Radar detectors typically operate in several levels or modes of sensitivity. You will usually find a choice between one or more City Modes and Highway Mode. Understanding the difference between them is simple, yet choosing the right mode for a given situation can be very important.
Simply put, when you are choosing City or Highway Mode, you are adjusting the sensitivity level of the radar detector. The level of sensitivity affects the type of signals the radar detector will detect and alert you to. City Mode is on the low end of the sensitivity scale, Highway Mode on the high end.
Setting your radar detector to City Mode will generally reduce sensitivity to filter out "false" radar signals generated by devices that typically operate on X-band and are usually detected when driving around town, such as automatic door openers. Some radar detectors such as those made by Beltronics, Cobra and Whistler have multiple City Modes in which the detection of such signals can be reduced by varying degrees or even turned off in an attempt to eliminate falsing altogether.
Highway Mode sets your radar detector to full sensitivity for maximum range of detection while on the road. When in Highway Mode, the detector is set to detect all types of radar that it can detect. While this can increase the number of false alerts in the city, the chances are much greater that those signals detected on the open road are real threats.
When driving around downtown, setting your radar detector to City Mode should keep you alert to potential threats while filtering out false alerts. When driving on the highway or in rural areas, it is best to set your radar detector to Highway Mode.