Week 5

Question 1

Traffic Flow Thoery

Answer 1

• tool that helps transportation engineers understand and express the properties of traffic flow
• used to evaluate the capacity of existing roadways, designing new roadways, or developing multi-purpose traffic simulation models

Question 2

Time-Space diagram

Answer 2

review in notes, learn how to read

Question 3

Elements of traffic flow

• speed
• volume
• density/concentration
• spacing
• headway

Answer 3

• speed (m/s)
• volume = number of vehicle observed/predicted to be passing a point at a specific time interval
• density/concentration = # of vehicle occupying a given length of lane or roadway, averaged over time
• spacing = distance between successive vehicles in a traffic stream as measured from front bumper to front bumper
• headway = corresponding time between successive vehicle as they pass a point on a roadway

Question 4

Relationship between TMS and SMS?

Answer 4

TMS >= SMS, unless the vehicles are all spaced the same distance from each other

Question 5

Microscopic measures?

Macroscopic measures?

Answer 5

Microscopic

• they describe characteristics specific to individual pairs of vehicles within the traffic stream)
• Time headway and spacing, etc…

Macroscopic

• they describe characteristics of traffic stream as a whole
• flow, average speed, density..etc

Question 6

Fundamental diagram

Answer 6

review notes, learn it, what are the assumptions?

Question 7

Two Macroscopic models, and assumptions?

Answer 7

Greenshields - assumes linear relationship between speed and density
Greenberg model - assumes non-linear relationship between speed and density

Question 8

review over greenshield/greenberg model

Answer 8

ok

Question 9

Boundary Conditions of Greenshields model?

Answer 9

Flow = 0 @ 0 density
Flow = 0 at maximum density
Mean free flow speed occurs at 0 density
Flow-desntiy curves are convex (i.e theres a point of max flow)

Question 10

When do Schockwaves occur? What does it result in? Where do they occur?

Answer 10

Occurs when the capacity between 2 adjacent sections change

• results in a compression/decompression of vehicles in a traffic stream
• occurs at any discontinuity in a state variable

Question 11

Types of Schockwaves

Answer 11

• Backward forming –> rear stationary
• -> forward recovery

frontal stationary—>backward recovery

Question 12

Define a backward forming shockwave and give a scenario of how it might happen

Define Rear stationary shockwave, and give as scenario of how it might happen.

Define Forward recovery shockwave, and give a scenario of how it might happen

Answer 12

A backward forming shockwave occurs when demand capacity > supply capacity resulting in upstream queue.

example: 4 lane highway –> 2 lane tunnel —> 4 lane highway
- during peak hours, demand capacity becoems higher than tunnel capacity, backward forming wave will be formed and will continue to move upstream

A rear stationary shock wave occurs when the arriving traffic demand is equal to the supply capacity in the congested region.

example:
-same example, but near the end of peak hours, the demand flow tends to decrease until it is the same as the tunnel capacity

A forward recovery occurs when supply capacity is constant, but demand decrease below that capacity.

example:
-when the demand flow rate is less than tunnel capacity, forward recovery shockwave is formed

Question 13

Define Frontal stationary Shockwave

Define Backward recovery

Answer 13

Frontal stationary shockwave occurs when capacity suddenly reduces to 0
example: during red light, or when highway is completely closed due to an incident

Backward recovery is when capacity is greater than the demand flow rate or when the restriction causing the capacity reduction is removed
example: when signal light goes from red to green, or when an incident is cleared