In the sense of performing as designed, most radar detectors do detect the presence of police radar in the immediate vicinity of a speeding vehicle. Whether or not that information will actually prevent a driver from receiving a speeding ticket remains to be seen, however. The problem does not lie with the detecting devices themselves, but the technology used by law enforcement officers to determine if a driver has committed a violation.
IN the US, early police radar units used one of two radio bandwidths assigned by the FCC, the X band and the K band. In order to be most effective, police radar units would have to transmit a constant signal in one of those two bandwidths. Radar detectors contained powerful antennas that could pick up the signal from an active police radar gun long before the vehicle came into range of the officer. The X band could almost always be detected from a distance, but the K band was much more difficult to detect. Only the best detectors on the market could warn against K band police radar.
The basic point behind these devices is for a speeding driver to receive a warning before driving into an area where police radar is in use. This information is only useful if the driver has enough time to slow his or her vehicle to a legal rate of speed. If the police officer is using a different radar technology than the detector, however, the driver may never get this warning in time, if at all.
Modern police radar units now use either a Ka bandwidth or a laser system, both of which allow officers to use an "instant on" technique. The traffic officer can literally point and shoot at a suspected speeder and get an official speed reading within a few seconds. Many radar detectors sold today can detect the presence of Ka band or laser police radar guns, but the driver usually doesn't get that information in time to make any significant reductions in speed. The end result could very well be a speeding ticket.
This is not to suggest that these devices are completely useless against modern "instant on" radar guns. If traffic on a highway is heavy, for instance, police officers may target the first few cars of a pack. If a driver at the rear of the pack has a modern detector, he or she may have a few seconds' warning to reduce speed. The situation may be completely different on a sparsely populated highway with a dedicated traffic enforcement officer, however.
Radar detectors have not become completely obsolete, but their effectiveness against modern radar units can be questionable. Some police departments still use the older X and K band radar units for speed detection, which means a good detecting device should provide enough warning to the driver. When dealing with modern Ka and laser "instant on" radar units, however, a speedy driver may just want to modify his behavior whenever traffic conditions suggest caution and good judgment.