OG15

A new phenomena visible at Managua's major intersections is waves of vendors and beggars, many of them children, who mob cars at the stoplights.

Anyone will help me analyze the underlying structure?

Many thanks.

国内的语法教材称之为“独立主格结构”，你可查阅相关语法书。

同意楼上的 

独立主格（名+分）解释现行词vendors and beggars 的，一般都用逗号隔开。

语法上独立于主句，但意义上和主句紧密相连。