On my drive in to work today I saw a couple of people dressed up like crash test dummies on the side of the road holding "Click it or Ticket" signs.

When I was 18 I was in a head on collison with another vehicle. I was maybe going 2-3mph, the other driver was going about 25 - 30mph. I had my seatbelt on and had no injuries. The girl driving the other car wasn't wearing her seatbelt and broke the windshield with her forehead.

So even at city speeds the seatbelt can help out.