MY RECOMMENDATION: I'm currently using the feature, and I'm the only one in my association. DO NOT START WITH BIG; UNLESS YOU HAVE THE EQUIPMENT AND MANPOWER TO DO THIS, as I currently have 70 [Event] Calls. I'm currently running calls with 8 fire stations, 4 EMS stations, 2 Police Stations, and 3 Hospitals. The calls currently are in an area consisting of 2 states, and 7 different counties because I decided to try it on Big. (I live close to a state line, with 3 rivers and islands.) It is a great feature and i'm enjoying all the calls. Especially with so many requiring use of each type of station.
You can run an alliance event every 7 days for free.
If you want to start it sooner then 7 days then it's 20 coins.
You have options for Small, Medium, Big; which is the size of the area you want the calls to be in (dictated by a shaded area on your map window). You can also move the map to fit the area you want to respond in better.
Then you can set it to have a new call every 30, 45, or 60 seconds.
The total "Storm" run time is 2-3 hours, I can't remember the exact total time.
You will need Fire, EMS, and Police Stations for the scenarios.
It is a great feature. I think we should keep it.
On a response side note: If you have to cross a body of water (river, lake, ocean) with a ferry, the game treats it just like it is a road.