PURPOSE: To contrive to prevent a rear-end collision accident at the time of a traffic jam on an expressway, by a method wherein when the vehicle speed of a vehicle running on an expressway is lowered to or below a predetermined speed due to a jamming, this condition is discriminated by a calculating means, and a hazard lamp is winked by a controlling circuit.

CONSTITUTION: When it it discriminated by a microcomputer 6 that the vehicle has entered an expressway from a general road and the vehicle speed is lowered, for example, to or below 40km/h, whether or not a winker switch is closed is discriminated within 30sec, and a hazard switch in the controlling circuit 7 is closed only when the winker switch is not closed. By this, when the vehicle is running on the expressway and the vehicle speed is lowered by a jam or the like, the hazard lamps 8 are changed over into a winking condition by the circuit 7. When the vehicle speed reaches or exceeds 40km/h during winking the lamp 8, the microcomputer 6 causes a signal for opening the hazard switch to be supplied to the controlling circuit 7, whereby the winking of the hazard lamps 8 is stopped.