Sub: So I don't go through the windshield if we have an accident?
Me: That's right little one. Put on your safety belt.
Sub: Let's discuss it sir. I'd like to come up with another valid way to stop me from going through the windshield.
Me: Why should we do this? What's wrong with my safety belt?
Sub: It's a fine safety belt sir. It's just not how I think we should stop ME from going through the windshield. I understand you place a lot of faith in your safety belt, but there are other ways to keep me in my seat.
Me: All right. I'll bite. How are you improving on the safety belt?
Sub: How about a giant sock that goes over my passenger seat, that cocoons me to the chair?
Me: Sounds like a safety belt. A giant, thick, hot, inconvenient safety belt.
Sub: Yes but it's not a safety belt sir.
Me: That's for sure. OK. Any other bright ideas young lady?
Sub: How about we run inside and collect all the pillows, and stuff them all in front of me? Wouldn't that stop me from going through the windshield?
Me: It will take time and energy to do that kiddo. A whole lot more than putting on your safety belt.
Sub: But you must admit that it's a valid way to make me safer while we drive together?
Me: Sure. It's not as immediately accessible as your safety belt, won't react as quickly as your safety belt, won't keep you as safe as your safety belt, but it's a valid way of making you safer in the car. Now put on your safety belt.
Me: The bottom line here is that we will go nowhere together until you get on board with the safety belt young lady. Your safety is my first priority. Even more of a priority than your company. Careful getting out of the car. Don't let me keep you from coming up with less efficient ways to keep you safe while driving with me. We can talk again when you're ready to strap in and use the time honored, repeatedly proven, currently installed safety belt that exists in my vehicle, while journeying with me.
(I wrote this in response to several people who proposed that safewords were not an essential ingredient to healthy and valid D/s dynamics)