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
