The best way to point out all the pitfalls of radar readings is to subpoena the radar unit’s instruction manual. The manufacturer will usually include a page or two on inaccurate readings and how to avoid them. If you study the manual, you may find a way to attack its reliability in court using the manufacturer’s own words. It is important to make sure that the manual is complete. Police departments have been known to tear out pages that discuss common radar screw ups from the radar manual before responding to a subpoena. So be sure to look to see if any pages are missing and, of course, point out any gaps you discover.
12. What are the common malfunctions and sources of inaccurate readings?
a. More Than One Target
Radar beams are similar to flashlight beams. The farther the beam travels, the more it spreads out. This simple fact often results in bogus speed readings, since it’s common for a spread‑out beam to hit two vehicles in adjacent lanes
It is very possible that the officer obtained a radar reading from another vehicle. At trial, ask the officer if his radar unit was on automatic. The chances of registering the speed of the wrong car go way up when an officer, points his radar unit at a highway and puts it on the automatic setting. This is true because the officer isn’t pointing his radar unit at a specific vehicle. Therefore, the beam angle width means the unit could be picking up one of several cars going the same, or even opposite, directions. In this case, ask the officer whether there was other traffic in either direction. If his answer is “yes,†ask him which direction. If there was traffic in the direction opposite you, follow up and ask him whether the unit responds to traffic in both directions. Either way, if there was other traffic, be sure to raise the possibility in your closing argument that the radar unit clocked the wrong vehicle.
b. Wind, Rain, and Storms
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