particle system that kicks up dust behind a tire

I am creating a vehicle that is driving on a dusty surface and I would like it to kick up a little dust when it is moving but it stops when vehicle is stationary. I have the emmiter all done. I just need to script it on and off when a direction arrow is pressed.

There is a ParticleEmitter.enabled variable that does exactly what you want. just put in your update script something along the lines of

var pe = GameObject.GetComponent("ParticleEmitter");
if(velocity > 1){
    pe.enabled = true;
}else{
    pe.enabled = false;
}

I probably got some of the syntax wrong, but hopefully you get the idea. See the script reference for the particle emitter: http://unity3d.com/support/documentation/ScriptReference/ParticleEmitter.html

Thats awesome. Thank you very much.

You might also want to use minEmission and maxEmission to scale the number of particles with the velocity of you vehicle.