RollerCoaster Tycoon was programmed at a time when Personal Computer processing power was severely limited. As such, Guest Pathfinding algorithms were severely limited so as to not unduly slow the game. Unfortunately this mean that players must be very cognizant of path layout in their parks.
Never disobey these caveats: Do NOT make
Because you do not want your Park Exiting guests to loose Happiness (and possibly even become a Vandal), always have a way for guests to be able to exit the park whenever they choose to do so. This means never violating any game pathfinding algorithms that would prevent this from occurring.
The game makes pathfinding calculations at every "decision point". A guest walking on a footpath will be doing only one of two things, they will either be wander around, or they will be heading for a specific attraction (park gate, ride entrance, or bathroom/drink/food stall). Wandering guests will only make decisions at footpath junctions. Guests heading for an attraction will make decisions both on the tile where they first get the idea to head to an attraction, and when they are heading for the attraction and meet a path junction. Note though that what constitutes a junction will differ between wandering and purposeful guests.
Wandering guests don't choose which path to take at a junction in a totally random manner. If the footpath continues forward through the junction with only one diverging footpath, a guest has a 75% chance of heading forward (with a 25% chance of taking the diverging footpath). If the junction has two diverging footpaths (i.e., a "+" junction), a guest has a 67% chance of heading forward (with a 17% chance of taking the right diverging footpath, and a 17% chance of taking the left diverging footpath).
When there is no footpath heading forward through a junction (i.e. walking up the leg of a "T" junction), a wandering guest has a random 50% chance of taking either the right or left diverging footpath. Note that regardless of the junction type, guests will never turn around and walk back the way they came (turning around only occurs at the end of a "dead end" footpath).
Any Ride Entrance (including queue), Ride Exit building, or Shop/Stall, that are abutting any but the last footpath tile, are considered to be forming a junction on the footpath, and regular footpath junction rules apply. Thus, since guest tend to move forwards on paths, an ideal location for a Novelty Shop or a Ride Entrance would be facing oncoming guests (rather than on the side of a footpath).
Junction choice so far has been assuming that none of the path choices are a "dead end". Before making their semi-random junction decision, a wandering guest first checks if any of the footpaths lead to a dead end (i.e., a footpath leg devoid of any junctions). If they do, then the wandering guest will never choose the footpath leg (exception: approximately 3% of the guests may take it anyway...oh well, no big deal). It's still counted as a junction, but essentially the junction now turns into whatever kind of junction it would be (including just a continuing footpath) if you erased the dead end legs.
Staff ignore the dead end exception entirely (just like they ignore No Entry signs). They also tend to wander aimlessly (except for Mechanics answering a call to work), and may even turn around while walking. Guests and Staff not on footpaths/queues will aimlessly choose a direction to move on each
Any dead end footpath with a Ride Exit on the legs last tile is still considered a dead end path. Thus you don't need to worry about adding No Entry signs to this Exit footpath, as mostly only ride exiting guests will use it. If you want its wandering guest use lowered even further, place the Ride Exit path on a side diversion, rather than facing guests strait on. However, do not add any other connections to this footpath (such as a shop/stall or another ride Exit), even it is also placed on the last tile, as its connection point is treated as a junction, and guests will head up the former portion of the ride exit footpath to that new junction point.
On the other hand, any dead end footpath with a Ride Entrance on the legs last tile is not considered a dead end footpath by the game. This can be used to advantage if you need to dynamically lengthen or shorten a queue line (although, depending on the junction, the entrance may not be as effective as one directly abutting a main footpath tile where guests are passing by).
Guests who are drinking or eating ignore the dead end exception, and treat all the footpaths on the junction in the normal
Uninterrupted path length, regardless of how many turns this path makes, is mostly irrelevant to the games calculations. The game instead looks ahead a specific amount of path junctions. In other words, guests at the beginning of two paralleling paths would see a differing amount of tiles forward along his path depending upon how many junctions connected to his path. What constitutes a junction differs for guests purposefully heading to an attraction rather than just wandering. Any Ride Entrance (including queue), Ride Exit building, or Shop/Stall abutting a footpath is ignored, and not considered to be making a junction. What does count as a junction is any tile where the footpath diverges (i.e., left, right, "+", or "T"). Note that Dead End footpaths are not ignored like they are with wandering guests, and always considered, so strategic placements of No Entry signs on some Ride Exit footpaths may sometimes be helpful for guest navigation (the game will ignore any footpath as being a junction leg if it is blocked at the junction point with a No Entry sign).
When a guest gets the idea to head to an attraction, the game searches up to 5 junctions away from the guests current tile to determine a path to the attraction. If a guest has a Park Map, then this search distance extends to 7 junctions away (so yes, Park Maps do make a difference beyond just informing guests about the existence of all the attractions in the park). Any guest who cannot find a valid footpath to the attraction will eventually give up and become a wanderer once again (taking a slight hit to their Happiness stat).
All guests heading for the Park Gate to exit (including those without Park Maps) will search up to 7 junctions away. If the game cannot find the Park Gate by searching 7 junctions, it will then search one more junction distant, before reporting that the guest cannot find the Park Gate to exit. At this point you must manually intervene, as guests will try the reach the Park Gate exit indefinitely.
Mechanics will always search 8 junctions to find a ride they have been radioed to repair. If they cannot reach the ride they will eventually give up, and the radio call will be put out for another Mechanic to respond (frustratingly the same Mechanic could be called once again).
After examining all possible travel pathways, the guest will choose to head for a footpath that leads to a point closest to their destination. If two footpaths at differing heights both get the guest the identical closeness to the attraction, the guest will choose the higher footpath if it is less than eight height marks higher than the lower footpath at the closeness point (a footpath at the closeness point which is higher than seven tiles above another footpath is considered not as close as the lower footpah option).
Every time a navigating guest encounter a junction, they will again check for a footpath that leads closest to their destination. To lessen navigation issues, avoid having many junctions in the wrong direction before a footpath heads in the correct direction. Also avoid building long dead end footpaths, and instead loop such potential dead end footpaths back into the main footpath somewhere.