Roller coasters are built to entertain and be enjoyed, however in order to be built they require a lot of capital. It is for this reason they are often built within amusement parks. Amusement parks have a few reasons for building coasters.
It enhances the park portfolio
Draws a specific target market into the park
Keeps consumers entertained and in the park longer
Decreases congestion (to a point)
Having a broad park portfolio is essential to park operations, as it draws in multiple groups of people. Think about the last time you visited a theme park, was there a "kiddy ride" section or group of rides? What about thrill rides? Water rides? Theme parks diversify their rides because it means there is more to see in the park, and more people that could enjoy it. Roller coasters are an essential part of that, as they are the first thing you think of when someone says amusement park.
With those objectives in mind, there are a few key factors or success markers for roller coasters. The first is the safety score, or amount of injuries or hazards on the ride. Safety is always the number one concern for the management, as a safe ride is one that can stay open, even if it isn't the most popular. If a ride has a lot of incidents surrounding it, no matter how popular the ride, management sometimes must shut it down.
An example of this was Dragon Challenge in Universal Studios Florida. While the ride was extremely popular for its first decade or so of operation, it was known for loose articles falling off the ride, which in the nature of the ride became dangerous fast. "Dueling Dragons" as it was originally called, featured two inverted coasters that would ride simultaneously and have "close" calls near one another throughout their tracks. With loose articles of clothing, and some guests having closer calls than others, this ride was eventually changed into alternating coasters. This change, while safer, made the ride much less popular, especially when Universal began to require lockers and a metal detector to avoid more loose items on the ride.
Due to the concerns with safety in the past, and the lack of popularity with previously mentioned decisions (along with theming changes), Dragon Challenge was torn down and replaced with Hagrid's Motorbike Adventure.
Another key factor of success is the throughput. This is how many riders can get on the coaster within an hour (also referred to as the "hourly capacity" by the industry). A high throughput means the queue takes less time, and allows more guests to get on the ride within the day. One main factor in roller coaster design is designing it so that the throughput ratio is high for the ride, especially if it is going to be a main attraction. Keeping in Islands of Adventure in Universal, Velocicoaster, the main attraction of the dinosaur themed section, can load up to 1,745 people an hour (average 1,344). That is a very efficient operation, as each train only holds 24 riders, meaning a train must leave the station (meaning unload and reload) every 56 seconds.
This means if Velocicoaster is running the entire time the park is open, it could theoretically have 20,500 riders in a single day! That is a lot of riders, and keeping queue times low keeps people in the park longer.
While not every coaster could or should have this high of a capacity, it is important to note that when it comes to high ratios, the Velocicoaster isn't even close. Gemini in Cedar Point has a theoretical capacity of 3,300 an hour. Because roller coasters are usually a main selling point for amusement parks, it's important that the ride loads fast enough that those who wish to ride will get the chance.
Hand in hand with this metric is the load time. This is how fast riders are on the ride and ready. This directly impacts the hourly capacity, and depending on the type of ride, may or may not be applicable. If a ride uses constant motion for loading and unloading (the ride never stops motion, guests hop on and off), then the hourly capacity is guaranteed and the load time is not applicable. An example of this type of ride is the Haunted Mansion, which has a capacity of 2,620 riders per hour (the highest capacity in Disneyland!), but no load time, as the buggies are constantly moving at a slow consistent pace for loading and unloading.
While not every coaster could or should have this high of a capacity, it is important to note that when it comes to high ratios, the Velocicoaster isn't even close. Gemini in Cedar Point has a theoretical capacity of 3,300 an hour. Because roller coasters are usually a main selling point for amusement parks, it's important that the ride loads fast enough that those who wish to ride will get the chance.
Hand in hand with this metric is the load time. This is how fast riders are on the ride and ready. This directly impacts the hourly capacity, and depending on the type of ride, may or may not be applicable. If a ride uses constant motion for loading and unloading (the ride never stops motion, guests hop on and off), then the hourly capacity is guaranteed and the load time is not applicable. An example of this type of ride is the Haunted Mansion, which has a capacity of 2,620 riders per hour (the highest capacity in Disneyland!), but no load time, as the buggies are constantly moving at a slow consistent pace for loading and unloading.