i am doing an animation with a few cars moving in the far background.
since they all move on a straight line i just have two keyframes for each car (start and endpoint). but it looks unrealistic, since every car maintains a perfectly static speed.
how can i make the cars movement speed up or slow down over time between the keyframes?
You could precompose each car layer and enable time remapping. This way you can set up keyframes in between the in and out points and move them to create a variation in speed. You will have to do this in a different way for each layer to get different movement for each car.