The only thing that slows drivers, at least that I have noticed, is tar and gravel, or wet white paint, I.e. things that mess up the cars paint job. Other than that, I might try something that creates a whooping cloud of dust to settle on the car like really dry dirt or a mud hole that splashes up
OP, the assessor's maps where I came from were retracings of the original Subdivision or Parcel Maps. There was always a disclaimer somewhere about the assessor's parcel maps not being warranted to be accurate. If your lot is old there may not be a better map than the assessor's maps and the road may not be called out as to whether it's a public easement or not.
Counties often won't maintain public easements that don't meet their minimum standards.
Our road was private and didn't show up at all on the assessor's maps, but was called out in great detail on the Parcel map for the subdivision. It wasn't called out in the legal description of the lot either.
Oddly enough we had some sort of agreement with the county to maintain the road (ie pave it ) that had been signed very soon after the place was subdivided in the '50's. (you better believe they don't do that any more).
Now, as far as slowing them down. Gates are a nuisance. If they can't get through they are liable to sit there and honk their horn to get the attention of the party further down to let them in, or they'll park in the middle of the road, leave the car, climb the gate etc..
As close as you are to the road any extra dust on the road will get into your house sooner or later. I hate dust myself.
A small water filled moat (ditch) will surprise the H out of them the first time and slow them down the next time, but that first time you could be waiting for the tow truck to show up to clean up the mess too. We had a creek crossing and if you can believe it more than once we had lost persons jam on the brakes - which would have been fine only they were leading a convoy of other lost persons and got rear-ended right there in my front yard.
We had good relations pretty much with most of the residents back behind us, but every year there was something.
I will tell you that after living there for over ten years the very first thing on my list was NO EASEMENTS through my place when we looked to buy here.