Roadblocks are a temporary installation set up to control or block traffic along a road. In peaceful circumstances, they are usually instantiated by the police, or road transport authorities; they are also commonly employed during wars and are usually staffed by heavily armed soldiers in that case. During protests and riots, both police and demonstrators sometimes use roadblocks.
Roadblocks are wedge-shaped concrete slabs most commonly placed by either the police or military on sections of road situated in between adjacent buildings in order to impede vehicle traffic in and out of a town and attempt to control the spread of the infection.
Roadblocks cannot spawn anything.
They are useful for jumping over if you are being chased by a zombie.