How to change a force field's strength by triggers when animating? How I can automatically change force field strength by triggers?

I working on animation where a car (rigid body) moves on the track. I need increase speed on few places (like speed zones in games). Force fields work fine, but the problem is I can't animate force field strength, because I need  to change mass, friction and other car values. So the car doesn't arrive to force fields in the same frames.
I need the force field to turn on (change strength from 0 to XX) when the car hits the trigger IN, and switch off (or change from XX to 0) when car hit the trigger OUT. How can I do this?
How about moving the object that represents the force field? 

When you cant't animate the force field strength, you could move the complete force field.