Calculate Density

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Example: for previous

D = _____1878 vph____ = 39.1 pc/mi/lane

48 mph

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LOS = E

Also, D = 39.1 pc/mi/ln, LOS E

Design Decision

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What can we change in a design to provide an acceptable

LOS?

Lateral clearance (only 0.4 mph)

Lane width

Number of lanes

Lane Width (Example)


How much does use of 10 foot lanes decrease free

flow speed?

Flw = 6.6 mph

Recalculate Density

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Example: for previous (but with wider lanes)

D = _____1878 vph____ = 34.1 pc/mi/lane

55 mph

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LOS = E

Now D = 34.1 pc/mi/ln, on border of LOS E

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Recalculate vp, while adding a lane

Example: base volume is 2,500 veh/hour

PHF = 0.9, N = 3

fhv from previous, fhv = 0.87

Non-familiar users, fp = 0.85

vp = _____2,500 vph _____ = 1252 pc/ph/pl

0.9 x 3 x 0.87 x 0.85

Calculate Density

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Example: for previous

D = _____1252 vph____ = 26.1 pc/mi/lane

48 mph

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LOS = D

Now D = 26.1 pc/mi/ln, LOS D (almost C)