To me the problem isn't people just bringing their phones on and leaving them in their pockets. Most of the time when there is an incident, it is because someone took their phone out to either get an unauthorized POV or to take a picture or something. And having dropboxes for phones wouldn't prevent this, since people want the footage. They would need to enact a policy of "all persons seen with a mobile device out on the ride will be removed from the park" to really discourage it.
Not only that, the people who are conscious that their phone needs to be secured already place it in a bag or in a zipped up pocket if available, with a slim percentage of those conscious saying "I think my pocket will be okay". It's usually the people oblivious to the potential danger of bringing something on the ride in your pocket. The same morons who would wear a hat or glasses onto a ride. There is nothing you can do about those people.
The reality is in a couple months this won't be a problem, people are just taking their phones out since it is a new ride and coaster enthusiasts are creaming over a local RMC. Once the hype is gone it'll be no big deal. The park should have been more aware that this was going to be an issue and prepared accordingly, with more signage or putting up netting in places where guests below could get struck. Not saying it is the park's fault, but it is like defensive driving, try to prevent the problem just so you don't have to deal with it.
