Roadblocks are wedge-shaped concrete slabs placed on most roads by either the police or military in the early stages of the apocalypse. It is unknown why exactly they were used, since any vehicle could easily get past them by driving onto the grass alongside the road.
Roadblocks cannot spawn anything.
They are useful for jumping over if you are being chased by a zombie.
Makeshift roadblocks are useful for people trying to prevent vehicle movement. Just throw down some building supplies, maybe arrange them and boom! You got yourself a roadblock.