Photo by Frank Santangelo 

By William Tauro 

The City of Somerville has ordered a supply of these water-filled crash proof barriers. 

Once filled with water via a fire truck adapter, they are designed to prevent a motor vehicle from entering and endangering pedestrians during outdoor events.  Once these barriers are interlocked into each other, they make it very difficult for a vehicle to penetrate the perimeter that it is protecting. 
These particular barriers were used for the first time in Somerville this past weekend during the Ignite event that was held in Union Square.