Highway mode provides full audio and visual warning of laser/radar systems while maintaining full sensitivity. Highway mode is the default setting of all Whistler's laser/radar detectors. City mode helps reduce the annoyance of automatic door openers and burglar alarm alerts by providing an initial alert (same distance as Highway Mode) then remaining quiet unless the signal strength becomes very strong. When the signal increases the unit will alert briefly to notify you of the change in signal strength.
Which do you use? It's really up to you, There is no performance advantage either way. If you have a quiet vehicle then using City mode will be fine. If your vehicle is a convertible or is noisy, then Highway is best so you will not miss any alarms.
NOTE: City Mode(s) does not affect laser reception.