I hope the following applet is what you want.
There are two cars in this simulation.
Those two cars have the same initial conditions.
When radio button labeled "all" is checked: (default state when "initialize" button is clicked).
You can adjust initial velocity, reaction time and friction coefficient for both cars with sliders.
If you radio button "2" is checked:
The sliders only change values for car number 2.
Both cars will move from left to right after you click "play" button.
The simulation assume at a random time (selected by random number), both driver saw something bad happened
and s/he will stars to hit the brake and stop the car (However, the reaction time and stopping are controlled by adjustable slider(reaction time/friction coef.) before you click play button.
The V-t diagram will save the last three sets of v-t traces so different conditions can be compares.
Let me know if you need more features!