Emergency vehicles as defined in subsection C of § 46.2-920 may be equipped with the means to flash their headlights when their warning lights are activated if (i) the headlights are wired to allow either the high beam or low beam to flash, but not both, and (ii) the headlight system includes a switch or device which prevents flashing of headlights when headlights are required to be lighted under § 46.2-1030. The provisions of clause (ii) above shall not apply in the City of Chesapeake, the City of Portsmouth, the City of Poquoson, or the County of York.