May 1956 Popular Electronics
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
In the opening scene of "Gladiators,"
Quintus remarks to Maximus (Russell Crowe), "A people should know when they've been conquered." Such truth
is applicable to society today regarding ubiquitous surveillance. Less than decades
ago the media was filled with stories of outrage over the discovery of some new
form of monitoring and reporting system having been installed on highways, in shopping
malls, along sidewalks, even bathrooms. Since the 9/11 terrorist attacks, anything
goes with government snooping. Count the numbers of freedoms you have lost and the
inconveniences suffered because of those
19 men with no identifiable common cause (wouldn't want to "profile"),
15 of which came from Saudi Arabia, and how they changed our lives forever. But,
I digress. This story from 1956 shows how long stealth installation of radar speed
traps have been in use. I was very surprised to see the detector disguised as a
street lamp at such an early date!
Radar on the Highway
By O. P. Ferrell
Photos by Maynard Frank Wolfe
A POP'tronics Exclusive Report on Radar Speed Meters
One day soon you may be speeding along a lightly traveled highway and find that
electronics has indeed arrived. In fact, it has arrived to the tune of a wailing
siren and a big, flashing red light - you have been caught by a radar speed meter.
Much to your chagrin, the poor motorist with the flat tire, parked at the side of
the road, was really a highway patrol car. A radar broadcast reached out and measured
your speed. Simultaneously, a printed record was made that is accepted as irrefutable
evidence in 99% of the traffic courts throughout the United States.
This new pattern of speed law enforcement is being subjected to widespread criticism.
In an effort to alleviate technical misconceptions, the staff of POP'tronics has
prepared the following report. Because of the nature of the material, it has been
written as a question and answer survey. The questions are those heard in law courts
or sent in by the many readers requesting this article. The answers were obtained
from engineers and technicians using radar speed meters, plus several visits by
the POP'tronics staff to the one manufacturer whose speed meters are in common use.
Q. How does the radar speed meter work? Is it really radar?
A. Yes, speed meters are radar devices.
But they do not operate like the radars our G.I.'s used during World War II.
Speed meters are radars using the Doppler shift effect. This effect is best understood
when applied to sound rather than to radio waves. It is the change in the pitch
of a train whistle, or automobile horn, as it speeds past a stationary observer.
The pitch heard by the observer is different from that radiated by the whistle or
horn. If the train is approaching, the pitch sounds higher; if it is receding, the
pitch will sound lower.
Most police officers simply mount the radar speed meter on a
tripod. Connecting cables permit the readings to be made inside or outside of the
patrol car. If the officer is inside the car, he is generally in radio communication
with a partner about one mile up the road.
Radar tape being shown to author.
A radar speed meter operates on the same
principle. It measures the difference in pitch by sending out a u.h.f. radio wave
and listening for the change after it is reflected by the moving car. If the car
is not in motion, the radar speed meter does not "see" it. As soon as the car moves,
the speed can be measured. The faster the car moves, the greater the change in pitch.
Technically speaking, all radar speed meters operate on 2455 mc. Speed is read
as an audio frequency by mixing the continuous wave radiated by the transmitter
with the reflected wave from the moving car. A speed of 100 miles per hour is equivalent
to a Doppler shift of 731 cycles. The audio frequency may be read directly from
a meter, or recorded on paper tape.
Q. Could a radar speed meter measure motion inaccurately?
A. In the vast majority of cases, the speed meter will read within plus or minus
two miles-per-hour of the actual speed. Field trials of the equipment by POP'tronics
editors always gave results lower than the car speed by about one or two miles.
Because of the principle of operation, it is practically impossible to make a speed
meter read more than actual car speed, but a faulty "zero" adjustment could be made.
This means that the speed meter would read five to ten miles per hour if no car
was intercepting the radio beam.
Q. How does the police officer know if his equipment is operating properly?
A. Officers are trained to assemble and calibrate the speed meter. In addition,
the speed meter has a tuning fork calibrator. It is simply "rung" in' front of the
antenna. The speed meter will then read. The frequency of the fork will be 7.31
times the miles-per-hour meter indication. This frequency is stamped on the fork
and cannot be changed.
