Reading through this thread it seems apparent that any crash in any formula is multi-factorial. If this happened this way and this contacted this, this would happen. However one area that has not been suggested yet, which I think is quite important is the safety around the track. By this, I do not mean the walls or the track layout, but the medical doctors, paramedics etc.
JRS and others, I think I am qualified in talking about this subject as I have been a motorsport doctor for many years and indeed even a F1 doctor at Silverstone for a couple.
When we have a F1 event, and I am led to believe the FIA impose stringent rules Worldwide, we have multiple (at least 3 medical cars), on average 20 medically trained doctors (all of whom specialise in Emergency trauma - so Anaesthetists/Emergency Docs) stationed around the track, separate extrication teams (3 in total) and a huge number of medical doctors at the Medical centre. All for the drivers, not for the public!
Now I have no idea what the medical backup situation was at the Indycar, but the response time for emergency responders seemed lacklustre and there didn't seem to be many around. I definitely am not suggesting that having more doctors would have made any difference to the outcome of Dan, but actually from where I am sitting it is a little difficult to even compare F1 to Indycar racing. For all the people saying the Indycar chassis from 2003 is as safe as a current F1 car I may suggest that this is far from the truth. The amount of money, R&D that has gone into protecting drivers in F1 is massive and it shows in crashes like Webber's & Kubica's.
Accidents will always happen, some will be horrific, some will be tragic, all we can do is try and make the odds for the drivers better. If you want an insight into what F1 has achieved in terms of safety may I suggest you invest in Dr Gary Hartstein's book 'Medicine in Motor Sport.' He happens to be a FIA doctor and FIA delegate that goes to all the races.