I've viewed collision or accidents as situations where the collective 'carelessness/risk' between all parties involved has exceeded an unknown threshold. Whether it was 1 person that contributed all of the 'carelessness/risk' or whether it was a culmination of the separate parties' activities.
And, if one party is diligent or careful enough, they may even compensate for some of the 'carelessness/risk' of the other party and be able to avoid the situation.
Imagine a person with a blind fold skating in a rink in a highly aggressive manner. If he meets another that is slightly aloof, their collective risk levels may culminate in an event. Then imagine a situation where everyone else is hyper-aware and vigilant of his carelessness and they do everything within their powers to successfully avoid him. Those are 2 extremes that illustrate the point of collective risk with quite a spectrum in between.