First and foremost, its a clearly visible deterrent. If the readers here cannot wrap their minds around that and understand what a deterrent means, I just can't help them. This deterrent would stop 99% of the trespassers.
Second, who would continue to drive over it to blow out all 4 tires? Makes no sense. Even if they did, then they would have to get a tow. Tow trucks operate everywhere. Shame on the 1% if they are too stupid and drive over spikes. Serves them right to wait and pay for a tow. Even the stupidest would then learn their lesson and not go back again. Pretty simple.