How do the police know how fast you're going, anyway? Today, there
are a number of methods that a police officer can use to measure your
speed. The most common way cops catch speeders is to use a radar
device. Radar uses radio waves that shoot out of the device and bounce
off of a moving object, such as your vehicle. The radar device
calculates the speed of the vehicle based on the change in frequencies
between the waves that are sent out and the waves that are reflected
back. Radar devices can either be handheld devices such as a radar
"gun" or larger devices placed in an officer's vehicle. "Speed cameras"
can also be attached to radar devices, but there are currently legal
restrictions on this practice in Texas. In June of 2007, a bill was
passed preventing Texas cities from using speed cameras to catch
speeders. The Texas Department of Transportation is currently the only
entity that is allowed to set up speed cameras. When they announced
plans to do so in September of last year, there was enough of an outcry
that they decided to hold off on the cameras for at least another 2
Another type of device used to catch speeders is laser. Laser devices
use pulses of laser beams aimed at your car to measure speed, with your
speed being determined based on how fast the light pulses are reflected
back. What about those signs that say "speed monitored by aircraft?"
No, the aircraft don't have super-strong radar guns or lasers...at
least not yet. Aircraft determine speed by calculating how long it
takes your car to get from one stationary point on the road to another.
Since the distance between the two points is known, it's easy for the
people manning the aircraft to figure out how fast you are going.
The same principle can be used to measure speed from the ground using
two hoses laid across the road. When you drive over the first hose, the
pressure of your tires triggers a machine that measures how long it
takes you to get to the second hose. The machine can use this
information to calculate your speed.
A decidedly less-high tech way to determine if someone is speeding is
called "pacing." To pace someone, all a police offer has to do is get
behind you, get his vehicle going the same speed as yours, and then
look down at his speedometer.