1 - Traffic Engineering Basics Flashcards
What is flow?
The average number of vehicles passing a given point per unit time.
q = N/T
What are the units of flow?
Vehicles per hour or vehicles per day
What is the journey speed?
The average speed of a vehicle on a journey, including stops
What is the running speed?
Journey speed excluding stops
What is spot speed?
The speed of an individual vehicle measured instanteuously at a given point
What is time mean spead?
The average mean speed of all the vehicles at a given point during a period of time.
What is the space mean speed?
The average speed of all the vehicles along the given road segment at a typical instant in time.
= length of road segment / average time to traverse road segement.
For steady flows, where the concentration and speed do not vary much from their means, what equation relates the time mean speed and the space mean speed?
Time mean speed = space mean speed + variance^2 / space mean speed
What is the concentration?
The average number of vehicles present at a typical instant on a road of unit length
K = N / L
units veh/km
What is ‘gap’ and what are its units?
Gap is the interval between the rear of a behicle and the front of the following vehicle
m if measured in space
s if measured in terms of time
WHat is the mean time headway?
The avergae time interval between successive vehicles, measured from head to head.
For steady flows, what does the mean time headway equal?
1 / q , q being the flow
What is the mean time headway if q = 2000 veh/hr?
Mean time headway = 1 / 2000 * 3600 = 1.8s
What is the mean distance headway?
h,d is the average distance between successive vehicles, measured from head to head
h,d = 1/K K being the concentration
What is the mean distance headway h,d if K = 60veh/km?
h,d = 1/60 * 1000 = 16.7m
What does the space mean speed * the time mean headway equal?
The distance headway
What is the fundamental relationship of traffic flow?
Flow = concentration * space mean speed
q = kv,s
As concentration increases, what happens to the speed of vehicles?
The spacing between vehicles decreases and drivers react by lowering their speeds
What does greenshields theory suggests happens to speed as concentration increases?
The speed decreases linearly
What is K,j?
The jam (maximum) concentration
What is the fundamental diagram?
The graph of flow vs concentration.
What concentration is the flow optimised at?
Kopt, the flow is known as the capacity , qmax of the road.
When analysing a Greenshields concentration - flow graph, what does the slope to the origin at any point on the line represent?
The space mean speed
When the concentration is greater than Kopt, what happens?
The traffic becomes highly congested and unstable
Name 4 physical characteristics of the orad that the fundamental capacity of the road depends on:
-width
-number and types of intersections
-alignment
-road surace
Name some environemental and operating condtions that effect the shape of the fundamental diagram:
-weather
-level of pedestrain activity
-number of parked vehicles
-frontage activity
What is free flow?
When speed and flow are ver high, speeds restricted by legal restrictions or highway geometry
What is unstable flow?
Speed is at near or optimal of the roads capacity, speeds fluctutate, momentary stoppages
What is forced flow?
Congestion, so low speeds, long and short stoppages, flow reduced to below capacity.
What is q,fmax?
The maximum flow rate at which the free-flow space mean speed is obtained
When flow exceeds capacity, what is used instead of space mean speed?
Link journey speed
What is the total time on link equal to?
Time taken to traverse the link at flow qmax + waiting time representing the effect of queuing at the link exit
What is the waiting time representing the effect of queuing at a lin exit ?
