LE 2 Concepts Flashcards
– number of vehicles passing a
point during specified period of
time
Flow rate or volume, q
– rate of motion in distance per
unit time
– space mean speed
Speed, u_s
– number of vehicles in a given
length of road at an instant
point in time
Density, k
– time interval between passage
of consecutive vehicles at a
specified point on the road
Headway
– instantaneous speed of a
vehicle
– time mean speed
Spot Speed
– front-to-front distance between
a vehicle and its leader
Spacing
Traffic Stream Model suitable for congested conditions
Greenberg’s Model
Traffic Stream Model suitable for free flow conditions
Underwood’s Model
region in traffic where one or more traffic variables spatially change abruptly from one state of traffic to another state
Shockwave front
motion or propagation of a change in density and flow
Shockwave
velocity of the front
Shockwave velocity
Common Examples of Shockwave Generation
- Stopping
- Starting
- Platoon formation
- Platoon dispersion
● the specification of
the position of the
vehicle at all times
● slope of the line is
the speed of the
vehicles
Time-Space Diagram
TRUE OR FALSE: Macroscopic and microscopic traffic variables could be
calculated using a time-space diagram
TRUE
Traffic flow is a _____________, with random variations in driver and vehicle characteristics and interactions
Stochastic Process
models of reality in which effects of chance variation are ignored or averaged out; any given input will produce an exactly predictable output
Deterministic models
allow random variation and look into probabilities or variabilities of different outcomes, not just average outcome
Stochastic models
Common distribution used to model vehicle arrivals in traffic flow
- Poisson Distribution
- Binomial Distribution
- Negative Binomial Distribution
- traffic is light and no disturbing factor such as traffic signal; behavior appears random
- low traffic volumes (~200-400 vph)
- mean equals variance
Poisson Distribution
- congested traffic
- ratio of observed variance over mean is substantially less than 1.0
Binomial Distribution
- high variance in observed arrivals; variance over mean greater than 1.0
- cyclic variation in flow (from light to heavy), e.g., downstream traffic from a traffic signal
Negative Binomial Distribution
Common distributions used to model headways
- Negative Exponential Distribution (low flow rate)
- Normal Distribution (high flow
rate) - Erlang Distribution (intermediate
flow rate)
If vehicle arrival is ___________ distributed, then headway follows a _________________ cumulative probability distribution
Poisson; Negative Exponential
a waiting line of persons or vehicles
Traffic queue
provides a way to assess the impact of traffic service facilities by determining the magnitude of delay and extent of queue propagated
Queueing Analysis
- means constant arrival rate, constant departure or service rate, with one server
- can be analyzed graphically because of the regularity
- arrivals and departures are represented by straight line with slopes corresponding to the rates of flow
D/D/1 queue
- Randomly varying arrivals and departures; but distribution can still be characterized by mean arrival and mean departure rates.
- Queue can form whenever the arrival rate is greater than the VARYING departure rate
M/M/1 queue
- Queue can form whenever the arrival rate is greater than the CONSTANT departure rate
M/D/1 queue
is one of the most important activities within a traffic engineering and management organization.
Traffic data collection and analysis
most basic field survey needed for any traffic study.
Traffic volume count
is used for determining annual travel; estimating expected highway user revenue; computing accident rates.
annual traffic
is used for measuring the present demand; programming capital improvements
annual average daily traffic (AADT) or average daily traffic (ADT)
8 hr, 12 hr, or 16 hr traffic volume used to estimate daily traffic
partial daily traffic
is used for evaluating capacity; geometric design or improvement of streets and intersections
hourly traffic (peak hour traffic)
min volume) is used to estimate maximum flow rate and determination of characteristics of peak hour volumes.
short term traffic
is the speed of a vehicle at a given instant of time
spot speed
spot speed study seeks to determine the range and magnitude of speeds (time mean speed) as a basis for operational analysis and design.
– establish maximum and minimum speed limits;
– determine the need for posting advisory speed signs;
– determine the need for school zone protection; and
– evaluate the performance of a geometric improvement or traffic control device through a ‘before and after’
spot speed study
- to get information on the
journey time and delays
along a particular road or
route - to determine travel
speed
Travel Time and Delay Study
traversing a route with fixed waypoints to record travel time; data recorded includes location, duration, and cause of stops and interruptions along the route
Test Car Technique
recording and matching license plate numbers and passing time of vehicles at two, or more observation stations along the route; useful when only information on travel time between stations is desired
License Plate Method
performed to get information on the number, usage duration, and turnover rate of parking spaces
Parking Survey
Obtains information on a road section, not only traffic volume, but speed and density as well
Moving Observer Method for Volume Count
