Lesson 1.2: Traffic Variables Lesson Summary
Lesson 1.2: Traffic Variables Lesson Summary
Lesson 1.2: Traffic Variables Lesson Summary
Lesson Summary
Uninterrupted flow can be described by major traffic variables — volume or
flow, speed, density or concentration. Uninterrupted flow can also be described
by other variables — time headway, spacing, and time occupancy. These
variables, however, are simply variants of the three variables described
previously.
Lesson Outcomes
After this lesson, the students must able to discuss the different traffic variables
and the parameters involved.
Discussion
MAJOR TRAFFIC VARIABLES
FLOW RATE or VOLUME
Flow rate is defined as the number of vehicles passing a point during a
specified period of time. It is often referred to as volume when measured over
an hour.
𝑵
𝒒=
𝑻
Where:
𝑁 = number of vehicles (represented by trajectories crossing line 1-1)
𝑇 = time
𝑞 = volume; will have a unit of like vehicles per hour and vehicles per day
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CEng 159: Transportation Engineering
SPEED
Speed is defined as rate of motion in distance per unit time. When describing
traffic stream, two types of speed are used:
1. Time mean Speed
Also known as spot speed. It is simply the arithmetic mean of the speed of
vehicles passing a point within a given interval of time.
𝑛
𝑖=1 𝑢𝑖
𝑢𝑡 =
𝑛
Where:
𝑢𝑡 = time mean speed
𝑢𝑖 = speed of vehicle
𝑛 = number of vehicles
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For instructional purposes only • 1st Semester SY 2020-2021 5
DENSITY
Density is defined as the number of vehicles in a given length of road at an
instant point in time. It is the number of vehicles counted at time t divided by
the length of the section L gives a measure of density in that section.
If 𝑛 vehicles are found within the section 𝐿, density 𝑘 is computed as :
𝒏
𝒌=
𝑳
TIME HEADWAY
Time headway is defined as the time interval between passage of consecutive
vehicles at a specified point on the road with a unit of time per vehicles.
𝟏
𝒉𝒕 =
𝒒
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CEng 159: Transportation Engineering
SPACING
Spacing is the distance between two vehicles measure from the front bumper
of a vehicle to that of another.
𝟏
s=
𝒌
TIME OCCUPANCY
Another useful measure of traffic flow is time occupancy. 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.
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and innovative technologies for sustainable communities and environment.
No.