Crowd control barriers act as a physical and psychological barrier,
used to demarcate "no access" zones, and to designate space for
lines. They are also used by riot police to control large
gatherings.Crowd control barricades are most commonly made of
steel. Barriers are most effective when they interlock, being
attached to each other in a line via hooks at the side of each
barricade. When barricades are interlocked, security personnel can
create impenetrable lines, because such lines of barriers cannot
easily be toppled over.