Some newer cars have cruise control with sensors to maintain a set distance behind the vehicle in front of you. I wonder what their operating limits are, both high and low, distance-wise.
Btw, an interesting (ok maybe not terribly interesting) read...
www.visualexpert.com/Resources/reactiontime.html
It mentions that when traveling at 55mph, you're covering 80 ft/second.
They suggest a time of 1.5 seconds to recognize, react to and then begin to brake for a given stimulus. If it takes a person 1.5 seconds to react to the brake lights of the vehicle in front of them, then they've traveled more than 120 ft, which at 40 meters is a good deal further than the 8 to 10 meters mentioned above.
Now, I like to think that I'm faster than that, but even if my reaction time is only half a second, I've still lost 13 meters before I can hit the brake.