How does Roller Coaster Tycoon come up with ride ratings?
I never experimented enough to come up with anything approximating an exact formula, but this was the impression I got:
- Excitement is determined by the variation in the track. A simple circle will get you a low score, whereas a track with twists, drops, and loops will get you a high score.
- Intensity is determined by the speed / force (see Raven's comment). If you start out high with a drop and add boosters, etc., you'll get a higher score here.
- Nausea is similar to Excitement, I think it has to do with a large amount of variation in a short space. A booster right before a series of tight turns will probably get you a higher Nausea score, and spacing it out lower. If the total variation is the same for both versions, I would expect the Excitement score to be the same.
Building a giant coaster high into the air and adding tons of drops, loops, and so on usually only adds intensity and cost to the ride. If you find yourself doing that hoping to get more excitement, it's better instead to focus on the specific elements enumerated below.
No-one has published the exact algorithm RCT uses; these notes are from my play, testing, and tweaking. Most of these rules are for coasters but many apply to other tracked rides (such as water rides or go-karts) and some apply to fixed rides (which often having fixed intensity and nausea ratings).
Excitement
To me, excitement is the fun bit of ride design to aim for!
All rides start with a base excitement. For non-tracked rides from the gentle Merry-Go-Round to the high-intensity Inverted Swinging Ship, the excitement value can't be changed much from the base; pretty much only changing with tweaks to ride parameters (such as number of spins for the carousel) and scenery. All rides gain additional excitement from the addition of nearby scenery, while tracked rides also gain with track features, tunnels, water, music, pathways, and interaction with other rides.
A short, flat coaster will have an excitement very close to zero. Adding a handful of drops, especially a big one, will get you up to 2.5 or so. Once you have all of the basic requirements -- several drops, at least one of them steep, and at least one point where the coaster crosses over itself -- the excitement will jump over 5. From that point, more drops (and the deeper the better) and cross-overs will continue adding excitement although with diminishing returns. Drops and crossovers alone can push a coaster's excitement over 6, although with wooden coasters it's tough to get much over 6 and especially without pushing intensity. Different coaster types have more interesting elements, such as the wooden coaster's water splash, but except for inversions most of them are equivalent to simple drops and, if you're maxed out on drops, won't add any excitement at all.
Adding tunnels can add up to 2 points of excitement; each underground section and the amount of turns and elevation changes that are done underground will increase contribution. I've found that a moderate lift hill that dives steeply into a tunnel is an inexpensive and effective way of adding a lot of excitement. Like with drops, the second tunnel section -- no matter how trivial -- will add another chunk of excitement. Further tunnels and more complex underground acrobatics add less and less to excitement, eventually adding only intensity and nausea.
Going over water -- even briefly -- can be an easy way to add a half-point, and the contribution of water will max out at around 1 point after a long (not necessarily contiguous) stretch of track is placed over water.
There's a limit to scenery's contribution, but it's hard to achieve. Variety and visual appeal of scenery matters to humans; the game only cares about the number of scenic elements near the ride, and that includes trees and shrubs that were present when you loaded the scenario. Paving every open space in your park with the same quarter-tile shrub is just as "exciting" as the most beautiful and grandiose multi-tiered scenic construction. For fixed-size rides, only scenery within 5 blocks of the ride's "center" contribute to that ride's excitement. Generally, four quarter-tile scenic elements will add more excitement than a single full-tile element -- but be careful with gardens, as they need to be watered in order to continue contributing excitement. Also note that the game only periodically updates the contribution from scenery; it might take 30 seconds from when you made a scenery change to when the ride updates its excitement.
Turning on music will add a small bump to excitement, although its effectiveness (in my experience) seems to be muddled with other scenic elements. I've seen some say that it adds 0.3 excitement but I haven't played with it much. Damn those noisy merry-go-rounds!
Interaction with other rides can add up to 3 points of excitement. This generally means having two coasters crossing over each others' tracks. The type of ride and the degree of interaction matters a lot, with interlocking loops being the most effective at adding excitement. Stacked turns and intersecting headchoppers work well; parallel track less so. You will need multiple points of interaction as well as multiple different rides to get the full 3 points here. The interaction is easiest to do with tracked rides; I've had a tricky time getting fixed rides (whether thrill or gentle) to add much to my coasters' excitement ratings. Also, for coasters, linked stations add a good chunk to excitement and is (I think) part of the 3 point limit.
Pathways are a bit tricky and are something I'm exploring in my own play right now. Having paths cross the tracks, having tracks cross pathways, and having the ride & the path in parallel will all add excitement. There's maybe 1 point to be had here, but you have to be very diligent about extracting it.
Intensity
In general, intensity is a measure of how extreme (outside of normal) a ride gets. High heights, fast speeds, steeper drops, wide ranges of g-forces, and continuous high-intensity features all add intensity. Flat curves are more intense than banked curves and back-to-back changes in curvature (such as with s-curves) are the most intense. Things like tunnels and going over water will add a bit to intensity, but generally it's the g-forcey stuff that adds the most.
If you're having trouble with too much intensity, slowing down the ride speed reduces g-forces across the board and is the cheapest solution. Otherwise you'll need to modify a lot of track, such as making turns wider, reducing drop heights, changing turns to banked rather than flat, and getting rid of jarring changes in turn direction. Moving track out of tunnels can help lower intensity but comes with a steep terraforming cost. Overlapping and intersecting tracks, especially from different rides, will also add to intensity and can only be 'repaired' through the generally expensive changes to segregate the bits of track.
Nausea
There are a few specific things that add nausea, somewhat like intensity, but the algorithm is different. In general, anything that increases intensity will also increase nausea. Things that add more nausea than intensity are very high g-force values, big changes in lateral g-forces, underground turns, inversions, and persistent intensity.
One non-obvious way to reducing nausea is to add rest breaks from high-intensity bits of the track. Keep your big drop followed by an inversion, but then add an easy banked turn and some low-speed bunny hops before the next bit of high intensity.
Excitement goes up for a while with intensity, but then goes down again. A ride that intersects itself has more excitement. A ride that comes within sight of a path gains excitement, and also draws more visitors, who see it and then decide to come and ride it.
I've used extra scenery to reach the excitement goals in some scenarios, such as putting flowers and shrubs next to and around as many track segments as possible. Scenery near the entrance is especially valuable. You can also built a pre-built ride on flat ground, remove a few flat sections of track, pull the land up, and then rebuild the flat segments for a nice boost to both excitement and intensity.