Let's Make Robots!

Car maintaining same speeds as the one ahead

I have previously posted in the beginners section but I feel this is more advanced than I had first anticipated.
Previously I was trying to code something that would react to a change in distance... but I've found a similar problem which would require the same bit of code however is much easier to explain and think about.

Say you decide to put a picaxe in your car so that when you're on the motorway you can switch a button and then the picaxe will accelerate/decelerate/hold-contstant-speed dependant on the car in front.  

React depending on speed?

I have an srf005 ultrasonic range sensor attached to a picaxe 28x1.

When you get too close to the sensor an arm rises, and when you are so far away the arm then lowers.

I want the arm to rise/lower at different speeds depending on how fast your approach it.

Is there anyway to guage speed using the picaxe?

Kind regards,
Luke