Dear chazwat-ga;
Thank you for allowing me to answer your interesting question. In
order to qualify my answer let me say that having been in law
enforcement myself for more than 25 years I have some working
knowledge of several types of speed detection and speed measurement
devices frequently used by law enforcement.
VASCAR (an acronym for Visual Average Speed Computer and Record) is,
as you say, a glorified stopwatch. The theory however is relatively
accurate though it does have some flaws that I will speak about in a
moment. The proven principle upon which VASCAR functions is based on
this formula:
(dt = r * t)
[The ?distance traveled? equals ?rate? (speed) multiplied times ?time?
so a vehicle?s speed is determined by ?distance traveled? divided by
?time? which are both known values.]
Most VASCAR devices today are digital, and a vehicle?s speed is
calculated in terms of AVERAGE speed, which of course is never higher
than the peak speed the vehicle reached within the measured distance.
In other words, like any average, the speed calculated by the VASCAR
device does not necessarily reflect the ?actual? speed of the vehicle;
rather it reports the ?probability? of speed within a proven
mathematical RANGE, assuming there is no operator error (again, I will
speak more about this in a moment). The assumption is that if the
calculated speed of the vehicle is not more than the vehicle could
possibly have been traveling, AND the lower range of the average is
high enough to indicate that the vehicle was likely exceeding the
speed limit, that the clocked vehicle was speeding. Here is another
way of looking at this:
Assume the speed limit is 55 mph. If the applied VASCAR formula
indicates that the vehicle was traveling at 55-60 mph, the average
speed is assumed to be roughly 57.5 mph. There can be no argument then
that the vehicle was traveling OVER 60 mph since the applied formula
itself proves the vehicle could not have been traveling that fast. By
the same token, barring any operator error, the reverse is also true;
the vehicle could not have been traveling less than 55 mph.
The problem with this type of measurement is that the result is an
AVERAGE. This means that there is a rather small probability that the
clocked vehicle could have been traveling at 55 mph, which is within
the posted speed limit. It seems trivial to quibble over a few miles
per hour, but let?s face it, at some point one goes from being a
lawful driver to a viable target and it is in this range that such a
determination is made. What makes the difference? More often than not
it is due to operator error, technical error or environmental factors.
Modern VASCAR units are quite sensitive and require a warm-up period
before use. They are also known to be sensitive to temperature
variation inside the police vehicle. Secondarily, the implementation
of the VASCAR principle requires the officer to clearly see the
vehicle reach two known points that are a known distance apart. This
does not always involve marked roadways or distance between signs
because the same principle can be applied by watching his odometer. If
the officer observes a car traveling at a known rate and clocks that
car at that rate for a specified period of time, he can do the same
math and calculate the rate of speed for a given distance traveled.
This of course requires the officer to have a professionally
calibrated speedometer and odometer and if he doesn?t have both of
these in his car, the mathematical conclusion can often be disputed.
Once again, this also requires the officer to accurately mark the
point on his odometer where the clocking begins and likewise relies on
the premise that he accurately marked the end of the clocking period
before applying the mathematical formula. In other words, let?s say
for example that the officer began clocking the rate of travel over a
one-mile distance when his professionally calibrated odometer reads
?10000.0 miles? and stopped clocking when his odometer reads PRECISELY
?10001.0 miles?. If this is the case, as it theoretically should be,
the officer could determine the speed based on the one mile traveled
in x-amount of time. If, on the other hand, the officer was sloppy and
began clocking at ?10000.3 miles? and stopped clocking at ?10001.0
miles? the end result would clearly be inaccurate. According to some
of my knowledgeable law enforcement colleagues who frequent the REAL
POLICE FORUM (who, by the way, also confirm your statement about the
200 feet recommendation of the manufacturer) ?an error of 20 feet
amounts to approximately a 1 mph discrepancy? in a VASCAR operator
error. This could obviously be a significant margin of error if one or
two miles per hour are at stake in determining if a driver is a lawful
driver or a violator.
REAL POLICE FORUMS
http://realpolice.net/forums/archive/index.php?t-41919.html
To further complicate matters, if the odometer is not professionally
certified as calibrated, the actual distance traveled could be even
more or even less than that. Even if the odometer is calibrated, a
simple thing such as changing the vehicle?s tires or tire pressure can
often throw the calibration off and bring it under question. If the
odometer is not re-calibrated after such a change the accuracy of the
calibration is questionable. In fact, without a calibrated odometer,
the distance traveled is essentially UNKNOWN, isn?t it? In this
scenario the conclusion that you were exceeding the speed limit would
be inaccurate and it would be impossible for the officer to testify
under oath that you were indeed exceeding the speed limit.
