Ultra Start 15xx Manual de usuario descargar pdf (Pagina 85)

10-3

The accuracy of this method depends upon several factors:

• The speed of the vehicle is assumed to be constant while driving from

the start line to the end line. If the vehicle accelerates or decelerates

while in the speed course then the accuracy will be reduced. You want

to pick an area where the vehicles are likely to maintain their speed,

if possible.

• The observer must press the buttons as accurately as possible. There

is always some error due to poor reaction times. Some observers are

simply better than others.

• The main source of error is caused by the difﬁculty in determining ex-

actly when the vehicle passed over the start line (assuming the observer

is standing even with the end line). A slightly elevated view usually

enhances the ability of the observer to see the start line.

• Other factors, such as the length of distance between the start and

end lines, the timing accuracy of the TDC Ultra electronics, and bias

caused by the drivers detecting ‘something going on’ and slowing down

all contribute errors to the results, but these errors are small compared

to the ones mentioned above and generally can be ignored.

Since the sources of error can vary greatly, it is difﬁcult to say what ac-

curacy you should expect. However, our test indicate that an error of about

2-3 MPH is typical at a site with good visibility and steady trafﬁc driving

at a constant speed.

What Data Do You Need to Collect?

Spot Speed studies can be done in one of two ways – with vehicle clas-

siﬁcation or without vehicle classiﬁcation.

The green key marked "A" on the template is pressed when a vehicle passes

over the start line. If you are not concerned with distinguishing between

types of vehicles when recording speeds, you can press the number 8 key

shown next to the “B” on the template when the vehicle passes the end

line. When the data is processed you will see one column of all the vehicle

speeds, from the ﬁrst to the last.

Chapter 10 — Spot Speed Studies

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Ultra Start 15xx Manual de usuario Pagina 85