Highway & Transportation (I)
ECIV 4333
Chapter (4): Traffic Engineering Studies
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).
Basically, there are two types of speed: the time-mean speed and the space-mean speed.
Space-mean speed is the length of a road section divided by the average travel time of
several vehicles over this specific section. The time-mean speed (spot speed) is the
average spot speed of several vehicles measured at a given spot.
Theory and Concept
The methods used for conducting spot speed studies can generally be divided into two
main categories: manual and automatic.
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.
Data Collection and methodology
1- 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.
Eng. Almotasembellah M. Abushaban
1
Eng. Safaa Abu Elaish
Highway & Transportation (I)
ECIV 4333
Chapter (4): Traffic Engineering Studies
X
A
B
This method is not very accurate.
Limitations:


Number of vehicles on the sample  100 veh.
Distance between the two specified points  100 m.
Veh.
No.
Time(s.)
(t)
Velocity (m/s)
V=(x/t)
Velocity
(Km/hr)
1
2
3
4
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2- Plot the frequency and the cumulative frequency distribution of the speed data.
3- Calculate the:
- Average Speed.
- Median Speed.
- Modal Speed.
- The 85th and 15th percentile speed.
- Pace Speed.
- Standard Deviation.
4- Check whether the speed distribution is reasonably normal.
Eng. Almotasembellah M. Abushaban
2
Eng. Safaa Abu Elaish
Highway & Transportation (I)
ECIV 4333
Chapter (4): Traffic Engineering Studies
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.
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. They are:
1. 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
where
u = arithmetic mean
Ii = number of observations in each speed group
mid value for the ith speed group
N = number of observed values.
Uj =
2. 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.
3. Modal Speed which is the speed value that occurs most frequently in a sample of spot
speeds.
4. 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.
5. 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.
6. Standard Deviation of Speeds which is a measure of the spread of the individual
speeds. It is estimated as
Eng. Almotasembellah M. Abushaban
3
Eng. Safaa Abu Elaish
Highway & Transportation (I)
ECIV 4333
Chapter (4): Traffic Engineering Studies
where
S = standard deviation
u = arithmetic mean
Uj = jth observation
N = number of observations
Eng. Almotasembellah M. Abushaban
4
Eng. Safaa Abu Elaish
Highway & Transportation (I)
ECIV 4333
Chapter (4): Traffic Engineering Studies
Eng. Almotasembellah M. Abushaban
5
Eng. Safaa Abu Elaish