The “principle that a filled distance appears longer than an empty distance is one of the oldest to have received formal recognition as a perceptual law” [1]. This “clutter phenomenon” or “filled space illusion” appears in a many conditions of spatial perception and cognition.

Estimating a route-distance between pairs of towns, either from memory or while looking at the map, produced longer estimations of routes when they passed through several cities and shorter estimations for routes passing through few cities [2]

When walking in pathways through a room (18m) while also attempting to remember names and information about several locations throughout the room, the more names and info a person can remember, the longer the total length of the pathway was estimated to be. This indicates a correlation of “increases in cognized distance with increases in stored route attributes” [3].

Similarly, routes with more turns are judged to be longer than straight routes [4]

Effects of clutter are similar to effects of higher-order regions where the more regions that a pathway crosses, the longer it will be judged to be [5].

1. Thorndyke (1981, p. 526)
   
2. Thorndyke (1981)
   
3. Sadalla et al (1979, p. 291)
   
4. Byrne (1979)
   
5. Allen & Kirasic (1985)