You would be surprised at how something so simple can go such a long way. If trimming trees around roller coasters could very well prevent these horrible accidents, then I would go out there and do the job, myself. I am all about helping saving lives and making things more positive.
A tree branch on the tracks caused the accident, so it's obvious that if the trees were maintained properly, that wouldn't have happened. The environment around the roller coaster is part of keeping it safe - even if the tracks themselves are perfect, you have to monitor for things like this. The trees should have been trimmed back.
This seems like a no brainier, but anytime there is a potentially dangerous machinery anything that could have an effect on said machinery should have some sort of preventative maintenance guidelines laid out for it. In this case, trees should be trimmed so they couldn't possibly interfere with the coaster.
While there is an inherent risk in riding a roller coaster, Six Flags could have taken more precautions for riders by staying up to date with their grounds maintenance. By cutting the branches so that they did not interfere with the ride, the accident could have been prevented, and the injuries would not be affecting riders.