To simulate motion of an object with constant speed or constant acceleration, two initial conditions are needed: i.e. position and velocity.
There are four vehicles, so distance between vehicles and velocitry of each vehicles are important parameters.
The [url=http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=137.0]reaction time[/url] for each driver also affect the timing the brake beening hit, along with the above information can be used to determined when the car are fully stopped or when the car collide with another one.
If the frition coefficient is \mu, the acceleration a=-g \mu, and V(t)=V(t=0)+a t=V(t=0)- g \mu t.

The impact force can be calculated if the time interval between collision happened and the car fully stopped is available. F=\tfrac{\Delta P}{\Delta t}.

Please check out [url=http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=138.0]Reaction time and car accident[/url] (may be you can find out the information you need.). 
[url=http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=365.0]Ejs version of traffic control simulation[/url]