The Liberty Police Department has nine, marked Ford Police Interceptor SUVs. You may not just see them coming, but feel them, too. The SUV patrol vehicles are equipped with the Rumbler - an intersection traffic clearing system that provides a secondary, low frequency tone that you can hear and feel.
The vehicles still have the siren you're used to hearing, but the Rumbler is activated when an emergency vehicle approaches an intersection. The sound lasts for about 8 seconds and is designed to be detected over other sounds drivers may be hearing, like the radio or a conversation, and also penetrates the soundproofing many late-model cars have. The Rumbler increases motorists' and emergency responders' safety.
The department replaces vehicles in its patrol fleet on a rotating 3 to 4-year cycle. Once retired from patrol, many of the vehicles remain in service and are are used by other divisions for several years.