In Mexico, just south of Brownsville / Matamoros: http://www.cnn.com/2008/WORLD/americas/06/02/mexico.bikers.ap/index.html 
That picture is shocking. The fact that anyone survived it, let alone almost everyone is surprising.
Be careful out there, even on a closed course car drivers have a way of sneaking past barriers, over cones and onto courses where they shouldn't be. Just as fast thinking and good reflexes prevented a tragedy when that kid ran in front of the break recently, so we should keep our eyes peeled as much as feasible for unexpected things happening.