I think andy said it. I've HAD a cable break sitting at a light with side traffic both directions. blind luck and tipping over saved me.
that was 60 years ago. About 10 years ago, my hat blew off, and since it was a good hat went back for it. Rather than shift into neutral, I pulled in clutch and I stopped, bent over to pick up hat, and end came off cable and of course, bars turned, over I went. And yes, my cables are well lubricated. Both times were on Ironheads, not Nortons.
As far as a car coming fast behind me- at what point do you pull into traffic? 30 feet? 40 Feet? more? Any less and it wouldn't matter. With hand over lever and foot on shifter. I contend within that distance the end results good or bad would be the same. Pulling in clutch and shifting takes very little reaction time when poised to do so, and the decision time, would be pretty much equal.