Transcript Calculation

High way and Transportation I(DIS)
ECIV 4333
Instructor
 Assistants

: Dr. Essam Almasri
: Eng.Mustafa abou foul
Spot Speed
Introduction
 Speed
is an important measure
of the quality of travel and safety
of road network. Speed by
definition
is
the
rate
of
movement of vehicle in distance
per unit time. A typical unit of
speed is kilometers per hour
(kph) or miles per hour (mph).
Theory and Concept
The methods used for conducting spot
speed studies can generally be divided
into two main categories: manual and
automatic. Since the manual method is
seldom used , automatic methods will be
described.
Theory and Concept
Several automatic devices that can be
used to obtain the instantaneous speeds
of vehicles at a location on a highway are
now available on the market. These
automatic devices can be grouped into
three main categories:
(1) using road detectors.
(2) by radar-based.
(3) using the principles of electronics.

Locations for Spot Speed Studies
Unbiased data
 Drivers be unaware
 Equipment concealed from the driver,
 Observers inconspicuous.

Time of Day and Duration of
Spot Speed Studies
depends on the purpose of the study.
recommended when traffic is
 free-flowing,
 during off-peak hours.
typically:
 the duration is at least 1 hour and
 the sample size is at least 30 vehicles

Data Collection and methodology
The data is normally based on randomly
sampling individual vehicle speed over
short time period.
 It depends on observing the time required
by vehicle to cover a short distance of
roadway.
Requirements:
 Stop watch.
 Measuring tab.

Manual Method
The following table shows the recommended
distance between points 1 and 2 for selected
average speeds on the road.


This method is not very accurate.
Calculation

The calculated mean (or average) speed is used
to represent the true mean value of all vehicle
speeds at that location. The accuracy of this
assumption depends on the number of vehicles in
the sample. The larger the sample size, the
greater the probability that the estimated mean
is not significantly different from the true mean.
It is therefore necessary to select a sample size
that will give an estimated mean within
acceptable error limits.
Calculation

Statistical procedures are used to determine this
minimum sample size. Before discussing these
procedures, it is first necessary to define certain
significant values that are needed to describe
speed characteristics
Calculation

Average Speed which is the arithmetic mean of
all observed vehicle speeds (which is the sum of
all spot speeds divided by the number of
recorded speeds). It is given as
Calculation
Median Speed which is the speed at the
middle value in a series of spot speeds
that are arranged in ascending order. 50
percent of the speed values will be greater
than the median; 50 percent will be less
than the median.
 Modal Speed which is the speed value
that occurs most frequently in a sample of
spot speeds.

Calculation

The ith-percentile Spot Speed which is
the spot speed value below which i
percent of the vehicles travel; for
example, 85th-percentile spot speed is the
speed below which 85 percent of the
vehicles travel and above which 15
percent of the vehicles travel.
Calculation
Pace Speed: the highest speed within a
specific range of speeds that represents
more vehicles than in any other like range
of speed. The range of speeds typically
used is 10 km/h or 10 mph.
 Pace – The 10 Km\h speed range
representing the speeds of the largest
percentage of vehicles in the traffic
stream.

Calculation

Standard Deviation of Speeds which is a
measure of the spread of the individual
speeds. It is estimated as
minimum sample size
Example 4.2
Histogram of Observed Vehicles Speeds
Frequency Distribution
Cumulative Distribution