Do you think that's enough to stop a truck filled with explosives travelling towards it at high speeds? The goverment seems to think so... don't know why.
You'd also have to spring load them too so they jump up in a second - do you think that people, even if they are speeding or elderly, won't notice something a couple of feet high extending out of the ground in 30 seconds?
You know, there used to be things that you can buy to stop burglars:
Little spikes that you put under your seat... well I say "little" - they're a few feet long. As soon as someone sits on the seat activating the pressure pad on it, the spike punches up through the seat and, well, you get the picture. But, of course, there's a little code box thing that you enter a code into to turn the spike off. Therefor, a car theif who doesn't know the code, or that the spike is there will sit on the seat and BAM! Instant Kebab.