With today's low-cost radar-receiving technology, many drivers who commute regularly carry some kind of speed detection to alert them to traffic enforcement. There are two kinds of detection: multi-band active systems and passive systems. Additionally there are a number of active combination detector/jammer systems on the market. Here is what they do, and whether these systems are legal in Illinois.
Active detectors scan for emitted wavelengths normally associated with mobile police radar guns. If your detector picks up one of these wavelengths the dash-mounted unit announces the fact by either a blinking light, an audible tone or both. If your unit is sensitive enough, you may be able to slow down fast enough to avoid a ticket, but typically if your car is "seen" in radar terms, you may get stopped. Active detection is legal for in Illinois personally owned vehicles, but illegal on any commercial vehicle exhibiting a gross weight above 5 tons.
Passive detectors do not scan, but simply wait until the mobile radar emission is picked up by your unit. Again, these devices usually announce that an officer is looking at your car, and if you are react quickly in slowing down, you might be able to fool the gun. However, more times than not, you're going to be signing for a speeding ticket shortly. In Illinois, passive devices are legal on both personal and commercial vehicles, primarily because they don't do a very good job of across-the-board avoidance.
Active detector/jammers pick up the pace in terms of active mitigation of radar signals. These systems scan for wavelengths normally associated with mobile police radar guns, then when the device picks up a likely wavelength, it jams the sending signal electromagnetically. These devices are generally illegal although there a few states still allow them. This is not the case in Illinois, however; jamming any kind of active law enforcement radar is illegal, regardless of the type and size of the vehicle.
