I'm with PolarBear. None of us here are allowed to use red emergency lights unless it's an actual emergency vehicle (even the volunteer FFs), but we all have amber lights and get a kind of "free pass" to run with them given a situation that would need it. (BTW, I have them on my vehicle because I am one of two dispatchers that are required to respond, and quickly, during high risk and large emergencies.)
If I'm coming on or immediately off work all I have is a .45 in my car. If I'm off duty a .45's in the car and a 9mm on me. (Getting the .223 for my trunk in a few days, too.) Emergency lights come on once I notice something out of place. If it's nothing then no harm-no foul, lights come off and I'm on my way. If I actually see something going on (e.g. BGs hitting the LEO) then the cellphone and gun come out at the same time.
I will be dialing 9-1-1 in my weak hand while I'm issuing commands at gunpoint with my strong hand. The dispatcher will quickly surmise that something major is going on just by listening to what I'm saying. If/when I get the chance to say something back to the dispatcher (who should stay on the line, btw) I'm telling them my location, that an officer on a traffic stop there is down/needs immediate assistance, to send an ambulance and hold it off until clear, and that I'm armed plus any suspect information (including if they're still there or in the area).
Clear the area around the officer, check the officer for consciousness/weapon, render first aid if necessary/applicable and give an update to the dispatcher on those results. Once I start hearing sirens I'm giving the dispatcher my description and advising that I'm not putting my weapon down until there are other officers there to assure his safety.