SCIENTIFIC SPEED DETECTION

    Speed limits are arrived at scientifically through the use of traffic engineering, speed limits are enforced via the use of scientific equipment, and convictions for speeding are accomplished by technical testimony about the scientific principles underlying detection by radar or analysis of skidmarks. Courts have also been willing to consider various eyewitness and "skilled witness" accounts under some circumstances, but science, in general, permeates the whole field of traffic enforcement.

A PRIMER IN TRAFFIC ENGINEERING

    Roadways are designed geometrically, for aesthetic purposes, and to be in harmony with their environment. The geometry includes consideration of such things such as cross section, gradient, curvature, sight distance, and clearances. The aesthetics includes consideration of such things as channelization, signs, and markings. Environment determines the type of roadway -- freeway, expressway, arterial highway, collector street, or local. All three are components of what is called capacity, a quantitative term.

    Volume is the most important item of measurement in traffic engineering. There are different ways to calculate it, but it generally refers to the number of vehicles passing a given point during a specific period of time. Another important item is density, calculated by dividing volume by the average speed.

    If traffic becomes so dense that a bottleneck, gridlock, or the technical term, congestion, occurs, then there will be little spacing or headway between vehicles, and the average speed will be less than 35 mph. Vehicles are not manufactured to run for any significant length of time at speeds in the 25 mph range without damage from the jerky stops and starts that sometimes require ignition restarts.

    Before building a new road is considered, however, qualitative analysis is done. This goes beyond looking at capacity measures and at a concept called level of service. This is the general quality of operation afforded the driver. Roads designed for certain capacities might have quite different levels of service, depending upon the drivers who use them. Origin and destination surveys are used to determine level of service. If the congested road is used by drivers on a regular basis and not just for holidays, sporting events, or to avoid construction, then another road will be built.

    Speed zoning is the term used to describe the techniques of traffic engineering that are behind the establishment of speed limits. Not all roadways are analyzed by speed zoning, only those that carry an appreciable amount of traffic, maintain high accident rates, or have enforcement problems. A high accident rate tends to be the most important, but not only, consideration for reducing the posted speed limit. Posted speed limits tend to, in probability terms, be designed for 70% of all drivers, with a standard deviation of 15%. Anyone driving outside this range is not considered a "safe and reasonable" driver. Some states allow local jurisdictions to have input or at least help coordinate the setting of speed limits, but the trend is toward uniform traffic laws where states abide by the 1968 Uniform Vehicle Code.

    Two schools of thought exist as to how speed zoning and speed limits should be translated into law. The first theory is known as "basic speed law" and gives a lot of latitude to drivers in terms such as they should always be "reasonable and prudent". The second theory is known as "maximum speed law" and holds that drivers should be informed exactly of what constitutes being "fairly certain to be dangerous". Some states have a combination of these two, with signs indicating a regular and upper (absolute) speed limit. Here's some examples of the typical statutory language:

    Occasionally, factors such as the need for nationwide energy conservation necessitate maximum speed limits such as 55 mph, but such limits are not necessarily unsafe from a traffic engineering standpoint. There are also numerous other standards in Traffic Law such as "reasonable care" (when making a turn), "adequate care" (when merging), but there hasn't really been much scholarly effort put into development of these standards beyond the "common sense" approach taken with things like lane changing and right-of-way. 

DETECTION OF SPEEDING

    The most effective devices are RADAR (Radio Detection and Ranging), VASCAR (Visual Average Speed Computer and Record), and LASER (Light Amplification by Simulated Emission of Radiation; sometimes called LIDAR). Law enforcement's latest technology is PHOTO-RADAR. Photo-radar equipment combines a camera and radar with electronic controls to detect and photograph a speeding vehicle. The unit can photograph the driver's face and the front license plate if deployed to photograph oncoming traffic or the rear license plate if deployed to photograph receding traffic. The license number of the speeding vehicle is extracted from the picture, and a citation is sent to the registered owner of the vehicle. 

    RADAR in law enforcement was developed as an offshoot of military technology following WW II, and is a special type known as "pulse" type radar, perhaps the simplest kind. It works by sending a cone-shaped stream of radio wave crests which is reflected or bounced back (the return is called the echo) by the object detected. The number of wave crests is the frequency, and if the object is stationary, the frequency of the echo is the same as the frequency of the transmission. If an object is moving (it doesn't matter if coming or going), the echo will have a different frequency which can be calibrated by a voltmeter on a mph scale.

    This change in echo frequency for moving objects is a well-established sound principle that was first established in 1842 by Christan Doppler, and is known as the "Doppler Effect".

    VASCAR was popular in about 30 states from 1958 to 1990, and may be still encountered in a few jurisdictions. It is a purely mechanical stopwatch-type device built into police cars which require the officer to start the mechanism whenever a moving object passes a first checkpoint and stop the mechanism when a second checkpoint is passed. The distance between checkpoints has been precisely measured, and it calculates speed on the well-proven principle that distance traveled equals rate times time (dt = r * t) so rate (speed) is determined by dt/t which are both known values.

    Modern VASCAR devices have a computer (digital) readout, and the speed is calculated in terms of average speed, which is never higher than the peak speed the vehicle reached within the measurement interval. They are equipped with tamper-proof circuits that cause the unit to shut down if bumped or banged around. They also need a warm-up period before use, and are somewhat sensitive to temperature variation inside the police vehicle. Some police departments use VASCAR with aircraft that spot for pre-measured markings on the roadway only visible from the air.

