[RMMV] HOW TO DO A CAR CHASE WITH DODGING

I've been fussing with this for far longer than I'd like.

-The map is, let's say 5 wide and 200 tall.
-The player (a car, jet, carriage, horse, Usain Bolt, whatever) starts at the top of the long map and automatically moves to the bottom of the map. You can't stop them from moving towards the bottom of the map.
-Enemies come from the top of the map and shoot at you (bullets, arrows, missiles, Usain Bolt with a machete, whatever) as well as obstacles appearing in front of you. You need to move left or right to dodge them. Remember, you never stop automatically moving down.

So what's killing me here is how to give the player the control to move left and right but forcing them to constantly go forward. I'll obviously need 8-directional movement (essentially 3-directional since moving left or right would likely need to be achieved by moving left-down and right-down) but I'm having trouble forcing one direction of movement while giving freedom for another simultaneously.

Any ideas?

Then what stops the player from going wherever they want and how do you get to the finish line without using an actual countdown clock?

Well crap Sated that's pretty much exactly spot on to what I wanted. Awesome! I'll check out what those option names got changed to for MV as soon as I can.

I'm kind of okay with input lag, or rather I was expecting that to be an issue anyway. LOL I even have a BS story-related explanation all ready to go:).

Thanks a lot for the help.