1 minute is 60 seconds, so 4 minutes is 4 * 60 = <<4*60=240>>240 seconds.
During this time there would be 240 / 20 = <<240/20=12>>12 big crashes.
And also 240 / 10 = <<240/10=24>>24 car collisions.
So in total during 4 minutes there would be 12 + 24 = <<12+24=36>>36 accidents.
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