04 Traffic Stream Characteristics (Traffic Engineering هندسة المرور & Prof. Saad AlGadhi)

CE 436 Traffic Engineering

Topic 4: Traffic Stream Characteristics

Prof. Saad AlGadhi

CE 436 –Traffic Engineering

King Saud University

email: [email protected]

CE 436_KSU_Prof.AlGadhi 2

Ch. 5 Objectives

Understand the underlying mechanism governing typical driving behavior (inherent variability and normal ranges)

Know how to describe vehicular traffic as flows

Understand traffic flow characteristics

Investigate the traffic flow relationships in Traffic Engineering studies


Type of Facilities

Uninterrupted Flow

traffic stream characteristics are based solely on the interactions among vehicles

for example: freeway and other highway segments more than 2 miles without fixed interruptions

Interrupted Flow

fixed external interruptions

for example: signalized intersections; STOP, YIELD

Major difference: impact of “time”


Traffic Stream parameters

Macroscopic parameters Volume or flow rate

Speed (TMS; SMS)

Density (concentration)

Microscopic parameters (time) headway

speed of individual vehicle

Spacing (space headway)


Basic Measurements (q, h)

Flow rate q (veh/s): number of vehicles passing a stationary point during a unit time interval (or veh/hr, veh/day)

Headway h (s/veh): time interval that two consecutive vehicles (front-bumper to front-bumper) pass a stationary point

h

q1


4 Types of Traffic Daily Volume

Average Annual Daily Traffic (AADT) Average 24 hour volume over a year (365 days)

Average Annual Weekday Traffic (AAWT) Average 24 hr volume per weekday over a year (260 days)

Average Daily Traffic (ADT) Same as AADT, but average over period less than a year, say, a

month

Average Weekday Traffic (AWT) Same as AAWT, but average over period less than a year, say,

a month

** Generally not directional


Recreational Road; ADT>AWT & see summer months


AADT/ADT

AADT Applications Estimation of highway use Estimation of trends Economic feasibility evaluation Planning Maintenance

ADT Applications Planning Measurement of current demand


Hourly Volume

Daily volume is useful for planning purposes, but cannot be used alone for design and operation purposes

Volume varies considerably over 24 hours (rush hour: AM Peak; PM Peak)

Peak hour volume is generally directional


AADT DDHV

DDHV = AADT * K * D

DDHV: directional design hourly volume

K: proportion of daily traffic occurring in the peak hour; 30th peak hour in a year (Urban: 0.07-0.12, Table 5.2)

D: proportion of peak hour traffic traveling in the peak direction (Urban:0.5-0.6)

K and D generally decrease in high-density area



Sub-hourly Volume/ Flow Rate

Traffic flow rates fluctuate within an hour

Short term peaks of flow within the hour may exceed capacity and create breakdown.

Volumes observed for periods < 1 hour are expressed as equivalent hourly rates of flow.


Capacity = 4200 v/h or 1050 v/15-min


Peak Hour Volume (PHV)

Theoretically (1.0 ≥ PHF ≥0.25) Practically (0.7 – 0.98)


Basic Measurements (TMS, SMS)

Time mean speed (spot or instantaneous speed)


Space mean speed (harmonic mean)



or




Excercise: 1-mile test track

Given: 1-mile circular test track, two vehicles with 60 and 30 mph respectively, without affecting each other. Calculate the TMS and SMS.


Basic Measurements (K, S)


Basic Measurements


Inductance Loop Detector


Occupancy Calculation


Basic Measurements


Basic Measurements


Time-Space Diagram

Applications:

Signal Coordination

Length of acceleration and

deceleration lanes of

freeway on-ramps and off-

ramps

Estimation of safe passing

sight distances


Using GPS Data to Construct Time-Space Diagram


Typical GPS Data


Relationships among Flow, speed, and density

V = S * D q = k * v

Flow (veh/hr) = Speed (mile/hr) * Density (veh/mile)

Proof: Consider a segment of freeway.


Field Freeway Data


Field Arterial Data


Speed, Flow, and Density Relationship


Relationships between Speed, Density and Flow

A: almost zero density, free-flow speed, very low volume

B: increased density, reduced speed, increased volume

C: increased density, reduced speed, max volume

D: jam density, min speed (crawling), very low volume


General Properties for any Traffic Flow Model

Need to satisfy four boundary conditions

Flow is zero at zero density

Flow is zero at maximum density

Mean free-flow speed occurs at zero density

Flow-density curves are convex (i.e., there is a point of max flow


Properties of Speed-Density Curve


Properties of Flow-Density Curve


Properties of Speed-Flow Curve


Typical Traffic Flow Models


Hydrodynamic Theory

HW # 2

Problems #:

5-4

5-6

5-7

5-8

5-10

Due: First Wednesday after Hajj
