The extensive freeway system of the Los Angeles basin utilizes a very large number of modern bridge structures to distribute traffic at freeway interchanges and to carry the freeways over and under the city street systems. Most of the bridges are of prestressed concrete or reinforced concrete design and commonly box girder construction is used although some arch and girder type bridges are employed. In general bridges are the major structures on 
the freeway system and high earth retaining structures and tunnels are not common. Retaining walls are used on some of the older sections in the central area of Los Angeles City.