You're driving home. You're behind a motorcyclist doing the speed limit. You want to go a bit faster. The highway's a bit busy, so you can't over take. You start tailgating. A hidden on-ramp hides an approaching truck about to join in front of the motorcyclist. The truck pulls out and the motorcyclist starts to slow down. You're a bit too close so have to hit the brakes hard. You skid. Nothing else happens.
That's not a near miss, it's a lucky survival.
You're driving home. You're behind a motorcyclist doing the speed limit. You want to go a bit faster. The street is a bit busy and there's only one lane. There's some space ahead where there are no parked cars. You get closer to the motorcyclist to make it easier to overtake on the left. The motorcyclists chain snaps and jams his back wheel. The wheel stops immediately and the motorcycle starts slowing down quickly. You're mind isn't prepared - you're still thinking about overtaking. You hit the back of the motorcycle at 60. The bike's doing 30. The motorcyclist comes off. He dies.
That's not an accident, it's manslaughter.