5 accidents on one street over a year period is hardly an epidemic. Amazingly enough there are around 290 thousand accidents per year in Michigan alone. The vast majority are single motor vehicle accidents.
I would certainly run your scenario of a vehicle catch by an attorney or homeowners agent first, especially since the intent of said object is now public.
To answer your question, there isn't an easy answer. As part of my job duties I investigate serious and fatal accidents. There are way to many factors to count to figure what will work. Because the vehicle is moving it will always want to "glance" off and continue unless it hits at a 90 degree impact. Because an object at rest wants to stay that way it won't take as much weight as you may think to completely stop a vehicle. Any object big enough to reliably keep a vehicle from deflecting off will be plenty heavy.
You don't mention the speeds involved and that is the primary factor to consider. When speed increases the stopping distance increases at a much greater rate. So stopping an object, in this case a vehicle is largely weighted towards speed.
Joe
JC Custom WoodWorks
For best results, try not to do anything stupid.
"So this is how liberty dies...with thunderous applause." - Padmé Amidala "Star Wars III: The Revenge of the Sith"