A method for notifying traffic of an approaching emergency vehicle is described. A series of alert units are mounted along a roadway so as to be visible to traffic. A signal is emitted from the emergency vehicle, which is detected by a receiver along the roadway. The receiver causes a light to flash and a relay signal to be sent to the next alert unit along the roadway. Accordingly, an alert signal is propagated along the roadway ahead of the emergency vehicle, alerting traffic.