MODULE 2 Flashcards
TRAFFIC FLOW CLASSIFICATION:
UNINTERRUPTED
INTERRUPTED
occurs at long sections of road
vehicles are not required to stop by any cause external to the traffic
stream
UNINTERRUPTED
occurs at intersections or driveways
vehicles are required to stop by any cause outside the traffic stream
External interruptions may include traffic signs (STOP or YIELD), traffic
signal lights, etc
INTERRUPTED
For Uninterrupted Flow
Traffic is defined by the following variables
A. Flow rate or volume
B. Speed
C. Density or concentration
D. Other Traffic Variables
o number of vehicles passing a point during a specified period of time
o Often referred to as volume over an hour
FLOW RATE
Also called SPOT SPEED
arithmetic mean of speeds of vehicles passing a point within a given time
interval
Length of road is required to measure the speed, called TRAP LENGTH
often used as basis for establishing speed limits
TIME MEAN SPEED
also called the HARMONIC MEAN SPEED
rate of movement of a traffic stream within a given section of road
based on the average travel time of vehicles in the stream within the
section
SPACE MEAN SPEED
o number of vehicles in a given length of road at an instant point in time
o May also be defined as the ratio of the flow rate to the space mean speed of
vehicles
DENSITY
o the time interval between passage of consecutive vehicles at a specified point on the
road with the unit of time per vehicle
TIME HEADWAY
o distance between two vehicles measure from the front bumper of a vehicle to that of
another
SPACING
o the total time a space is occupied by a vehicle divided by the total time of
observation
TIME OCCUPANCY
A dimesional analysis of the units show that flow rate is simply the product of density
and space mean speed
Relationship of Flow, Speed, and Density
Observed Relations
A. Speed – Density Relations
B. Volume – Density relation
C. Volume – Speed relation
is defined as the maximum hourly rate at which persons or vehicles can reasonably
be expected to traverse a point or uniform section of a lane or roadway during a given time
period under prevailing roadway, traffic, and control conditions.
CAPACITY
is a qualitative description of how a certain facility is performing.
LEVEL OF SERVICE
Traffic engineers rely on __________________ to determine the width and number of lanes when planning for new facilities or when expanding existing facilities that
are already experiencing congestion problems.
capacity and level of service analyses
Free flow, with low volumes and high speeds. Drivers are virtually
unaffected by the presence of others. Little or no restriction in
maneuverability and speed.
LOS A
The level of comfort and convenience provided is somewhat
less than at LOS A. Zone of stable flow with operating speeds
beginning to be restricted somewhat by traffic conditions. Drivers will have reasonable freedom to select their speed but there is a decline in freedom to maneuver within the traffic stream from LOS A.
LOS B
Still in zone of stable flow, but speed and maneuverability are most closely controlled by higher volumes. Most of the drivers are most closely controlled by higher volumes. Most of the drivers are restricted in the freedom to select their own speed, lane changing, or overtaking maneuvers. The level of comfort and convenience declines noticeably at the level.
LOS C
Approaches to unstable flow. Speed and freedom to
maneuver are severely restricted, and driver experiences a
generally poor level of comfort and convenience. Small increases in traffic flow will generally cause operational problems.
LOS D
Flow is unstable, and there may be stoppages of momentary condition. Represents operating conditions at or near capacity level. All speeds are reduced to allow but relatively uniform value. Freedom to generally accomplished by forcing a vehicle to “give way” to accommodate such maneuver.
LOS E
Forced or breakdown flow. The amount of traffic approaching a point exceeds the amount that can traverse the points. Queues from behind such as locations. Operations within the queue is characterized by stop-and-go waves, and extremely unstable. It is the point at which arrival flow causes the queue to form.
LOS F
models that are used to describe traffic flow will be developed
fluid flow analogy
traffic flow has a tendency to SLOW DOWN.
c is positive
traffic flow tends to go FASTER.
c is negative
(n = 1)
Greenshields’s Model
(n = -1)
Greenberg’s Model:
(n = 0)
Parabolic Model:
– provides ways of assessing impacts of activities by knowing the magnitude of vehicular
delay and extent of queue propagated
– models are derived on assumptions related to ARRIVAL and DEPARTURE patterns and
prevailing QUEUE DISCIPLINE
QUEUING THEORY
QUEUING SYSTEM
INPUT
SERVICE STATION
OUTPUT
Characterized by arrival patterns given by arrival distribution
INPUT
Depends on queue discipline and service mechanism at the SERVICE STATION
OUTPUT
manner customers are served at the station
SERVICE MECHANISM
Examples of queue discipline
- FIFO (First-In-First-Out)
- Most common type - LIFO (Last-In-First-Out)
- Limited applications to traffic flow
- Popularly used to describe a QUEUING SYSTEM
KENDALL’S NOTATION
TRAFFIC PATTERNS/ DISTRIBUTION
a. MARKOV (M)
b. DETERMINISTIC (D)
o Random process or non-uniform distributions
MARKOV (M)
o Processes characterized by regular or constant (uniform) arrivals or departure
distribution
DETERMINISTIC (D)
MODELS OF TRAFFIC DISTRIBUTION
- UNIFORM DISTRIBUTION (DETERMINISTIC)
- POISSON DISTRIBUTION (NON – UNIFORM)
- NEGATIVE EXPONENTIAL DISTRIBUTION
o Assumes all vehicles arrive equally or are uniformly spaced
UNIFORM DISTRIBUTION (DETERMINISTIC)
Assumes that vehicles arrive at random
POISSON DISTRIBUTION (NON – UNIFORM)
o A non- uniform distribution
o Assumption of POISSON vehicle arrivals also implies a distribution of time intervals
between arrivals of successive vehicles (time headway)
NEGATIVE EXPONENTIAL DISTRIBUTION
- Regular arrivals and departures with single server; limit queue is at 100
- Arrival and departure are easily represented by straight lines with slope
corresponding to their rates (flow)
D/D/1
Combination of Markov and Deterministic processes
- Assumes that arrivals follow a negative exponential distribution (random) and
regular departures; has a single server
M/D/1
- Random arrival and departure(service rates)
- Single server; Infinite queue (no limit)
- Negative exponential distribution for both arrival and departure
M/M/1
- Random arrival and departure(service rates)
- Multiple servers;
- Infinite queue (no limit)
- Single queue for arriving vehicles; vehicles proceed to a vacant server
M/M/N
Stalled vehicles, traffic accidents, parade, or any other temporal activities will cause
abnormal traffic flow and will definitely reduce the capacity of the roadway.
Such occurrences lead to a long queues extending several kilometers that can only be
dissipated long after the obstruction has been removed.
SHOCK WAVE THEORY
is simply the motion or propagation of a change in density and flow.
Shock wave
