How is movement explored in Piers Plowman (with particular reference to the first vision and the prologue)? I'm confused as to how exactly movement is conveyed within the text. I refer to movement both as spatial movement (therefore Will's movement between places and what it means, eg. his shift from one place to another to convey morality) as well as structural movement (a lot of research papers focus on William Langland's use of alliteration to evoke a sense of movement and stasis. Are there some actual examples of alliteration being employed this way?)

In the Prologue to Piers Plowman, Will falls asleep and dreams of a tower and a "deep dale" beneath the tower: 

"A[c] as I biheeld into the eest an heigh to the sonne,/I seigh a tour on a toft trieliche ymaked/ A deep dale bynethe, a dongeon therinne,/With depe diches and derke and dredfulle of sighte" (lines 13-16).

This movement from the tower, representing heaven, to the dale beneath that contains a dungeon with deep and dark ditches (representing hell), conveys both spatial and moral movement. As the dreamscape moves from heaven to hell, the poem moves in space and in morality. 
The rest of the Prologue contains the sights in Will's dream, in which he moves along the social hierarchy and sees different kinds of people. He starts with the simple plowman, or farmer, and describes the plowman in the following way: "Somme putten hem to the plough, pleiden ful selde,/In settynge and sowynge swonken flu harde" (lines 20-21). The plowman may be at the bottom of the social scale, but Will portrays him as the most moral kind of person in the hierarchy, as the plowman plays little ("pleiden ful selde") and sets and sows while working hard. 
As Will moves up the social hierarchy, the people he meets decline in morality. For example, he refers to jesters as "Judas children" (line 35), and he moves up the social ladder, making fun of the greed of the clergy (for example, in line 59, he refers to friars as "Prechynge the peple for profit of [the wombe]," or preaching for their own profit). He reaches the court of St. James, at the top of the social pyramid, and there finds a king who is a cat and a court of rats. Therefore, Will's movement witnessing different types of people as he goes up the social ladder is accompanied by a downward movement in morality, and the movement in this dream is both upwards towards social privilege and downwards towards immorality.
Piers Plowman also uses what is called an alliterative long line, which is a line with four stresses and three alliterative words (Baldwin; see the source below). According to Baldwin, the line "Under a brood bank by a bourne side;" has only 11 syllables rather than 14 syllables and its stresses are placed alongside each other, suggesting stasis and Will stopping in his wandering. The line "I slombred into a slepyng, it sweyed so murye" has 14 syllables, and its stresses are separated by unstressed syllables, suggesting the movement of the flowing water. Therefore, alliteration is also used to express stasis and movement.
Source: Baldwin, Anna. A Guidebook to Piers Plowman. Palgrave-Macmillan, 2007.