What I noticed about the front collision warning is that it's not just a static distance. It's more of a dynamic time-to-impact metric that varies with your speed and the speed of the vehicle in front. It's looking at the distance to the vehicle in front and calculating dynamically the time to impact and giving you the alert if it calculates that you may not have enough time to stop. So at lower speeds you can follow closer, but the faster you go, the more space it wants you to leave. Which ain't a bad thing, really.