POLICE TRAFFIC RADAR ERRORS
In court, radar suppliers and radar enforcers raise their
right hands and swear that police radar never makes mistakes.
If you have a radar ticket, you’re guilty.
But when the court is in recess and those same radar suppliers
are pitching new equipment to those same enforcers, you
hear a different story. Kustom Signals, Inc., of Lenexa, Kansas, has
been making speed radars for about 30 years.
In touting its new TruTrak feature, Kustom says
"…TruTrak eliminates common speed anomalies such as combined
speeds, splitting speeds and shadowing."
"Anomalies" is the salesman’s term for "errors."
The errors Kustom admits to are as old as moving radar
a test of radar units in 1980, the National Highway
Traffic Safety Administration identified "shadowing" as
a common problem with moving radars. A few years later, the Texas Department of Public
Safety produced a comprehensive training manual in which
it warned of "Vehicle Interference Error," an umbrella
category that includes "shadowing" and other errors.
Moving radar is much more complicated than stationary
radar, which needs only to read moving targets within
range. Moving radar must make two measurements, the relative speed
between the target vehicle and the moving patrol car,
and the actual patrol-car speed.It then calculates the target
vehicle’s speed by adding to or subtracting from the patrol-car speed.
If you’re thinking this process is subject to lots of electronic
confusion, you’re absolutely right.
Not long ago, moving radar worked only on oncoming traffic,
and it erred in that simple job. In the last five years, the addition of "same lane"
capability gives readings on vehicles moving in the same
direction as the patrol car. They can be either ahead of the patrol car or behind,
which multiplies hugely the possibilities for error.
No matter whether the target is oncoming or traveling
in the same direction, it can be clocked accurately only
if the radar gets a correct reading on the patrol-car’s
speed. "Vehicle Interference Errors" including "shadowing"
all result from the wrong reading for patrol speed. For example,
as moving radar looks for reflections
from terrain that would allow it to calculate patrol speed,
it sometimes grabs on to the stronger reflection from,
say, the back of a truck moving in traffic.
If the patrol car is moving at 45 mph, and the
truck at 25, the radar calculates the patrol speed relative
to the truck instead of the ground.
It wrongly decides the patrol speed is 20 mph.
Let's say the target car is also approaching at
45 mph in the oncoming lane. The radar correctly reads
the relative speedbetween it and the patrol car as 90 mph, then subtracts
the erroneous patrol speed of 20 mph, and pops up a target
speed of 70. The red lights flash on and the unlucky motorist gets nailed
for 25 mph over his actual speed.
Other common radar errors identified by the Texas Department
of Public Safety include: Antenna positioning error; Look-past
error; Cosine error; Double-bounce error; Beam-reflection
error; Road-sign error; Radio-interference error; and
Perhaps Kustom really has eliminated some of radar's errors
as it claims. But what about all of those older Kustom units
still in service, still bearing false witness against