discussions Flashcards
The number of vehicles (n) passing some
designated roadway point in a given time interval
(t)
Flow (q)
- Units typically vehicles/hour
- Volume typically refers to flow in an hour
Flow (q)
Flow is constantly _____
varying
peak 15-minute flow
within the hour of interest.
Analysis flow rate
How much space do you leave
between vehicles?
- 2 chevrons!
- Depends on speed
- Varies for each person
The distance (ft) between successive
vehicles in a traffic stream, as measured
from front bumper to front bumper
Spacing
the number of vehicles over a length of
freeway
Density
– Measured by loop detectors
– The percent of time the loop is covered by a
vehicle
Occupancy
The time (in seconds) between successive
vehicles, as their front bumpers pass a
given point.
Headway (h)
The average speed of vehicles that pass
by a specific point in space over a specific
time period
TMS
Time necessary for a vehicle to travel a
length of roadway
SMS
– Taken at a specific point
– Average of instantaneous speeds
Time mean speed
– Harmonic speed
– Look at a segment of roadway
– Average speed of all vehicles in that segment
Space mean speed (u)
- Arithmetic mean of speeds observed at some
point - Easy to measure
Time Mean Speed
- It is the harmonic mean
- Used in traffic models, but harder to measure
Space Mean Speed
- The number of vehicles (n)
occupying a given length (l)
of a lane or roadway at a
particular instant - Unit of density is vehicles
per mile (vpm).
Density (k - konzentration)
- Number of vehicles in
length of segment - Inverse of average spacing
Density (k)
- A model for the relationship
between flow, density, and
speed - Represents idealized
behavior and fundamental
relationships - Useful for traffic analysis
Traffic Flow Theory
The speed at which vehicles will travel
unimpeded
Free-flow speed (uf)
The density of vehicles in stopped traffic
Jam density (kj)
The maximum flow a section of roadway can
maintain
Capacity (qm)
______ can easily be measured by remote
sensing, but has historically been difficult to
measure
– Use occupancy obtained from loop-detectors
Density
TMS more easy to measure than SMS
– Use correction or approximation
– Easy to measure with remote sensing (GPS)
true
- Flow is easy to measure
- Occupancy is measure of density
- Only need to measure 2 of 3
true
_____________ decreases when a car is on top of it.
Loop inductance
Single loops can measure:
– Occupancy (O): % of time loop is occupied per interval
– Volume (N): vehicles per interval
How do you estimate speed from a
single loop detector?
- You know how long it took the object to
pass over the detector - You know the percent of time (in a short
interval) the loop was covered by a vehicle
PHF
peak hour factor
used to account for the variation of traffic flow within the peak hour itself
peak hour factor (PHF)
defined as the maximum hourly rate at which persons or vehicles can reasonably be
expected to transverse a point or uniform section of a lane or roadway during a given time period
under prevailing roadway, traffic, and control conditions.
Capacity
a qualitative description of how a certain facility is
performing.
level of service (LOS)
PHPM
Philippine Highway Planning Manual
uses qualitative measures that characterize operational conditions within a
traffic stream and perception of these conditions by motorists and passengers.
LOS concept
defined as the number of vehicles in a given length of road at an instant point
in time.
Density
the time interval between passage of consecutive vehicles at
a specified point on the road with a unit of time per vehicles.
Time headway
the distance between two vehicles measure from the front bumper of a vehicle
to that of another.
Spacing
It can only be measure,
however, if a detector is installed at a specific point on the carriageway. It is defined as the total
time of a detector is occupied divided by the total time of observation.
time occupancy
the motion or propagation of a change in density and flow
Shock wave
occur when there is a change in the travel condition
on the roadway that affect the stream flow.
shock wave
are associated with a particular vehicle in the stream
slowing down or stopping
Shock waves
might be associated with the pressure being
released and a traffic jam dissipating
shock wave
