A roadblock is a temporary installation set up to control or block traffic

Traffic

Traffic on roads may consist of pedestrians, ridden or herded animals, vehicles, streetcars and other conveyances, either singly or together, while using the public way for purposes of travel...

 along a road

Road

A road is a thoroughfare, route, or way on land between two places, which typically has been paved or otherwise improved to allow travel by some conveyance, including a horse, cart, or motor vehicle. Roads consist of one, or sometimes two, roadways each with one or more lanes and also any...

. The reasons for one could be:

• Roadworks
  Roadworks
  Roadworks occur when part of the road, or in rare cases, the entire road, has to be occupied for work relating to the road, most often in the case of road surface repairs...
  
  
• Temporary road closure during special events
• Police chase
• Robbery
  Robbery
  Robbery is the crime of taking or attempting to take something of value by force or threat of force or by putting the victim in fear. At common law, robbery is defined as taking the property of another, with the intent to permanently deprive the person of that property, by means of force or fear....
  
  
• Sobriety checkpoint

In peaceful circumstances, they are usually instantiated by the police

Police

The police is a personification of the state designated to put in practice the enforced law, protect property and reduce civil disorder in civilian matters. Their powers include the legitimized use of force...

, or road transport authorities; they are also commonly employed during war

War

War is a state of organized, armed, and often prolonged conflict carried on between states, nations, or other parties typified by extreme aggression, social disruption, and usually high mortality. War should be understood as an actual, intentional and widespread armed conflict between political...

s and are usually staffed by heavily armed soldiers in that case. During protest

Protest

A protest is an expression of objection, by words or by actions, to particular events, policies or situations. Protests can take many different forms, from individual statements to mass demonstrations...

s and riot

Riot

A riot is a form of civil disorder characterized often by what is thought of as disorganized groups lashing out in a sudden and intense rash of violence against authority, property or people. While individuals may attempt to lead or control a riot, riots are thought to be typically chaotic and...

s, both police and demonstrators sometimes use roadblocks.