I have also released an edited version of Marvine's Ladder that prevents sims from going through each other with this mod. UPDATE 2: Fixed an oversight from the previous update.

Otherwise, it was possible for sims to end up stuck and unable to route. Here is the most important new feature: if after using the stairs, multiple sims end up on one single tile (mostly happens if you order them to the top or bottom tile of the stairs when free will is off), then the game will now try to find a suitable spot for the sims nearby and set their location there, just like it does with spiral stairs. When a sim is trying to use the stairs, if an elder sim is already using the stairs, if neither the new sim nor the elder is running, or if both the new sim and the elder are running, then the game will make the new sim use the stairs at elder speed. When a sim is trying to use the stairs, if all the other sims on the stairs are running, then the game will allow the new sim to run, too. I may make a new mod to fix the spiral stairs from Apartment Life in the future, but I think it is not a priority. This mod currently only affects regular stairs. It looks like Maxis copied the code from regular stairs into spiral stairs, and didn't bother adding any checks.

Also, I realized that sims do walk at different speeds on all types of stairs depending on age, not just on regular staircases, which causes non-elders to sometimes go through elders on spiral stairs from Apartment Life. UPDATE 1: This is a very important update that adds new features and checks. Otherwise, sims would run through walking sims on the staircase. If a sim is running, and if there is another sim using the staircase, the new sim will always walk, and will continue running, if need be, after they ascend or descend. This mod makes it so that if the staircase is already being used, then every new sim who starts using the stairs will ascend or descend at elder speed, because otherwise they walk through elders. Elders and adults walk at different speeds on regular staircases, unlike other types of stairs. However, there are a few things that had to be done in order to account for certain special features of regular staircases. Even then, if the sim attempts to ascend or descend while another sim is already using the stairs, the sim throws a route failure tantrum. Maxis attempted to make it so that sims would be able to wait in queue, but this only partially works if the sim is attempting to descend. In an unmodded game, only one sim can use one side of a regular staircase. Stair Straight Mod: This mod allows multiple sims to use regular, straight stairs. There are a number of mods on this page related to stairs.

Sims 4 custom content stairs