carAccident.evt

<aura:event type="COMPONENT" description="Event template" >
    <aura:attribute name="msg" type="String" access="GLOBAL"/>
</aura:event>

CarAccidentHandler.cmp

<aura:component >
    <aura:attribute name="msgFromNotifier" type="String"/>
    <!-- give same name as that of notifier registerEvent name attribute -->
    <aura:handler name="newCarAccident" event="c:carAccident" action="{!c.handleNotification}"/>
    <!-- Handler should contains notifier component-->
    <c:carAccidentNotifier />
    <div>message from Notifier : <b>{!v.msgFromNotifier}</b></div>
</aura:component>

CarAccidentHandlerController.js

({
	handleNotification : function(component, event, helper){
        var sentMessage= event.getParam("msg");
        component.set("v.msgFromNotifier", sentMessage);
    }
})

carAccidentNotifier.cmp

<aura:component >
    <aura:registerEvent name="newCarAccident" type="c:carAccident"/>
    <h1>Car Accident Example: Fire Event by Clicking on Button</h1>
    <ui:button label="Accident Took Place" press="{!c.fireCarAccidentEvent }"/>
</aura:component>

carAccidentNotifierController.js

({
	fireCarAccidentEvent  : function(component, event, helper) {
		var accidentEvent = component.getEvent("newCarAccident");
        accidentEvent.setParams({"msg":"there is car Accident. Send ambulance."});
        accidentEvent.fire();
    }
})

TestApp.app

<aura:application extends="force:slds"   >
	<c:CarAccidentHandler/>
</aura:application>