Speeding Tickets: If They Get You, Have They Really Got You?

Recently law enforcement agencies across the nation have come under scrutiny for methodically decreasing their tolerances for speed enforcement. This move has been seen by many as an attempt by cash strapped cities to generate additional revenue through fines. What goes into a prosecutable ticket?

The most common device used by police officers to measure a suspect's speed is traffic radar. Radar (Radio Detection and Ranging) predates World War Two. It works by sending out a cone of energy in the form of radio waves (image a flashlight, but instead of light, radio waves are emitted). Much like our imaginary flashlight, objects in the beam are detected when they reflect energy back to the sender.

Traffic radar uses the same principle with a twist. Remember the Doppler Principle you learned about in 5^th grade science? This principle basically states that when one object is emitting wave energy and is in motion relative to another object, the frequency of the reflected energy will be changed. Look at it this way, if you were sitting in a chair and could magically throw a tennis ball at a wall 30 feet away at the same speed every second, you would get a tennis ball "reflected" back to you every second because each ball would travel the exact same 30 feet away and 30 feet back. Now, imagine the wall was moving toward you slowly. The first ball would have to travel 30 feet, the second only 29 feet, the third, only 28 feet, and so on. Because the subsequent balls have to travel less distance, you will get a ball back faster than every second. The frequency of the returning energy has changed. Traffic radar measures this change, whether higher or lower, to determine relative or closing speed.

So that's the concept, what could wrong?

Well, plenty, actually. As we stated before, traffic radar sends out a cone of energy much like the beam of a flashlight. There are two "modes" for traffic radar, moving and stationary.

We'll talk about stationary first:

In stationary mode, energy that is reflected back at the same frequency as it was sent represents non-moving objects and is ignored. However, any object that is moving and reflects energy is going to provide speed data for the unit. This means everything from wind-blown debris, to other cars, to bicyclists could be measured. The radar unit makes a determination as to what target to display based on the strength of the reflected signal and the amount of frequency change. It's not a perfect system because sometimes the delivery truck two blocks back will reflect more energy than the Honda half a block away. Officers must be very diligent when determining what it is their radar unit is reading. In addition, radar units are electronic devices. They are subject to malfunction and interference.

In moving mode, things get even more complicated.

Since the Doppler shift in moving mode will reflect both the patrol car's speed and the target's speed (ie- a patrol car going 50 miles per hour approaching a target going 40 miles per hour in the opposite direction have a closing speed of 90 miles per hour), the radar unit measures two sets of Doppler shifts. One is the strong signal sent back by all the stationary items the patrol car is passing including the texture of the road surface. The other is the closing speed of the target. The radar unit then subtracts the patrol speed from the closing speed and displays the result as the target speed. This is fine so long as the unit is showing accurate patrol speed. If the patrol speed is calculated low for any reason (such as a large, slow-moving vehicle in front of the patrol car providing the Doppler shift for the patrol speed calculation), the target speed would be elevated. Moving mode radar is subject to a number of other "effects" that could result in incorrect target speeds.

So what do officers do to prevent these potential false readings?

First, radar speed measurements are not sufficient evidence in themselves to result in a speeding conviction. Officers are trained to use a four part "tracking history" when operating traffic radar. This consists of:

Visual speed estimation. The officer visually estimates the target vehicle's speed independent of the radar unit.

Audio. The radar unit emits a tone consistent with the Doppler shift it is displaying. Officers are trained to listen to this tone to make sure it is consistent with the reading and is steady.

Radar Speed Measurement. The officer observes the target speed displayed along with the patrol speed in moving mode. These displays must be consistent with the officer's estimation, the audio, and the patrol car's speed as indicated by the speedometer.

Follow-through. The officer continues to watch what happens to each of these as the target vehicle passes out of the radar beam.

To combat potential electrical problems, officers are required to check the calibration of the radar unit before, and reasonably soon after, each stop. This is a three part test. The first part uses a button on the unit to light up all of the light segments on the display to check for burned out sections. The second test is an internal electrical test the unit performs to check its circuitry. The third and final test uses two tuning forks, at least one of which must be certified accurate annually, to externally check the calibration of the unit.

Failure to follow any of these procedures should render a radar speed measurement inadmissible in court. This won't help you overcome the officer's observations, however.

In addition, it's important to note that most posted speed limits in many states are not absolute speed limits. The actual speed limit is that speed which is reasonable and prudent for the circumstances existing at the time. The posted speed limits reflect the speed that would normally be presumed reasonable and prudent. However, special circumstances could exist which would alter that presumption.

Of course, the best way to beat a speeding ticket is to follow the law and not speed!