When upgrading signal bulbs from incandescent bulbs to LUMENS HPL LED bulbs, regular signal flashers that come with the vehicle, will 'hyperflash' (flashes very quickly) because of the change in resistance from a standard bulb to LEDs. LUMENS HPL LED Flasher relays are used in conjunction with LUMENS HPL LED bulbs to slow the signalling speed back to normal. If the vehicle has a signal relay, using these relays would be more cost effective than installing resistors to maintain the flashing signal speed. On top of that, it's much easier to switch a relay than to solder in multiple resistors into the vehicle. Please note that some vehicle may have multiple signal relays (ie. one for driver side, one for passenger side, one for hazard lights, etc).