Bethel Maine
E & W Truck Speed %
of Trucks vs. Speed 2001
The JOHO image illustrates a
different arrangement of date from the other traffic images at this web site.
Using the same WIM sensor data files as the other WIM images of the Bethel Type
10 traffic, we made this image showing truck speeds. The image is an example of
the result of a simple change in the directives telling JOHO which fields
to read in the data file. Here you can quickly discern patterns in the truck
speeds at the WIM site.
Please see the description of the Kittery
November image for a more detailed description of JOHO image
concepts. Remember that image has a different layout!
This image has the same layout as the truck
speed image. Only the meaning
of the value in the cells is different.
This image shows the percentage of trucks (FHWA
Types 4-14) at each speed passing the WIM sensor in Bethel, Maine for all 2001.
The image contains 2,304 data points. This is far more information than
traditional graphics programs can display.
Image Layout
The image has twelve panels, each showing one
month in 2001, starting in the upper left and ending in the lower right. Each
column of a panel represents one hour in the day, starting with the hour from
midnight to 1 AM in the left column (as labeled in the December panel). The
right column of cells represents the hour from 11 PM to midnight. Each column
counts all the trucks in the month that passed the sensor at that hour, no
matter what day of the month they passed. The speed of the trucks is measured
vertically, from 30 mph on the bottom row to 100 mph on the top row.
Each colored cell indicates the percentage
of all the trucks that passed at that speed, in either direction, during that
hour, sometime during the month. We added the cells in each column in the truck
speed image to get the total number of trucks that passed the sensor in that
hour of that month. Then we divided each cell by the total of that column (and
multiplied by 100) to get the percentage shown here.
The legend at the right indicates the relation
between color and percentage of trucks. The image indicates 20% or less as the
dark blue color at the bottom of the legend. At about 50% the color changes from
green to orange. The white at the top of the legend indicates a count of 100%
trucks.
Image Interpretation
Because we
are looking at percentages, this image carries no information about the number
of trucks. Thus the September
decrease in traffic is no longer visible. Also the variation in the number of
trucks through the hours of the day is no longer visible.
The most
obvious feature is that the third row from the bottom (46-55 mph) has the
highest percentage trucks. This is to be expected because the speed limit at the
WIM site is 50 mph. Note that the cells this column have values from 60% to 70%.
This means that 30% to 40% of the trucks are going at some other speed.
The broad red peak in the images during working hours indicates here that a
higher percentage of the trucks are going the speed limit in the daytime.
Now consider
the fourth row from the bottom (56-65 mph). There are very interesting patterns
here. Note that from May to December there is a peak in the percentage of trucks
that are speeding in the morning between 4 AM and 6 AM (shown by a brighter
green bar). Note also that for September, October, and November, the percentage
peak also in the hours between 9 PM until 9 AM.
This does not occur during the other months.
Analysis of
such site-specific patterns can be very useful for enforcement, and can lead to
better understanding of the traffic situation. JOHO
is the best tool available for analyzing these large traffic data sets.