Calibration is a very important factor in clock-style speed devices.
This is especially true in cases where both the officer and the target
vehicle are mobile. In this scenario the officer?s vehicle and the
VASCAR unit must both have a current certification for professional
calibration. Obviously in a stationary scenario, the officer must be
able to testify that the vehicle reached two points of a known
distance within a certain period of time so his vehicle?s calibration
is not a factor, however, in court (if asked to do so) he must produce
not only the current calibration certification for the VASCAR unit he
was using, but he must also produce any personal certification that HE
holds showing that he is current, knowledgeable and proficient in the
use of the device. When an officer cannot produce the necessary
certifications the court will often dismiss the citation due to lack
of proof of the officer or equipments? proficiently and accuracy.
Now for the bad news: Can the mobile clocking scenario take place in
far less than a mile? Yes. An officer can begin clocking when his
calibrated odometer reads ?10000.0 miles? and stop clocking when his
odometer reads PRECISELY ?10000.2 miles?. While some more lenient
courts may consider this an insufficient period of time to reach an
accurate conclusion, there is technically no specific MANDATORY
minimum distance (unless the department policy dictates one) from
which the officer can base his calculation. The fact that the
manufacturer suggests a minimum of 200 feet upon which to base a
conclusion is the recommended distance, it seems that you very well
might have a valid argument in your defense. What a case like this
will all probably boil down to is the credibility of the officer vs.
your testimony of the facts. I should mention however that these US
DEPARTMENT OF TRANSPORTATION reports indicate that the clocking of
vehicles for mere ?seconds? is common practice and not entirely
outside the range of acceptable times.
New VASCAR-plus Timing Mechanism Test Results
http://ntl.bts.gov/lib/6000/6800/6873/896.pdf
(If that link doesn?t open try this cached copy)
http://64.233.167.104/search?q=cache:M21P5rtFmpsJ:ntl.bts.gov/lib/6000/6800/6873/896.pdf+vascar+%22200+feet%22&hl=en&gl=us&ct=clnk&cd=2
Analysis of VASCAR
http://ntl.bts.gov/lib/6000/6800/6867/706.pdf
(If that link doesn?t open try this cached copy)
http://ntl.bts.gov/lib/6000/6800/6867/706.pdf#search=%22vascar%20%22200%20feet%22%22
In answer to your question then about error rates, the rate of error
cannot be determined by poll alone. There are a host of factors
ranging from preparation, calibration and other technical issues all
the way to operator error and environmental issues that set the stage
for errors and any one of them can impact the accuracy of the device
or the mathematical calculation thereof. Suffice it to the say that
overall the VASCAR is deemed quite reliable in terms of what it can
determine, *IF* it is functioning properly and is used correctly by a
certified operator and *IF* the certifications are CURRENT (that?s a
biggee) and available for inspection. VASCAR citations are
historically very tough to beat and in my opinion the best defense is
to attack the credibility/proficiency of the user, the policy and
conditions under which the unit was used, and the
credibility/reliability of the unit, vehicle and other onboard
equipment themselves (Vascar certification, proof of odometer
calibration, tire pressure, proof of vehicle maintenance in
conjunction with calibration, etc). In other words, one can sometimes
turn the tables and refrain from using as their defense that the
calculation was simply wrong (an argument that most people probably
can?t successfully win), rather they can sometimes use the defense
that the equipment and procedures do not meet policy guidelines,
certifications are out of date or affected by alterations to the
vehicle, or are questionably accurate or downright dysfunctional based
on those standard operating policies and procedures.
I hope you find that my answer exceeds your expectations. If you have
any questions about my research please post a clarification request
prior to rating the answer. Otherwise I welcome your rating and your
final comments and I look forward to working with you again in the
near future. Thank you for bringing your question to us.
Best regards;
Tutuzdad-ga ? Google Answers Researcher
INFORMATION SOURCES
SCIENTIFIC SPEED DETECTION
http://faculty.ncwc.edu/TOConnor/205/205lect09b.htm
HOW VASCAR-PLUS WORKS
http://www.vascarplus.com/Pages/How_it_works.htm
SEARCH STRATEGY
SEARCH ENGINE USED:
Google ://www.google.com
SEARCH TERMS USED:
VASCAR
CERTIFICATION
POLICY
DISTANCE
FORMULA
PRINCIPLE
CALIBRATION |
