Spot Speed Study
Before anything else
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Read the manual carefully. It is your
homework
What is covered in the following slides
is just general
General
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The data is to be used for assessing
general speed trends and for setting
speed limits.
The equipment for our data collection
procedure is a hand-held radar.
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Radar usage
Radar checkout
Data Collection
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Name of the safety officer
Choose section of road
Not close to intersection
Off-peak period during daylight hours
on a typical weekday
Hide 
I will contact Police to cover you
Data collection form
Data analysis
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What should you do with the collected
speeds?
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What else, statistical analysis!
Remember that old stuff: Mean,
standard deviation,…
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Plus a few more things
Speed Statistics
Average
speed
Speed data
Grouped
Not grouped
u = uj/N
Standard
deviation
Speed data
Grouped
s=
f(ui – u)2
N-1
Variance
s2
Not grouped
Types of Speed Measurements
Median speed
The speed at the middle value in a series of spot
speeds. Or, 50th-percentile speed
Modal speed
The speed value that occurs most frequently in a
sample of speeds
ith-percentile
speed
The spot speed below which i percent of the
vehicles travel, e.g. 85th-percentile speed
Pace
The range of speed that has the greatest number
of observations; usually 10-mph range
85%
50%
Frequency histogram example
Northbound Frequency Histogram
35
Frequency (f i)
30
Modal Speed = 31 mph
Modal Speed = 33 mph
25
20
15
10
5
0
22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44
Speed Class (mph)
Frequency distribution curve
Frequency Distribution
12.00
Frequency (%)
10.00
8.00
6.00
4.00
Pace
28-34 mph
2.00
0.00
10
15
20
25
30
Speed (mph)
35
40
45
50
Cumulative frequency
distribution curve
Cumulative Frequency Curve
110.00
100.00
90.00
P85 = 35.5 mph
Frequency (%)
80.00
70.00
60.00
50.00
Median Speed=P 50= 31 mph
40.00
30.00
20.00
10.00
0.00
0
10
20
30
Speed Class (mph)
40
50
60
Helping calculation table
Interpret and conclude
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Answer questions in manual
Prepare your presentation