A life size cutout of a cop car.
A hutch of bunnies.
A life size cutout of a hutch of bunnies on a real cop car.
Even better, you want a Ho-Ho - it’s like a Ha-Ha, only deeper 
Si
Was this thread recently moved to GQ from elsewhere? I don’t remember seeing it in the two weeks it’s been up, and it really seems more like a survey of suggestions and advice, than any sort of factual point. Can someone clue me in?
And to invoke a real GQ question: Several posters said that placing boulders on one’s property would be illegal. I can’t imagine why. In what way would a large rock be more illegal than a rock garden or a fence or gnomes?
I knew there was a similar story not too long ago. Just found it.
It depends on where you put it. For safety, there are limits on what you can do within a certain distance of a public road.
In the thread Loach linked to, the corner of the house is incredibly close to the road. There’s nowhere to put a boulder that isn’t in the road’s right-of-way.
In the OP’s case, he could (very likely) place a boulder or three far enough back from the road to be legal, yet still prevent cars from driving over his lawn.
I did. A bougainvillea
They don’t even have a yard. The street is only a few feet away from the front of their house. I’m surprised it was legal to even build a house that close to the street. Anything they put in front of their house would be in the ditch. That isn’t the case with this thread.
IIRC (and without going back to re-read it), the city (or county) reconfigured that part of the road, moving it closer to the already-existing house.
True, it’s a much more extreme case. But the fact remains, placing an object to damage a vehicle and possibly hurt someone is not a good idea. If not expressly illegal in your jurisdiction it would certainly open you up to civil liability. Even in your own property. See the cases in which the homeowner was sued and/or charged with booby trapping burglars. Which is why I suggested the reflective posts you can get at Home Depot for $2.
You know, a string of reflectors along the fence line actually strikes me as reasonable. They are high visibility, so cars can’t easily miss them. They are lightweight, so if someone does hit them, they don’t cause a safety hazard. People will not intentionally run them over, because they will damage the car enough to be an annoyance. Paint scratch! OMG! And they are relatively cheap to replace.
When will the OP let us know what he decided on & the results?
Bags of mulch are a wonderful stopgap solution 
.
Results man! Has anyone tried to drive through the mulch?
It depends on how thick the mulch is. A mound of mulch that is 1 foot high is easy. Three feet still wouldn’t stop a big truck.
Okay, this thread is ancient and we never did find out what happened to the original poster.
If you’re still out there, IceQube, we’d like to know what you ended up doing.
Meanwhile, I stumbled across this info from 2 months ago on YouTube which could be relevant, at least to people in similar situations as the OP. It’s a report about a decision handed down by SCOTUS. "Fortified" Mailbox Owner NOT Liable for Driver's Injuries - YouTube
Enjoy!
–G!
The case has been much discussed on SDMB. The decision came not from SCOTUS, but from SCOTSOO (Supreme Court of the State of Ohio).
And thats an example saying you can’t even put a boulder there, you can’t install a garden, you can’t install a fence or a gutter or a golf sandtrap/bunker… Its got to be kept as safe lawn.
Thats because thats the public right of way , a road easement , to create the road there. In some parts the private land title goes to the roads centre ,or to the roads edge, but essentially thats pointless to the landowner, unless the road was to be disused, but even then the public right of way would likely remain, its just its not a road … its a reserve.
Anyway, it seems while you have that land on the title, and you maintain the lawn, there’s a public right of way declared on it… its up to the county to control traffic on it. You have to keep it as lawn and not assault the users of that lawn or even modify it from lawn in anyway. The public should also just move along it and not damage the lawn. There’s consideration for accidents or … misunderstands, given its just lawn.
One possibility is a lightweight fence , tomato stakes and string, not likely to injure anyway, with night visible signs, saying glow in the dark paint on cardboard, saying “keep off the lawn”. Because anyone could just ignore it, or move it, if they need to, its not creating the danger of restricting a firetruck or blocking the county/utility equipment from accessing that land…