    LASERs come in solid state or other (Maser) varieties, and the most common ones for speed detection are the same kind as found on gun sights. They work by projecting a tightly-focused beam of invisible light and producing a dispersed or reflected beam of light upon contact which is picked up and read by a calibrated speedometer. Law enforcement considers LASER ten times better than RADAR, mostly because no known countermeasures exist, and laser beams don't start spreading out until 2000 feet while radar beams spread out at about 550 feet.  Their effectiveness is limited by outside weather conditions.

SCIENTIFIC TESTIMONY

    Some jurisdictions will admit non-scientific evidence of speeding, even by lay witnesses (under various opinion rule exceptions) and by police officers (under various skilled witness standards), and a few states have passed statutes dispensing with scientific testimony altogether on the grounds that if speed detection devices were used, this is prima facie evidence of speeding, subject to showing proper training, operation, and instrument testing by a qualified operator. Most police officers are certified speed detection operators, as this type of training tends to begin at the basic academy level.  More commonly, however, the court relies upon the "public record" and takes judicial notice of other decided cases in which scientific speed detection devices were used. Courts do not always make clear exactly what public record they are talking about, and in any event, technically, they only dispense with expert testimony on the underlying scientific principles. Scientific speed detection evidence normally requires the following:

    While some defense challenges may involve a speedtrap or police quota argument, more successful arguments will be based on one of two areas affecting the reliability of devices: (1) operational error; and (2) technical error.

OPERATIONAL ERRORS

    (1) Not knowing your weapon -- the practice of judicial notice mentioned above which dispenses with the need for scientific testimony only applies if the same manufacturer model of device has turned in a successful (and unchallenged) record of performance in the same or similar jurisdiction. If the police department has just upgraded its radar guns or switched to a different model, they can't rely upon judicial notice nor manufacturer hype. Therefore, the defense can easily challenge or at least ridicule the credibility of an officer who doesn't know the difference between the model 8 or 10, for example.

    (2) Not being certified or recertified -- the police department may have not complied with its own training schedule or some state statute that always seems to be changing involving certification standards.

    (3) Not calibrating, checking or testing -- in most places, a check of some kind must be done every 30 days, and police departments use one of three methods, or combination thereof: (a) tuning fork calibration - these are available for purchase and come preset to generate waves in various mph speeds; (b) internal circuitry check - this requires a technician to be called in, and a 1% error in the oscillator equals a 20 mph error rate in the readout; (c) road test - the police department simply calibrates one device by comparing it to another known unit, this being by far the most common method used.

    (4) Bad location -- speed detection devices don't work well in certain areas where there are power lines, billboards, and various other terrain features around. Also, in the practice called "stationary radar", where one police car runs the equipment and another police car is the chase/catch vehicle, there's a risk that the wrong speeder was identified. With moving radar, "blocking" sometimes occurs in heavy traffic, especially if there are lots of trucks on the road.

    (5) Bad marksmanship -- speed detection devices require constant attention while in use. If the operator sits back and puts the "auto-lock" feature on, the device will only record increases from an average traffic flow, but the unit will be more sensitive to "spikes" and other power surges.  Marksmanship is highly important with LASER technology.

TECHNICAL ERRORS

    (1) Range/Targeting problems -- many technical errors overlap with operational errors when long range speed detection is involved. Most devices are also affected by weather, but at long range, beams spread out as much as eight lanes of traffic. It then becomes unclear if technically or operationally, you:

(a) stop the largest vehicle
(b) stop the fastest vehicle
(c) stop the most beam-reflective vehicle
(d) stop the closest vehicle -- this being the policy of most police departments

    (2) Bumping error -- this occurs when the police vehicle which is running radar slows down, as when to turn around and give chase in the opposite direction. The turnaround dampens the patrol speed calibration and artificially "bumps" up the suspect's speed on the readout.

    (3) Shadowing error -- this occurs when the police vehicle passes slower cars while in pursuit of a speeder. The radar temporarily focuses on the closer, slow-moving vehicles and then causes the suspect's speed to be overestimated when the chases returns to a clear line of sight.

    (4) Batching error -- this occurs with sudden acceleration of the police vehicle. The patrol speed calibration has to catch up and does so by artificially increasing suspect speed readout.

    (5) Cosine error -- speed detection works best if straight ahead or behind, but if on an angle, this tends to underestimate (most of the time; it can also overestimate) suspect speed. 

    (6) Ghost error -- many devices will produce unreadable readout (a series of indecipherable dashes) in the presence of high-voltage lines, microwave transmitters, neon lights, and some CB radio and mobile phone transmissions.  There's mixed evidence over whether radar detectors (illegal in Virginia, DC, and Connecticut) cause problems or not. Certain terrains, like those high-wall noise barriers along residential subdivisions, can cause an overestimation (doubling) of speed. Other features, like billboards, wood, plastic, fiberglass, paper, and vehicle bras tend to involve radar-absorbent materials and will most likely result in an underestimation of speed.

INTERNET RESOURCES
Idiot's Guide to Doppler Radar
NHTSA's List of Speed Detection Devices
The Speedtrap Bible
Speedtrap.com's The Problem with Radar and FAQ on Lidar

PRINTED RESOURCES
Goodson, M. (1985). "Technical Shortcomings of Doppler Traffic Radar" J. of Forensic Sci 30: 1186.
Hand, B., A. Sherman, M. Cavanagh. (1980). Traffic Investigation and Control. NY: Macmillan.
Moenssens, A., J. Starrs, C. Henderson & F. Inbau. (1995). Scientific Evidence in Civil and Criminal Cases. Westbury, NY: Foundation Press.

Last updated: 12/20/04
Syllabus for JUS 205
Instructor Home Page