I'm interested in designing public spaces. I want to do it, to show how many benches are poorly placed in public parks.
Here is a great movie about it:
http://vimeo.com/6821934@James C4S
I know i can count people, or pay someone but it's a lot of benches and it will take a lot of time/money to do it right :~
@Robin2
I could place pressure pad on the bench, put i think no one would sit on it. I live in Poland so in summers we have quite hot, checking temparature may be unreliable.
I want to suggest moving benches in better places, not just removing

@Grumpy_Mike
Video processing sound like it will be complicated an not scalable (many benches)
@radman
In Poland we are not so strict about placing things in public spaces

What is sensitivity of a piezo sensor, will it detect vibraton if i will stick it to a bench like this ->
http://ks366875.kimsufi.com/p3/201006/11/67186335.jpg ?