How would I make a hair particle travel across a surface? I am trying to create a scene where several vehicles travel across an uneven surface. To get the effect I'm looking for I put the vehicles into a hair particle system, however I dont know how I would get them to travel across the surface in a decent manner without having to animate each one individually.



Pretty much all I want is for the vehicles to travel in a straight line forwards over the course of a few seconds.

Is there any way to make the particles travel across the surface in a decent manner?

EDIT: After following the advice of Lemon I have something similar to what I was hoping for, however there are still a few small issues I need to resolve before its finished. 


Sometimes the Vehicles will stop then disappear or disappear into the terrain. 


Or float above it when the landscape dips downwards, as well many of the different particles spawn inside of eachother.

In addition i'm wondering if there is a way to have one of the vehicles to be less common while being in the same particle system or whether it would need to be in its own.

I'm unsure as to how to solve these issues, and am hoping there is a way to without having to have each vehicle be its own mesh.
In this approach :

The field in green does not emit particles but has a collision setting :



The emitter is a band along the field side :



The particles are driven straight forward by a curve set as curve guide field :



As we want the vehicles to stay above the terrain, their center is set at the bottom part of them :