How do I stop my sims from doing this whacky behavior?

graceymanors​  What do you want your sims to do: drive all the way to the subway, not use the subway at all, only use the subway at other times...?  Can they actually drive to the station where they enter the subway?  If you delete this station (and you don't need to save your progress; this is just a test), do the sims drive all the way to where you want them to go, or do they still run down the hill?

The problem may be how the game is calculating when sims should use the subway, which is fixable to some extent, but I'd rather be sure what you're asking before making any suggestions to that effect.

So for my household sims, I want them to use their car all the way, and not use the subway at all.. Deleting the subway worked with the hill area. However, they drive to another part of town, get out and hop in a subway down there. 

graceymanors​  Play with this number in Retuner:

https://www.nraas.net/Which-Mod-FAQ#I-would-like-my-sims-to-take-the-subway-more-less-often-522

until you get the effect that you want.

Note that some Retuner settings don't take effect until you've quit and reloaded, so you may want to save just before the sims are going to take one of these trips where you don't want them to use the subway, then keep trying progressively lower numbers (less than and further away from zero) until they stop using the subway on this trip.  Or you could just pick a very negative number and bring it back up if and when they refuse to use a subway when it's the option you'd want.

My sims never use the subway unless they are in walking distance from the subway rabbit hole in recent gameplay...they always prefer the car if it's in their inventor.

They own the vehicle and make it a designated vehicle for traveling in game.

This is good to know though.

In my games I decided the solution was to eliminate subways. But my sims still find ways to do similar things without the assistance of a subway.