Why do you start slowing down for a red light that is about half a mile out? Are you afraid that your Prius would not brake on time? Are you see that your gas mileage is 100 when you step off the gas and you know that you need to stop anyway for the next light, why not get the 100mpg for the half mile?
Are you aware of how much it annoys the driver behind you? Have you ever given a thought about their brake pads? What if they expect you not to slow down before such distance from their next red light? How much of a road rage it can cause?
I really don't know about others, but it frustrates me quite a bit to be behind a Prius driver who drives like a grandma (no offence to grandmas) when they can do much better. It makes me wonder what goes on in your mind.