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From the above methods, the collected data have to present into the some
representable form, this makes its calculation and analysis simpler and easier.
The following methods to present the spot speed data:
After the collection of data in the given conditions, arrange the spot speed
values in order to their magnitudes.
Then select an interval speed (e.g. 5 kmph) and make grouping of data which
come under this range.
Now, prepare the frequency distribution table.
For each speed group, the % frequency of observations within the group is
plotted versus the middle (mid-mark) speed of the group(s).
As shown in Fig 6.5. From this curve the modal speed and pace of traffic flow
can be determine.
Generally the shape of the curve follows the normal distribution curve, this
because the most of the vehicles move on road near by mean speed and very few
deviate from mean speed.
For each speed group, the % cumulative frequency of observations is plotted
versus the higher limit of the speed group (Fig 6.5).
The cumulative frequency distribution curve, however, results in a very useful
plot of speed versus the percent of vehicles traveling at or below the
designated speed.
For this reason, the upper limit of the speed group is used as the plotting
point.
In both the distribution curve, the plots are connected by a smooth curve that
minimizes the total distance of points falling above the line and those falling
below the line.
A smooth curve is defined as one without.
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