I don’t know the law, but I did see a “severe tire damage” sign at a parking lot today, along with some convincing looking (and presumably retractable) spikes. They were trying to convince people to not enter the wrong direction and thus avoid paying the parking fee.
The tricycle is a far more clever idea, however.
I like the tricycle idea too. Maybe you could rig up a cable on some pulleys that moves it to the side of the driveway automatically when your garage door opens, so you could use the remote control and drive right up and into the garage without getting out to move the trike. 
Is there any reason you can’t park your car in the driveway? I’m assuming you have one. Just pull it out till it’s near the street. Can’t pull in if there’s no room.
Can’t get sued, either.
Make a hole in the blacktop in the middle of the driveway, 2 feet from the curb. Insert a 4 inch section of steel pipe, 2 inches in diameter. Pour cement around it to hold it in place. We shall refer to this as the socket.
Get a 4 1/2 foot length of steel pipe, 1 inch, or 1 1/2 inches in diameter, which we shall call the pole. Spray paint the pole Day-Glo Yellow. Let cement & pole dry.
When both are dry, insert pole into socket.
This will be a highly visible barrier that can block driveway access without being a booby trap. It is also easy for you to remove.
Add a “Private Driveway: No Tresspassing” sign, & all should be well.
The OP is not suggesting that the offending driver be sprayed with bullets when he/she drives on his property, only that his vehicle be disabled. I hardly see how such a scheme, with proper signage, constitutes os boobytrap or mantrap. Some car-disabling mechanism, installed with the intent of protecting the homeowner’s familys from a persistant and potentially lethal threat, seems every bit as legit as a parking lot’s “severe tire damage” spikes, and those seem to be pretty common and (presumably) legal.
Since the police have agreed to monitor. Have them start ticketing people for Improper turn or trespassing. That should be the most effective deterrent.
How far do they pull into “your driveway”? The reason I ask is that here in CA, the part of “your” driveway up to the edge of the side walk is usually considered public “right-of-way”. Thus, yes, they can pull into “your” driveway up to the edge of the sidewalk (by that, I mean the edge closest to your home). Often here in CA, we would have (starting at the street)-1. The curb. 2. a “strip” about 3’ wide with grass & maybe a tree 3. The sidewalk 4. Your lawn/yard. The driveway portion from the curb to your lawn likely doesn’t belong to you (belongs to the City in many cases), and is very likely public right of way- although you are also likely responsible for maintenence. You have no right AT ALL to block any portion of “your” driveway from the curb to your lawn in that case. In fact, here in CA, you can get a ticket for blocking the sidewalk, even if you’re only parking in “your” driveway. Thus, if they pull no further than the edge of the sidewalk- they are legal- although I admit annoying. Anything blocking that area- tricycles, tape, cones, whatever- would be illegal.
Whatever you do- no booby traps. Scumpup & Lynn are very very right.
I think you are wrong (IANAL but I sleep with one). Even if someone is injured on your property while committing a heinous crime, guess who’s potentially liable? That’s right kids, the homeowner who erected the (insert bright idea here).
For og’s sake, consult a knowledgable, expert attorney before doing anything like this to your property!
I like the idea of the tricycle with this slight alteration:
-Keep the tricycle hidden behind some bushes
-Place a small, lifelike, child-sized dummy in the tricycles seat.
-Have a length of strong fishingline set up to put the tricycle into the path of a car that crosses into your driveway.
About the chain thing:
One time I put my car in a parking lot and was not able to retrieve it before they closed the lot for the night by placing a chain across the exit. Well…it being 2 am, I did not feel like waiting around for someone to remove it so I basically drove through it, snapping it like the ribbon at the end of a 50 yard dash. Can I sue for the scratches on my POS car?
Let’s look at some scenarios: 1. A neighbors kid rides across your driveway and blows his bike tire, falls down and hurts himself. The kids parents can sue, and can collect.
The UPS truck with a delivery for your house pulls into your driveway, and blows a tire. They can sue and collect.
Police or fire pulls into your driveway for an emergency- ditto. I’ll bet you could be criminally charged.
Go back to scenario #1. If the kid falls and hits his head into one of those caltrops, and dies- does the term “manslaughter” mean anything to you?
To answer your question, Dr. Deth, We are actually in CA also (not far from you). And I do realize that they are legally allowed to pull in the driveway to the top edge of the sidewalk, but some people put their entire car into the driveway, pulling up almost to the garage door. This is where I have a problem.
p.s. In case there is any doubt, this is pencil pushers wife–the vindictive one
Try my suggestion, P.P., or P.P.'s Wife, whichever is applicable.
Easy to do, cheap, & effective.
In addition to the other issues already noted, it is worth pointing out that caltrops, being small and portable, could easily be removed from your driveway and used for malicious purposes elsewhere. That being said, the factual answer to the second part of your question is: e-mail me, and I’ll let you know where to buy them.
PP, if they pull that far in, then it is definately a matter for the police- they are trespassing. A sign would be good.