Q. Should the motorist make the arresting officer prove his radar is calibrated
A. If the motorist is positive that his speed was considerably less than that
indicated by the meter, it might be worth an attempt. In some states, motorists
have a legal right to request this information; in most of the others, it would
be a courtesy on the part of the officer. The philosophy employed by most courts
and police officers is that the radar speed meter is just as accurate as the speedometer
in a patrol car. Speedometer errors are known to exist and often exceed five or
six miles per hour over actual speed at 60 miles-per-hour.
Q. How many radar speed meters are there, and where are they located?
A. Radars must be licensed by the FCC. At this writing, there are about 1600
radars in use. They are scattered throughout all 48 states. Most of the longer freeway,
turnpike, or expressway police patrols have one or more radar speed meters in operation
Q. Can the speed of one car be measured from a police car that is in motion?
A. This is a popular misconception. The radar speed meter must be standing still.
It should be mounted about three feet above the ground and pointed down the highway.
Speed meters cannot be used in moving vehicles.
Q. How does a radar speed meter distinguish between cars?
A. Present-day equipment does not make an attempt to distinguish between cars.
This is left to the officer monitoring the equipment. His paper tape recording will
register the top speed of the cars closest to the speed meter. He must pick out
the speeder visually and correlate it with the meter reading.
Q. How can a speed meter be hidden?
A. There is a variety of ways of hiding speed meters, with the favorite being
a cutout in the metal lid of the trunk compartment. This cutout is then covered
with plastic and painted to match the color of the remaining trunk lid. The radar
looks out through the hole when the police car has stopped at the side of the road.
Police team on the Baltimore-Washington Parkway flag down speeders.
A radar detected the speeders about two miles down the road. (Washington Post
A technician from Eastern Industries, Inc., manufacturer of the
radar speed meters, checks the recording during the POP'tronics field trials.
The future in radar speed control is seen hanging over a highway.
This radar sentinel does not tag or arrest speeders. It feeds data on traffic conditions
to police headquarters for evaluation.
Q. Would hiding a speed meter be called "entrapment"?
A. In some courts, yes, but this is a legal point now under investigation It
is important to note that the State of Ohio requires a notice to be served on the
public not less than 750 feet in front of an operating speed meter. Most states
will not regard speed meters as "speed traps."
Q. Can the radar speed meters be jammed like wartime radars?
A. Yes, but only by operating. an illegal (unlicensed by the FCC) transmitter
on the same frequency (2455 mc.) as the speed meter. Radio amateurs might "accidentally"
jam the speed meter using their 2450-mc. band. In any case, this would be of doubtful
advantage since the "jamming" equipment would become costly to and operate continuously
Q. Could a police officer detect a "jammed" speed meter?
A. Yes, the indicators would either become totally dead, or else they would swing
Q. Can a speeder leave a trail of aluminum foil to upset the radar?
A. Scattering aluminum foil has no effect on radar speed meters. This type of
radar will only react to motion.
Q. What about grounding the car to ground out the signal?
A. There would be no effect whatsoever on the speed meter indication.
Q. What other devices have been used to fool radar speed meters?
A. A wide variety of gimmicks has been sold to the public. They have included
special devices to ground the car through the hubcaps and an absorption shield mounted
behind the radiator. None of them will have any effect on the speed meter.
Q. Couldn't a microwave detector be built to warn the motorist?
A. Yes, a detector could be built, and several designs were considered during
the preparation of this article. All of them are costly to build and have a limited
Q. Will POP'tronics publish such a design?
A. If there is sufficient reader interest, we might assign the task to a project
engineer for development.
Q. If radar speed meters continue to gain acceptance, what will be the future
of this device?
A. Police authorities are now anxious to test several new devices which promise
greater control over speeding. One such example is shown in the photograph at the
right. The innocent-looking street lamp is actually a complete radar unit. It is
beaming radar waves up and down the highway. The information from this radar is
fed into a master control station at a local police headquarters. This enables under-manned
police departments to dispatch patrol cars to areas where traffic is flowing either
too fast or too slow.
Within the next two years, several express highways will be "saturated" with
these radars. The data will be reviewed by an electronic computer to measure the
flow and density of traffic. An experimental installation will be made on the Merritt
Parkway in the area of Westport, Connecticut, by the end of this summer.
Posted May 10, 2022
(updated from original post on 4/13/2016)