Indy cars are much more dangerous as compared to formula 1 cars.
Keeping safety in mind the engines of F1 cars were changed from V12 to V8's meaning these cars generated less BHP as compared to previous hence less straight line speed. Also there is a rev limiter in F1 cars which prevents the drivers from blowing up their engines.
As for Indy cars pushed onto the track by the centrifugal force of the banking, the cars complete each lap at 360km/h, faster than Formula One cars reach on the longest straights.
While most Formula One overtaking moves are made under braking from high-speed straights into slower corners, in Indy Car overtaking is almost continuous. Racing flat-out in top gear, the drivers use the slipstream of the cars around then to gain an advantage.
It means racing in very close proximity. In Las Vegas, the majority of the field were racing in a pack three-abreast just centimeters apart. There are no run-off areas either. A banked oval track is surrounded by a deformable barrier, a concrete wall and a debris fence, designed more to protect the spectators outside the track, rather than the drivers on it.
That is the reason that there have been no major accident accidents in F1 in recent times, however for American motorsports it is a different story.