Stopping Distances

Investigate the effect of different speeds on stopping distances, dynamically - watch the cars slow down and investigate how speed changes over distance (rather than simply over time). Can be used alongside
If you could stop just in time to avoid a crash when travelling at 20mph, how fast would you be going at impact travelling at 30mph? How does this change for 70mph / 80mph, etc? Why is this? How much of your speed do you lose during the first 3/4 of the distance?