how to let an object drive on a road?

following situation: i have a road mesh generated by script. i already tried gravity which works so far but my problem is
If the object drives up on a hill, the speed decreases and if it drives down the hill it increases. but i want it to have a constant speed that i can manipulate. i want it to exactly follow the road.
i hope you understand what i mean.

You can disable gravity and set the object’s transform.rotation to be perpendicular to the ground surface normal under it. Then, you can just set the velocity to whatever you want.