Reaction time : the lapse of time between stimulation and the beginning of response. 
You are driving on the high way and listening to the music you like most. 
Suddently, you see the brake light of the car in front of you just turned on. 
You will try to hit the brake and slow down your car. 
But, there is a small time delay before you really do that--- your reaction time. 
During that period of time, your car is still moving at the same HIGH speed! If you do not want something VERY BAD happened, 
What is minimum diatance between front of your car and the rear of the car before yours? 
If both cars need the same distance to fully stopped, the miniuum distance is  ( the velocity of your car )* ( your reaction time )        

    This java applet help you to determined your own reaction time (how much distance to keep between cars when you are on the highway)
  1. Hit the start button: the red car starts to move with initial velocity selected from slider bar
  2. The traffic light will turn from green to yellow, then to red at randomly selected interval
  3. Hit the brake button when you saw the red light turned on.
  4. The red arrow indicate car's position when the red light was on.
  5. The black arrow indicate car's position when you hit the brake
  6. Distance between those two arrow is the minimum distance to keep
  7. Your reaction time, brake time/distance are all shown in the text field.
Drive safely!